Archive for the ‘Deep Sky Astronomy’ Category

Losmandy HGM TITAN mount tracking speed and PE Measurements

Those of you familiar with this blog will be well aware of the tracking problems I’ve had with tracking speed descreptancies with the Titan mount. I finally decided to quantify accurately the amount of drift, using the free version of Pempro software, so that there can be no question left regarding this issue, as I wanted to send a report to Losmandy. People usual answer to my problem (Titan’s owner who do not use their mount or measure anything mostly) is that surely, I do not know what I’m doing, since Losmandy would never release a buggy mount to the public (yeah right!).  Also, I often read and hear real nonsense on forums regarding  mount’s Periodic Error (PE) and this is a good occasion to quantify it with real and accurate numbers and curves.

1. Background

1.1 Setup

The Observatory was equipped with a C14 EdgeHD on a Losmandy titan Mount in August 2010. The mount is fitted with a Gemini 1 Level 4 controller. The mount is a Titan 1:50 type, fitted with Maxon stepper motors. The observatory is located in the French Alps and its main purpose of the observatory is prime focus CCD astrophotography. The equipment was bought from Optique & Vision, Juan les pins.

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 Figure 1: Setup

In addition to the C14edgeHD, the mount is loaded with an Orion 120/1000 refractor used for guiding in Hyperstar configuration. The counter weights consist of four 20Lbs weights.

1.2    History

From earlier on, despite accurate polar alignment, it was noticed that average auto-guider corrections over a long period of time was not nil, as would be the case, if the auto-guider was only correcting for worm screw Periodic Error. CCD images were recorded at the time, with mount alignment off by a few degrees in azimuth so as to produce a drift on the DEC axis. The images clearly showed that tracking speed was not correct (RA is Y axis, while DEC is X; notice the average drift in RA).

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Figure 2: RA drift, Image Y axis is parallel to RA axis, and X axis parallel to DEC axis

The image above was taken in Sidereal tracking mode at 0° of Declination around the Meridian. At the time, it was thought that the root cause of the drift might be bad Gemini settings, electronics or stemmed from a mechanical problem. After ensuring the settings were correct, the electronics were changed twice and the entire mount once, across a number of months. Finally, a number of precision screws, manufactured by “Optique et Vision” were used. Although Periodic Error, while not formally measured, seemed improved, the tracking speed discreptancy remained. At this point, everything having been tried, no further work was performed in an attempt to solve the problem until summer 2013. The next idea was to see if, using Comet Rate/ User Defined setting, Tracking speed could be improved in order to minimize auto-guider’s corrections during exposures.

 

2  Tests and Measurements

2.1 Method

In order to measure Tracking speed discrepancy accurately, specific software from CCDWare, PEMPro, was used, as it allows accurate measurements using a CCD camera. In this case, an Atik 11000-CM camera, placed at the C14edgeHD prime focus, was used. In addition, ASCOM driver version 6 SP1 and Gemini.net version 1.0.58 were used to control the mount from the PC and perform calibration. Prior to testing, Mount polar alignment was refined and checked with PEMPro. The polar axis is off by 3’ to the West and 3’ to the South as given by PEMPro. These values were confirmed by the Gemini, upon building an accurate model using 15 stars, visible across the entire sky. The tests were made across 4 nights and starting on the 21th of July 2013.

2.2 Initial test on tracking speed error

Initial tests, performed on the 21th of July, only aimed to determine the amount of tracking speed error and see if the error could be cancelled out using User defined tracking mode. PE is not of interest at this point.  PEMPro was calibrated using the Atik 11000-CM sensor, and Sidereal tracking mode was used on the Gemini. The tests were performed pointing the C14 at an area  3° East of Meridian, at 0° Declination.

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Figure 3: Tacking in Sidereal tracking mode

Figure 3 above shows 3 worm cycles (or turns), lasting 15.95 minutes in total (1 turn is 319.1262 seconds). From the beginning of the blue trace to the end of the pink one, there is  about a 20” drift (apparent angular drift of the guide star). The tracking speed is too fast (East is down, West is up on the PEMPro graph). The mount was then put in Comet tracking/User defined tracking mode, as this mode allows for RA Divisor timer adjustment. Figure 4 shows an extract of the Gemini 1 manual, explaining the use of the RA Divisor Parameter.

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Figure 4: Gemini Manual extractdown

In order to slow the tracking (37398 being the RA Divisor value in Sidereal tracking mode) the value of RA divisor was slowly increased. After each increase, a new acquisition was made by PEMPro to evaluate the drift.

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Figure 5: Gemini advanced settings panel

With a RA Divisor value of 37500, it was found that the drift was fairly well corrected (Figure 6).

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Figure 6: Tracking with RA Divisor value of 37500 on the 21th

At the time, it was assumed that 37500 was the correct setting. However, during further testing on the 26th of July the value had to be adjusted to 37485 (Figure 7).

