Atik 11000-CM Colour CCD

Being accustomed to the ease of taking  pictures with a DSLR camera, I was not quite ready to start messing about with filter wheels and to require several nights to acquire a single object… So, the obvious choice for me was to get a color  24mmx36mm full frame sensor. ccd I went on the forums before taking the plunge and almost everybody was crying wolf, and telling me I was making a big mistake as color sensors are not nearly as sensitive as monochrome. While that may be true, when you have a 14″ aperture, you harbly notice it. I strongly believe that colour cameras are going to take the lead in CCD imaging as sensors become more sensitive and make monochrome sensors obsolete as these are more expensive too (filters and wheel on top of CCD cost). Being able to acquire Luminance and colour channels in one go really is fantastic and make everything easier. And, Astro-imaging should be a pleasure, not a hassle.  At the moment, people are so used to monochrome cameras that it will take time to change minds. The one drawback I find to a color camera is that, should you decide to binne the pixels, you lose colour information. Atik offers with the Atik 11000-CM, a full frame sensor at a very reasonable price, the drawback being of course, that you need an additional camera for guiding with an off axis guider (unlike SBIG). For guiding, I got an Atik 16-IC. My off axis guider is an old Celestron one I used with the Canon on the C11 telescope. It works reasonably well. A word of warning though: The field delivered by the C14 EdgeHD is absolutely flat, as Celestron claim, but the sensor has to be about 140 mm back from the black baffle nut on the back of the C14 telescope. I had initially tried to use my Orion 120mm aperture for 1000mm focal length refractor as guide scope for the Celestron telescope, by mounting it using Losmandy’s “DSBS” H plate in parallel with it. Unfortunately, I could guide for more than 3 minutes properly so I went back to the old off-axis guider solution.  CCDComments on the pictures delivered by this combination are welcomed. Image processing with colour CCD cameras are of course different from that of monochrome cameras. I personally use MaximDL to acquire and to process images. I am still experimenting, so any advice about colour image processing is welcomed. With MaximDL softfware, acquisition really is a piece of cake, since it manages main imaging sensor as well as guiding sensor. I usually take 10 minutes exposures at -20 Deg C sensor temperature using Maxim’s “Autosave” function. Focusing in my case is done manually by looking at faint stars FWHM (full width at half maximum) and adjusting the focus knob to make it as small as possible. Celestron telescope tubes are made out of aluminium which has a strong expansion coefficient. So, if temperature changes by a few degrees while imaging, it means that focus will have moved some and that focus has to be checked every so often… If I know, from experience, that temperature won’t drift during exposures,then I’ll just go to bed while setting my alarm clock to get up at the end of a sequence. But, I think that, more important than a perfect focus, a perfect guiding is required for deep sky imaging, because, focus usually is better than turbulence or guiding, in terms of arc seconds of resolution on the sky. With a focal length of about 4 Metres, which is that of the C14, guiding is not always easy to achieve, especilly if “seeing” is bad.

Colour CCD aquisition and processing technique

Single exposures length will depends on the object being photographed. For example, a planetary nebula will require a shorter 1 minute exposure time so that it does not get “burnt” while a faint galaxy, with a lower surface brightness , might require 10 minutes, maybe even longer, should the background allow it. It is also necessary to acquire darks, offsets and flats in order to calibrate individual exposures. Sensor temperature has to be the same as to that of the actual deep sky exposures. I usually set sensor temperature at -20Deg Celsius as noise at such temperature is quite low. Darks and offsets are made with the lead on so that the sensor is in the dark. Darks have an exposure length equal to that of the exposures to be calibrated, and offsets have a very short exposure length, such as 0.002 seconds for example. Making flats with a large telescope such as the C14 is not easy.  Usually,  a “light box” made of LEDs on a white background, positioned at the business end of the telescope, is used to acquire flats (see image below right). But, making such a box for  a C14 is not very practical, so I use the sky at sunset, before stars come out, to make flats. flatamono It allows me to dispose of a fairly uniform, not too bright light source. If the source is too bright, Sensor pixels tend to saturate. I then have adjust flat’s exposure length in order to get around 6000 ADU (Analog to digital units). I usually use 0.01 Seconds exposure. It is extremely important to obdurate the telescope as soon as the “start” button has been pressed because otherwise, the CCD matrix carries on being exposed while the flat is being downloaded to the computer. Fortunately, downloading takes around 30 Seconds with USB2.0 so there is plenty of time to put the lead on the telescope.  Also, keep in mind that the sky is blue, which means the flats acquired have to be converted to colour first, them to Black&White, using Maxim’s Convert to Mono function. You need to make a number of Darks, Offset, and Flats equal to the number of exposure. Once these are made, the calibration wizard in MaximDL allows to select the relevant files. The actual deepsky exposures are calibrated using “calibrate all” function, then, they are converted to color (parameters: red=80%, Green=100%, blue=100%). It is not possible to align the deepsky exposures before converting them to color because that would cause the bayer matrix blue, green, and red pixels to get  mixed with one another, and the colour information would be lost. ngc2841brute ngc2841traite

