Archive for the ‘General Astronomy’ Category

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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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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Crayford focuser

A crayford focuser? on a SCT? why for?

Well, Crayford focuser can be very useful for SCT but, I strongly believe, it won’t replace the built it focusing system which works by moving the primary mirror. Why? because, if you’re like me, you can do loads of different things with your telescope, such as:

  • use it for visual observing with eyepieces, barlow lenses etc…
  • use it for planetary astronomy with a webcam or a webcam style camera on top of a barlow lens.
  • use it for deepsky astrophotography, in which case you have an Off axis guider, a CCD for imaging and a CCD for guiding

And the focuser cannot do all that.

crayford

The main benefits from a Crayford focuser is that it allows you to adjust focus extremely accurately. So, really, in an ideal world, it’s useful in all cases mentioned above. But, as usual, there is a catch. First of all, even though you do get focusers with different travel courses, the overall length of your focuser is important, especially for deepsky imaging where it is critical, especially for an Edge HD, where the field is flat 127 mm back and nowhere else. Also, the deepsky imaging setup weight to about 4 pounds, which is slighly too much for most Crayfords focusers. As a final nail to the deepsky imaging coffin, the only way to position the main imager to the correct back distance, is without crayford, as it’s too far back otherwise. Which is why, in my case at least, I can only use the crayford for visual observations and planetary imaging. The crayford is great in those cases, because, it allows me to adjust the native focus knob  to a point which is mid-range to both applications, and then, lock the primary miror in place so that nothing moves, especially collimation. Also, if your telescope is subject to shifting, as in, the image moves while adjusting the focus knob, then using a crayford will cure that.

As you can see, a Crayford focuser can not do it all on a Celestron telescope, so, thinking is required before buying one. Also, it has to be said that if you buy a cheap one of poor quality, you might endup with a focuser that is worst than the original arrangement.

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Losmandy Gemini sideral speed discreptancies

We all assume, when we buy a new mount, that tracking speed is spot on don’t we? Well, the same applies to me, and I never questionned my Gemini Titan accuracy. I did notice that my guider was correcting more in one direction (right ascension) than the others, but I assumed the cause to be a slightly misaligned mount… That is until I spent some time fine tuning my mount alignment, and surprise: the discreptancy was still there. I was aiming at some galaxy in Coma Berenices, and once I finished taking exposures, I decided to switch off the guider while taking a 10 minute exposure, and let the mount do its thing. you can see the shot below: traks

I was careful to align the camera X an Y axis to the mount right ascension and declination. So, on the picture, right ascension is vertical, and declination is horizontal. You can actually check that on a starmap, the galaxy is NGC4565. So, what you can see here, is that first, the mount is well aligned, since there are no drifts in declination at all, and that periodic error of the mount manifest itself with blobs on the star tracks were the starlight stayed a little longer on the CCD chip. So, what this shot would seems to indicate, is that my tracking speed, is too slow… I have tried to check for flexures, in my Off Axis guider, and I have done other tests with the Celestron primary miror locked in place with exact same results. I have also tried the same experiment on objects around 45° declination, and the results are the same albeit tracks are shorter (as it should be). Needless to say, I have double checked all the mount parameters and everything is as it should be. I contacted the company who sold me the mount, and they spoke about this with scott losmandy, so, to be followed. I’ll keep you updated.

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Welcome to the Home of Astronomy

Welcome to this Website. You will find valuable information related to Astronomy and Astrophotography.

Serge

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