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Figure 7: Tracking with RA Divisor of 37485 on the 26th

Atmospheric turbulence on the 26th was fairly good, allowing accurate acquisition to be made. Testing on the 27th led the RA Divisor to be adjusted to 37475 to cancel drift out. Again, all testing was done using the same area of sky. On the 31st the proper RA divisor value was found to be 37460 (Figure 8). Because previous measurements were all carried out using an SCT telescope (with a clamped primary mirror) which can be subject to flexions, Figure 8 test was done using the 120/1000 refractor mounted parallel to the SCT (refer to Figure1) by means of a a heavy duty H plate.

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Figure 8: Tracking with RA Divisor of 37460 on the 31th, 120/1000 refractor Atik16IC

2.3 Tests on Periodic Error

Tests carried out on the 31st of  July focused primarily on evaluating Periodic Error of the mount. An analysis of curves acquired on the 26th of July (Figure 7) was added at the end as seeing was particularly good that day.

It had been noticed during previous tests that PE did not have the same amplitude when flipping Meridian (hence when the tube is West or East of its peer), while pointing  an area of the sky 3° East of Meridian at 0° declination. It was also noticed that mount balancing had an impact, meaning if telescope tube and counterweights balanced each other perfectly or not. 5 sets of PE analysis were carried out:

  1. PE test1: Tube positioned East, Mount balanced
  2. PE test2: Tube positioned West Mount balanced
  3. PE test3: Tube positioned East, Mount slightly “tube heavy
  4. PE test4: Tube positioned West Mount balanced “tube heavy”
  5. PE test5: Tube positioned East, Mount slightly “tube heavy” previously acquired 26th of July

2.3.1       PE test 1

This test (Figure 9) is performed pointing an area 3° East of Meridian, 0° Declination, Telescope tube East, mount balanced.

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Figure 9: Test 1 Periodic Error

The result of the test is a +5.3/-6.9 arc-second PE. Frequency spectrum of the PE is shown Figure 10.

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Figure 10: Test 1 PE curve Harmonics

2.3.2     PE test 2

This test (Figure 10) is performed pointing an area 3° East of Meridian, 0° Declination, Telescope tube West, mount balanced.

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Figure 11: Test 1 Periodic Error

The result of the test is a +10.1/-10.2 arc-second PE. Frequency spectrum of the PE is shown Figure 12.

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Figure 12: Test 2 PE curve harmonics

2.3.3       PE test 3

This test (Figure 13) is performed pointing an area 3° East of Meridian, 0° Declination, Telescope tube West, mount slightly tube heavy.

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Figure 13: Test 3 Periodic Error

The result of the test is a +10.1/-9.1 arc-second PE. Frequency spectrum of the PE is shown Figure 14.

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Figure 14: Test 3 PE curve harmonics

Please note that stray harmonics amplitude is greatly decreased.

2.3.4       PE test 4

This test (Figure 15) is performed pointing an area 3° East of Meridian, 0° Declination, Telescope tube East, mount slightly tube heavy.

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Figure 15: Test 4 Periodic Error

The result of the test is a +5/-7.2 arc-second PE. Frequency spectrum of the PE is shown Figure 16.

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Figure 16: Test 4 PE curve harmonics

Please note that stray harmonics amplitude has increased.

2.3.5     PE test 5

This test (Figure 17) was performed pointing an area 3° East of Meridian, 0° Declination, Telescope tube East, mount slightly tube heavy.

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Figure 17: Test 5 Periodic Error

The result of the test is a +5/-7.2 arc-second PE. Frequency spectrum of the PE is shown Figure 18.

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Figure 18: Test 5 PE curve harmonics

Please note that stray harmonics amplitude is quite high, even though PE is very good.

2.3.6      Periodic Error Synthesis

Below is a summary of the PE measurements:

PE test

PE (p to p)

PE (RMS)

Tube position

Remarks

1

12.2”

1.208”

East of peer

Mount Balanced

2

20.3”

1.159”

West of peer

Mount Balanced

3

19.1”

1.304”

West of peer

Mount slightly tube heavy

4

12.2”

0.817”

East of peer

Mount slightly tube heavy

5

5.5”

1.237”

East of peer

Mount slightly tube heavy

Looking at the summary table above, it’s obvious that the side of the worm  in contact with the teethed wheel has an impact on PE. That, and also, the amount of pressure on the worm due to Tube/CounterWeight imbalance. In my case I get far better PE measurements when the scope is East of the pear. What’s important, is that, for a given mount, depending on the tube balance and on the position of the tube, PE can vary quite widely. This means, that, whenever performing PE measurement to assess a worm, several measurement should be carried out with, different balance on both sides of pear.

 

3  Conclusion & Analysis

3.1  Tracking speed error

For the author, there was no doubt whatsoever that standard mount tracking speed be it in Sidereal mode or King mode, was not appropriate since a drift occurred. What came as a surprise is that it is impossible to set a correct speed by setting a fixed RA Divisor value, since this value changes from night to night to get proper tracking. Across 4 nights, RA Divisor value ranged from 37460 to 37500… Setup; sky area tested on, was exactly the same every night, the only difference being a slightly different temperature, ranging from 15°C to 20°C. But, looking at the values there doesn’t seem to be a relation between temperatures and RA Divisor value. In any case, it is difficult to explain how the Gemini quartz controlled time base could drift by that amount by a 5°C temperature change.