Raw exposure left, processed right.

Exposures are then aligned and stacked. For stacking, “median” works well especially if there is a satellite track  on one of the pictures, or if there are some “hot” pixels left on some of the exposures despite calibration. It has to be noted that cosmic rays will leave saturated pixels where they hit the CCD chip, and that is not corrected with calibration. Also, while it is very tempting to use a “dark” library and dispense with making them for every single shot, the CCD chip will change with time, by having more and more defective pixels. “Average” stacking mode is great but requires defect free exposures. Then, in order to correct “burnt galaxy centers, one can use DDP filter, or logarithm “stretch” correction. Finally, light curves can be adjusted in “photoshop, “The Gimp”, or Paint Shop Pro. Warning! A single “raw” exposure weights 20.5 Megabytes, and a converted color shot 62.5 Megabytes, so a computer with loads of memory if not especially fast is required. maximscreen

MaximDL acquiring

Equatorial mount alignment

Mount alignment is a requirement to proper tracking and guiding for astrophotography. It will not matter how good your mount is is your alignment is bad. To achieve proper alignment, there are several techniques at your disposal, such as:

  1. Using a polar axis scope
  2. Bigourdan method
  3. King Method

Using a polar scope

Using a polar scope (which my Titan mount does no have by the way it’s an option) usually allows you to get a rough alignment at best. You can easily live with it especially with a mount such as a Gemini equipped mount, as you need to train it and it will compensate for alignment errors in its goto function, but not in tracking (meaning that it is not going to make any adjustment in declination while tracking). if your mount is misaligned and your guiding software still manages to keep up with it, you will get field rotation in the long run, centered on your guide star. What polar scopes are good for, however, is to get your mount roughly aligned so that you can use one of the drift methods below to get it spot on. That said, I know that some of you who practice astronomy in a nomadic fashion will tell me that using a polar scope to align their mount is all they do and I respect that.

Bigourdan method

This is the method I use. First and foremost, you will need a cross-haired eyepiece of fairly short focal length (usually is 12.5 mm). The method consist in aiming at 2 stars, one at the meridian to the south, to adjust the pointing of the mount in azimuth, and the other, 6 hours away from the meridian, either to the East or to the West, to adjust elevation of the mount. Corrections are made looking at drift direction of the star in relation to the cross-hair.

Adjustment in Azimuth

Point a star to the south meridian, as near to the celestrial equator as you can get on, and orientate the cross-hair in a East-West direction (horizontally). Keep in mind that in a Schmit-Cassegrainian telescope, up an down are reversed. place the star on the horizontal hair, and observe the way it drifts for a few minutes. The directions take into account the fact that the image is reversed.

  • if the star drift toward the horizon (hence to the south), move the top of the right ascension axis to the WEST.
  • if the star drifts toward the zenith (hence to the North), move the top of the right ascension axis to the EAST.

Once you are satisfied there is no up or down movement of the star for a few minutes, the orientation of the right ascension axis is good.

Adjustment in Elevation

point a star to the East or West, as near to  the celestrial equator as you can, and adjust the cross hair in a North-south direction (on a lign  polar star-south horizon). Again keep in mind that up and down drift direction are reversed. observe the drift of the star, in relation to the north-south hair. The directions take into account the fact that the image is reversed

  • if the star drifts down, under the north-south hair, lower the top of the right ascension axis until the star is back on the line.
  • if the star drifts up, rise the top of the right ascension axis, until the star is back on the line.

proceed in the same manner until there are no drifts and the star remains on the line. The picture below illustrates the Bigourdan method:

bigourdan
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