Also, what seem a bit “strange” is that during testing, while RA Divisor value seemed appropriate for the first couple of worm cycles, the last cycles seemed to show drift appearing again (figure 19).

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Figure 19: Tracking speed drift across time

At first, it was thought that this was caused by the telescope passing Meridian, but, since the mount was “tube heavy” there should not be any change. Also, what is important to consider is that the RA stepper motor is used in closed feedback loop, where an 4096 step encoder ensure that the motor runs at the speed dictated by software. So, unless there is an inaccuracy within the Gemini time base (Gemini controller was changed thrice), which should logically have been spotted by other Gemini Titan 1:50 users, it only leaves one cause for the drift: Atmosphere. Indeed, the observatory is situated on the flank of a large mountain, and it is quite possible those atmospheric temperature gradients are not that of a plain. Since refraction calculations show quite a large impact on object apparent position (minutes of arc) declination wise, it is quite possible that for “non standard” atmospheric temperature gradient to impact apparent sidereal movement. Also, this could explain the difference in RA Divisor values changing in time. Unfortunately, the telescope being at fixed station, it is not possible to move it to a different location and acquire data to ensure this.

But, at the end, the mystery remains… Can minor changes in the atmosphere refractive properties affect visible star movement to a few seconds of arc? I would appear to be so, or, otherwise, Earth rotation on the planet I live is not as regular as my Titan mount would like it! If you have any idea to explain these results, please let me know!

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Hyperstar for the C14 EdgeHD!

That’s it ! I finally got the chance to try the Hyperstar from Starizona on my C14, after a 3 month wait due to poor weather… But, as they say, good things come to those who wait. At first glance, I was very impressed with the manufacture of the Hyperstar. It came in a very nice “Pelican case”, and it looks very well engineered. Everything is machined, not cast and the large optics are superb. To install it, the secondary mirror has to be removed and is placed  into the bottom threadcap of the hyperstar. By instinct, one would expect the hyperstar goes onto the secondary housing in the same way as the mirror, but that is not the case, it just screws itself onto the secondary lockring threads: fronthyp

The Hyperstar possesses three sets of screws: a set to allow for rotation (handy to orient an object on the sensor) and the last two sets which are a common pushpull setup to allow for collimation. Yes, apparently, on most scopes, the hyperstar has to be adjusted so that the stars are nice and round across the entire field. On that subject, I feel extremelly lucky as my stars are fine across the field without having to tweek the setup screws. Speaking of field, Starizona states that full frame sensors such as my Atik11000-CM are not supported as the field is only corrected to a 24mm by 24mm (roughly an inch by an inch). Well, what I found is that it still works extremelly well with my sensor… Stars are ok in the corners even if there is some vigneting, but flats will take cae of that.

For guiding, since the focal length of the Hyperstar C14 is 700 mm, I choose to mount my Orion120 EQ in parallel to the main tube using the heavy losmandy H plate:

sethyp

notice that I have changed the native orion crayford for a Teleskop Service one which is alot moe rigid and accurate. The focal of the guider is 1000 mm, which works well to guide the C14.

mounthyp

You can see details of the H plate on the picture above. The whole assembly is pretty rigid but still requires 80 Lbs of counterweights on the weight bar.

So, I was ready for a test last night. I took a bunch of flat field shots at dusk as is my habit, and they seems to work quite well. Then, I went for 20x 5 minutes exposures on IC1396. There seemed to be good signal on the raw exposures, but, the background seemed to come up quickly, thanks for the combination being so quick! Imagine, a 14 inch scope opened at 2! It has to be said that I did no choose the easiest target, as my skies are not very dark. Anyway, below is the processed full rame image I acquired last night:

ic1396small

The main problem I get working without filters is the shear number of stars… Anyhow, there is a new “Hyperstar” sub-Section to the piture galleries where I’ll be putting the newly acquired images.

Clears skies

Serge

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Imaging lessons learned: aquiring flats and dithering

It has been over 2 years since I started using the Atik 11000-CM, and I can say it has been a steep learning curve. When you consider there are plenty of tutorials out there, why bother writing this at all? Well, for a start, very few tutorials available across the nets focus on the real basics of image aquisition and are more centered on image processing. The thing with astroimaging is that if you put garbage in you get garbage out… Knowing this, I felt some people may get some valuable information from my collection of mistakes (largely due to my abysmal ignorance) to improve their own images. While this posts is focused on CCD images acquired with a colour Atik CCD camera, I see no reason why it might not be applicable to monochrome CCD as well.

So lets start with an image of  NGC 3945 I acquired not so long ago, and see what is wrong with it and how it could have been better, processing aside (please note that you require a good monitor to view the example images):

ccdprocex

At first glance, suffice to say this image is not very esthetic… Ok, someone very new to astroimaging might get excited about it, as it clearly depicts the subject, but, when you compare it with pictures of the same object taken by more accomplished astroimagers, the difference is great. So what’s wrong with it? Guiding is pretty good, resolution is average for a deep-sky image, and anyway, there is not much to be done about seeing on a given night. What jumps out is that the background is very noisy. Also, there is an ugly vertical column to the bottom left of the image…

Noisy background.

let’s start by looking at a magnified section of the image above:

procnoise

the noise is pretty obvious… The main question is “where does it comes from?”.  The picture above results from median stacking of 21 sub-images, each calibrated with 15 flats, 21 offsets and 21 darks (if you are not familiar with image calibration, google it up).  Before  you jump to the conclusion that I did not use enough flats, I did try to use as many flats as subs one some other images, with the same result.  What tends to happen, is that noise pops up whever I try to reach low during processing, trying to reveal extremelly faint details. On the subject of flats, they have to be themselves calibrated by substracting the offset. I did hear that some astroimaging software had some bugs which caused offsets to be substracted twice from the flats, which did cause them to be noisy. However, I did check that MaximDL, which I use for calibration as well as processing, does perform calibration properly.

So, I stretch my head as to where that noise might be coming from… Darks frames? Well, darks are pretty simple to acquire, I mean, you make sure the CCD temperature is right, that you have no light leaks, and away you go. Bias (offset) frames? Same thing as darks, If I can mess these up, I better change hobby. This leaves flat field frames, and I started thinking. I do not have a specific flat screen, as it’s very expensive for a  C14 sized telescope, so I use the evening sky for my flats. It’s extremelly tricky. If you try do do flats while the sky is still too bright, then, some columns of pixel are much brighter that others, even though the image is not saturated due to a very short exposure length. It is something to do with the fact that some pixels on the CCD chip have an allowable maximum light level exposure, regarless of how short the exposure time might be. So, I do wait for the sky to be fairly dim (at dusk or down) and I usually set around half a second exposure which usually leads to an average of 4000 ADU (ADU:Analog Digit Unit), on 65578 ADU,full scale. I did read somewhere that the proper level for a flat frame should be about 30% of total dynamics. Hmmm… So, I tried to increase my exposure duration for flat field to reach about 20000 ADU intensity. On the same image, I used some of my usual flats from the day before to calibrate the subs, processed the image, then repeat the processed with the “new” flats. And guess what? the background was now alot nicer noise wise! Even when going extremelly deep within the picture. Problem solved… It just shows that, for astro photography, everything has to be just right for a good result.

Image Dithering.

Again, let’s look  at a magnified section of the image above:

deadpix

See that very dark column? Well that comes from a column of dead pixels on my class 2 CCD chip. Usually, since all subs have some form of shifts among them, it disapeared upon stacking. The shift is usually caused by minute shifting from an image to the next (that is the reason why subs registration is required) . But, in my case, even though the Titan mount has not got a great PE or tracking speed accuracy, Off Axis guiding does it’s job and keeps everything on their “assigned pixels”. It also keep the dead column in the same spot of the image, as a consequence…

So, to get rid of that problem, the solution is very simple: DITHER! Dithering means, that every 5 subs or more, it depends on total number being aquired, you need to shift the image by a few pixels on your sensor, so that the CCD chip defects disappear upon median stacking, just like a satellite trace on one of the subs would. What I do to achieve this is that, in between subs, I switch off the auto guider for a few seconds, and that is sufficient to shift the image by a few pixels, thanks to PE (periodic error) or as it is the case for me, sideral speed discreptancies. Of course, when you do, like me, go to your bed after having setup everything to acquire images for the rest of the night, it is easier said than done…

Clear Skies

Serge

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The secret to guiding with MaximDL

Few people have expressed interest due to the fact that I am able to guide properly and accurately with such a long focal length as that of the C14 (4 Meters or 13 Feet). Indeed, some people thinks it’s a feat or an achievement to be able to track a star with anything longer than 1.8 Meters (8 Feet). Well, I have learned imaging techniques with my current setup on my own, so I do not think much of it, especially since my only other guiding experience was using a C11 with a DSLR guided by hand!…

So, what’s the secret to proper guiding? As I see it below is a list of preequisite, from the most important to the least important:

  1. A very stable and robust mount well sized for the weight of the OTA.
  2. The mount has to be properly aligned using King or Bigourdan alignment method.
  3. Camera setup must be solid and very rigid so that here aren’t any flexures.
  4. The guiding/imaging Software must be free of bugs and well calibrated.

My current setup fulfill all the above. Losmandy Titan mount is massive enough (it can withstand 90 Lbs loads if I remember correctly) to support the C14 EdgeHD and all it’s accessories without effort. I try to keep the mount well aligned using Bigourdan’s technique (detailed in another post). Finally, even though my OAG (Off Axis Guider) is old, it support both the Atik 11000-CM imager a well as the Atik 16-IC cameras very rigidely and MaximDL works reasonably well. That being said, the most important factor to accurate guidance is: A good Calibration!

Calibration

Calibration is used by the software that is beeing used for guiding, be it MaximDL or other, to tell it by how much and in what direction to move the mount in Right Ascension or Declination in order to put the guide back onto it’s assigned sensor coordinates. So, in that respect it is critical. I usually try to align the guiding camera axes with the axes of the mount as it makes it easier for myself  to see what the mount is doing, and it what direction the guide star tends to drift. That tells me things on how well the mount is set up, but it really is not necessary as any guidance software is smart enough to decide which motor of the mount to act upon. Also, a guide star, with a well set up mount, will only tend to drift along the Right Ascension (RA) axis, because of  ”periodic error” (which is an inherent defect of the wormscrew mechanism) thus it should only necessitate to correct the mount along that axis.  Let’s look at what the calibration window looks like in Maxim:

calibs

What you see here is what the calibration window looks like just after the calibration has been completed. Prior to this, the “expose” radio button will have been used after each small movement of the mount in an attempt to locate a guide star (which is not always easy). I usually use a 2 to 4 sec exposure time to locate a guide star. Of course, if you’ve already performed a calibration on an area of the sky not too far away from the area where you are planning to image, and the cameras have not been rotated, then there is not point to do it all over again. For calibration, what the Software does is pilot the mount to move a few seconds in declination, then the same amount of time back, before doing the same in the RA direction. The amount of time used can be set under  the “setting” button as seen on the picture above under “agressiveness” (see below).

Guide_window

You should aim at displacements around 100 pixels in each direction and adjust the time accordingly. Now, as you can see on the picture, the axes of calibration are nearly aligned with the x and y axes of the guiding camera, since I try to align it with the scope axes. Due to mechanical inaccuracies of the mount and motors, such as backlash and periodic error, it may happen that the star does not quite always go back to it’s original position in between calibration steps, but, there is no much that can be done about it apart from trying to set a “backlash” value, again, under “settings”. Also, because the software has to be able to clearly “see” the star, it has to be bright enough, otherwise, it might pick on a hot pixel of the camera window. If that happens, the best thing to do is to increase exposure time to make the star brighter.

Ok, so now Maxim is calibrated meaning  that it can relate guide star errors in pixel to amount of movement to send to the mount to set the star back to its assigned position.

Guiding

Now is the time to select the radio button “track” and see what happens. In track mode, a small window with the guide star in the center will pop up (see picture below).

The size of this window is selected under “options” in the guidance window. Generally, if all is well, the guide star should remain firmly in the middle of the window so it does not have to be that big. But, if you knock the mount or if for any other reason the star was to drift out of the window, then Maxim “can’t see it” anymore and is therefore unable to pilot the mount to put it back in the center. Normally, the star will drift by only a few pixels around the center of the window (right click on the window and select “crosshair visible”). It may happen, because of turbulence, that the star jumps around the center of the guidance window. There is not much that can be done about it appart from trying to increase guidance exposure time to try to average out turbulence. Exposure time will also have to be set according to how quick periodic error of the mount is causing the star to drift. If periodic error curve is very steep, then it might be necessary to reduce exposure time in order to correct the mount more often. For example, with a 4 second exposure time, the mount is only corrected once every 4 seconds, whereas, with a 0.5 Second exposure, the mount is corrected twice per second…

Tracking can also be improved by setting “agressiveness” parameters to their correct values. No, do not worry, even if you set it too high, the mount is unlikely to bite you, but it sure will destroy your image… What agressiveness does,  is multiply calibration parameters some so that the mount is over or under corrected. For example, if you notice that after each correction made, the star does not move back to the center of the crosshairs, then it is undercorrected and agressiveness has to be increased for the applicable axis. On the other hand, if aggressiveness is set to high, on oscillation can occur where star movement is over corrected and star moves around the crosshair. I generally find that a value of 5 on both axes works for me.

Guiding is not such a dark science, even trying to guide with a C14 native focal length. But, tyrying to understand what the software is trying to do goes a long way toward reliable and accurate guidance… Remember to set the appropriate audible alarms in case guide star is lost, use “autosave” function to program the number of exposures, set the alarm clock,  and go for a snooze, either in your bed or under the stars!

When you get back up again, the chance is that the guide star is still being faithfully maintained in the center of the crosshairs, and your hardrive will be full of wonderful deep sky sub images!

Happy Imaging

Serge

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My best image of Jupiter as Yet with the C14EdgeHD!

On the 11th of november, the scope reach thermal equilibrium at about 10pm, thanks to the temperature difference between day and night being pretty small these days. Just in time to have yet another go at Jupiter with the modified webcam. I first did a quick check on a nearby star, and the collimation was about spot on and only a small tweek was required. The picture below was taken at about 22:58 UT, just as Europes shadow (visble below the South equatorial band) started transiting on the planet. Seeing after that quickly deteriorated. Anyway, this is my best ever image so far!

jup13proc
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Fitting thermometers to ensure Thermal Equilibrium

If any of you have followed discussion on Yahoo C14EdgeHD group, or some of my older posts, you’ll have noticed that getting a C14 in its EdgeHD variety thermally balanced is extremelly difficult. While thermal equilibrium is not all that important while doing deep sky imaging, it is absolutely crucial for imaging planets. The reason is that for planetary imaging, you need to get as near to the theorical resolution of the scope as “seeing” of the atmosphere will let you. While the primary mirror has not reached the same temperature as the rest of the telescope, there will be convection currents of air either warming up or cooling down on the primary, and that will destroy maximum resolution to some extent.

Because the EdgeHD  telescope serie has that lens inserted into the baffle tube to correct the field of curvature of the image plane, the tube is effectively sealed and there is no room for circulating external air through the baffle tube as can be the case with “standard” C14, either forced through a fan, or natural, through convection. The vents at the back of the tube do not seem to have any cooling effect on the primary mirror whatsoever… a telltale sign of internal tube air convection currents is slowly moving areas of brightess or shadows while looking at a unfocused star. In any case, until a few days ago, I was not able to get the scope in thermal equilibrium, due to the fact that the days were alot warmer than the nights and that the primary mirror would remain “hot” for an extremelly long time as to the extent of making the scope next to useless for planetary work. These days, because the difference between daylight and nightime temperature is less, I found that the scope will reach equilibrium within about 4 hours (just in time for Jupiter!).  Now, beeing able (with a bit of experience) to tell whether the tube is thermally stable by looking through the eyepiece is OK, but it is still subjective. What this post is about is to show a non destructive way to rig two 10 dollars thermometers to establish whether or not the primary mirror has reached the same temperature as the tube…

thermor

You can see on the picture above that the thermometers used are cheap digital ones with a long sensor wire. The first thermometer is located by the corrector plate; and the sensor is taped to the aluminium tube using think silicon tape. The second thermometer measures the primary mirror temperature. there is no need to drill any holes as the wire is fed through on of the “cooling ports” and its sensor is taped to the back of the primary using the same tape as for the other sensor.

Well, as far as I can tell it works! What I did is monitor tube currents the usual way through the eyepiece (looking at a unfocused star) after installing the thermometers, and sure enough, until both sensors read  0.2 Deg C difference, some convection currents were visible. Once the primary reached ambiant temperature, I used the webcam on Jupiter and the result was one of my best picture so far…

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Extreme deepsky astrophotography

Hi folks.

Well it has been 2 weeks since I touched the scope, even though usually, regardless of work the next day, I will open up every time the skies are clear. This has to do with the fact is that, due to a combination of Myself/My site/my tube, I got extremelly frustrated with planetary imaging, that I needed a break from my scope…

So, these past 2 nights, I decided to get back to what I do resonnably well, namely deepsky astro-imaging. And, to celebrate my reunion with the telescope, I decided to shoot difficult targets: NGC7814 and IC5146. I had never done either of them so I thoroughly checked data on these objects and decided to spend a night each, since they are not very bright. For NGC 7814 (aka Caldwell 43), I decided to use 24x 600 Secs exposures which amounts to 4 hours in total. Transparency that night as well as seeing were average. Because the core of this galaxy is very bright and the extensions are very faint, it is quite a tricky subject to process, and I used Maxim’s ddp to adjust the galaxy:

NGC7814s

The result is quite pleasing to the eye. Also, the reason this post is titled “extreme astrophotography” is because, looking at the full-res picture (available in the C14 gallery) there are litterally 100′s of small galaxies on this picture… I managed to identify one of the brighter one whose magnitude turned out to be 21.46, so my guess is that the faintest magnitude visible here is at least 22. Mind you, I have never exposed anything for so long. I am generally never satisfied with my pictures, but I have to admit I really like this one.

Then, about IC5146 “cocoon Nenula”; I saw some stunning pictures of it on the NASA website, realised it was just the right size for the C14, so I decided to have a shot at it last night. After the first 10 minute exposure, I was not sure if I was going to get anything out of it, as the nebula was very faint. I went for 18×600 Sec exposures or 3 hours total exposure time. Because the more exposure, the better signal to noise ratio gets, once I finished combining all 18 shots, pushing the cursors a bit actually showed something. I then tried different techniques, filter, DDP with maxim, but I got the best result simply by adjusting the image curve… This is one very difficult object which necessitate very good transparency, as was the case last night. The resolution is quite good as well, since the smallest double stars are fully resolved.

IC5146curvess

This of course, is not a NASA picture, but I think it could get published by some astronomy magazine. Because it is a difficult object, you do not see amateur photographing it so often, as it is quite small and requires a long focal length to show as many details as possible.

Then, I wanted to do NGC 660 which is a beautiful galaxy, but, my sensor glass window decided to fog up after 5 exposures, so, it will have to wait. Tonight may be?

Serge

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How does a colour CCD camera interpolate Pixels?

The other day,  Some integrist from the “Filter wheel Clan” left a comment about my article on “Mono vs Colour CCD Camera”. This person claimed that Colour cameras were less sensitive than equivalent monochrome ones by as much as a third! Because I once compared performance of the Atik 11000CM camera with that of its monochrome brother, the Atik 11000M, on the same night, on the same scope and on the same galaxy, I know for a fact that he’s wrong. His logic was that both cameras have the same amount of pixels of the same size, but a color camera pixel only absorbs a third of the light “falling into it” because of the Bayer Matrix (green, red or blue filters in front of each pixels) . Well, I have to admit that it’s pretty logical. Take white light, for instance, and assume it’s made of one third green, one third red, and one third blue light; each green, red and blue pixels only absorb it’s corresponding wavelength which is a third of the total available to a  monochrome camera, compared to a monochrome camera where each pixel absorb the entire amount of white light due to it’s lack of filters. Lets look at a Bayer matrix to illustrate this:
bayer

Now, what happens if we are photographing a red H alpha emission nebula, whose wave length falls spot on into the passing bandwidth of red pixels? 80% of the light will be recorded by the red pixels of the colour CCD, the 20% loss being due to transmissive losses of the bayer’s matrix red filters, but nothing will be registered by the surrounding blue and green pixels. So, in essence, we have 80% sensitivity of a monochrome camera, but now, the resolution has been devided by 4! So how does a color camera compensate for these shortfalls, since as mentioned at the beginning of this article it’s been compared to a monochrome camera and it obviously does… Well, what color conversion software does is use a pretty smart algorithm. To illustrate this, let’s look at a raw Oneshot color camera picture with some hot pixels:

b&n

This example is very useful because it simulates a bunch of photons hitting one single pixel of it’s “bayer matrix” group (made of 2 green, one red and one blue) and it allows us to see what it does with the processing to reconstitute a color image. On this picture, you can easily see the matrix with the blue and green pixels being dark while the 2 green pixels in diagonal are lighter due to light polution. So now let’s look at the processed color image interpret the hot red pixel on the right by the star:
color

Hey, what happened? well adjacent blue and green pixels have been “filled” with red color! So we have indeed lost contrast…  What happens is that the software looks at adjacent pixels, and if they are dark, it will assume the light is monochromatic and fill these pixels with higher levels of red color. Also, the hot red pixel is now white! This is called “Synthetic Luminance”; as the software interpolate light levels by averaging pixels and blocks of pixels. What’s also interesting to note is that the  hot pixels on the left, not being as intense, have kept their original hue, and the adjacent pixels have been only sightly tinted… So, the conclusion is that we have  not completely lost the original resolution, but that the contrast of the image has been degraded compared to a monochrome one… As for sensitivity, because the software “adds” adjacent pixels (red, blue and green) to determine final pixel level, this goes to some distance toward restoring sensitivity compared to a bare mono sensor, but at the cost of lower resolution.

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Tribulations with the C14 EdgeHD optics!

Hello everyone,

it’s been a couple of month since my last post; mainly because of the weather which has been absolutely crap here, partly because, the scope was sent back to the importer, Médas, for a check and service. Here is why:

It has been about a year since I started using the C14 EdgeHD, and I have to say in all honesty that I still have not been able to get an half decent planetary image out of it. The main reason for this is that I live and observe in a “washing machine”, meaning on a mountain flank. What that does is that winds and mountains interaction causes alot of air turbulence, reducing seeing.Turbulence is easily identified looking through the eyepiece and watching a star “dance”. That said, it is hard to believe that, in the course of one year, there has not been a single night of good seeing. So, the second reason for not getting good images may be the scope itself; and that is the hardest part to evaluate. So, last March I really wanted to have the optics thoroughly checked and drove the telescope down to a company with an Haso optical testbench. What they did is measure up the wavefront accuracy at different wavelengths and metalback distance to quantify error of the C14 optical chain. They also tried to rotate the corrector plate to select the best position, which ended up being 120 Degrees from original’s. Here, you will find the test report. What it shows, when you do get used to looking at measurements in nanoMeters, is that the optics are quite decent for a scope that size roughly at Lambda/4. So, with newfound confidence in my telescope quality, after performing what I thought was  a good collimation, I set out to image planets, with the same result as before…

Since there is no substitute for my own eye, and I have never managed to see a star Airy disks on a real star, I set out to perform an extremelly precise collimation using an artificial star placed 50 meters away. Only that, with a focal length as long as a C14′s the star would need to be at least twice that distance from the scope in order to focus on it, which the terrain does not allow me to do… So, the solution is to place the eyepiece at a rediculus 30 cm (1 foot) distance from the scope metal back. To achieve this, I used a crayford, a barlow, a diagonal mirror and finally the eyepiece… And… For the first time, I was able to see diffraction rings! Here they are:

Then I was horrified! Indeed, those of you fluent in optical speak, won’t fail to notice that there is alot of trefoil and spherical aberration present, looking at the star. Apart from the fact collimation can be improved that is… How can that be, since the report shows that the optics are good? Well, this is because a C14 is not a microscope so, looking at a star so near it will magnify original optical defects tenfold. So, apart from allowing me to do a proper collimation, which is relative since the tube is horizontal, this sort of test is no use to evaluate optics on an SCT.

Still, the aspect of the star kept nagging at me, and I really wondered if the corrector plate might not had been better of left alone and not rotated. So, to set my mind to rest, I contacted the Celestron importer who is equipped (litteraly) to check SCTs and talked to them. They got the scope collected, and did a thorough check on it. What they do is that they own an high quality reference C14, stick a light at the back end, and place the tube to be checked in front of it. So, a fairly flat light wave front comes out of the reference instrument, which look like a star to the instrument beeing checked… Pretty crafty stuff which is a variant of auto-collimation. Then, you just stick an high quality eyepiece into the tube being tested, and you realign everything so that you get the best looking Airy pattern you can. It’s just about as good as an optical test bench without computer if you ask me. When I finally got the scope back after a couple of weeks, the corrector plate seemed to be back to it’s original position (hum… I wonder why).  Now, suffice to say that beside striving to achieve perfect collimation, I am done with messing around with my SCT, as the optics ARE GOOD. it’s the place where it’s being used as well as the user who are crap!

Now there is a very important point to be mentioned here about the EdgeHD variant of the C14 that impact performance tremendeously. this is what a unfocused image of the artificial star looks like after 4 hours ouside; notice the nice heat plume:

bilanun

Cooling and reaching thermal equilibrium is nearly impossible to achieve on an EdgeHD scope, because of the lens that now obstruct the baffle tube. There are no air currents inside the tube to help cooling, and the inbuilt air vents are there because Celestron probably knows that, but their effectiveness is next to nil. Also, the new fancy mirror cell casting won’t allow for rigging up cooling fans. The only way around I see is to make a fan assembly wich fits to the faststar cell, blowing air into the tube then out of the vents. That, and sticking a temp probe to the back of the primary through one of the vents and one stuck to the aluminium scope tube. When the 2 reads the same temperature, you can then be fairly confident your scope is at equilibrium. I’m planning to do this asap, so, to be followed…

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The differences between CCD and digital SLR cameras

It will be a year next month since I started using a CCD sensor… I know the number of CCD pictures taken might say otherwise, but no, it’s only been a year, give or take… Prior to that, I was using a stock, then baader modified, Canon 350D SLR mounted behind a C11, or with my Astrorubinar piggybacked on the C11. When using the main scope, I was guiding by hand using an off-axis guider and a 12.5 reticule eyepiece, but, knowing what I do now, the setup was much too inaccurate  to guide effectively. Using the 500mm/5.6 Rubinar piggybacked and guiding manually with the C11 did, on the other hand lead to good results. This is why I did keep some of these pictures posted in the Rubinar gallery. It is quite fun to retake nebulas I covered with the Canon with the Atik11000-CM CCD and compare the differences. A prime example of this are “The veils” in Cygnus. I had originally taken the eastern veil because I could not fit the whole thing on the Canon sensor which is not fullsized. So, last night, I started centering the CCD equiped Rubinar in order to get the same shot. Only that I forgot that the Atik sensor was much larger that the canon’s and now, I could get the entire nebula to fit on the same picture. Here are the two shots, the “original” (13×6 min exposures processed unde Iris software):

NGC6992

and the newer one (9×10 min exposures processed under MaximDL software):

Dentellesraw1s

The difference in sensor size really is apparent isn’t it? Also, while the Canon image is quite good by my standards, the CCD picture is alot crispier thanks to a better focus as the 350D lacked any tools (FWHM) to achieve perfect focus. Also, the colors of the CCD seems more “true life”, if there is such a thing. Click on the pictures above to see them full size.

Another example for my American friends is “North America” Nebula taken first taken with a 200 mm Sigma telelens (as it would not fit at all on the canon sensor with the Rubinar):

ngc7000

it is the result of 13x6min exposures on 350D as well. Below, the result from 9x10min CCD exposures:

Americaddp2

Lucky are you to have a dedicated Nebula for your country (I’ve yet to look for an hexagon shaped nebula)! Anyhow, here, even though the field of the Canon shot is much larger than that of the CCD (thanks to the short 200mm telelens focal length), there just is no contest with the CCD camera. In conclusion, it is much easier to get good result with a CCD than with a SLR CMOS sensor for obvious reasons:

  • Signal to noise ratio is much better with the CCD since it is cooled to -20°C or so. This is the reason why 6 minutes exposures at 400 ISO (or whatever settings since ISO sensitivities are completely artificial in a SLR; the sensing chip has ONE sensitivity full stop…) is the maximum for a 350D at 20°C.
  • Quantum efficiency is much better as well for a CCD compared to a CMOS chip, so it takes a shorter exposure to record the same amount of photons.
  • Even though that has changed with the newer Canon 450D and the Liveview software, there was no way to achieve perfect focus with a 350D apart from trial and error.
  • It is much trickier to capture proper colors as the original Infrared Canon sensor filter has to be removed or replaced with a third party in order to record Halpha wavelength (pretty obvious on “America” above which has a strong red tint to it.

But, to be fair, the internet is full of examples of what can be achieved with a well used SLR. Some of them are indeed impressive and do rival with what CCDs do. Also, one item where an SLR cannot be beaten  is price. It is true that an SLR, even modified, will cost a few hundred Euros/Dollars whereas an CCD costs a few thousands. Also, you can’t take pictures of your familly with a CCD! For these reasons, I think SLRs are the best way to start astroimaging, especially is you are not sure you will last long in the hobby. When the urge to photograph the sky is over you can still carry on snapping the wife and kids! But then again, more often than not, after that first picture, you’ll be completely and hopelessely hooked…

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