Imaging Equipment Setup

After countless nights of experimentation, many equipment changes, and lots of new technique additions and improvements; this article was written to describe a method of acquiring and processing astronomical images.   My methods can certianly be improved upon but currently result in acceptable image quality.   

Many budding astroimagers begin their journey by holding a digital camera over a telescope's eyepiece, hoping to hold it still long enough to snap a picture.  This actually works and some decent shots have emerged from the uncountable bad ones.   

It quickly becomes apparent that better equpiment means better images.   Then, not so quickly apparent, is learning how to get the most out of the equipment.  Excellent polar alignment and focus are absolutely essential.  The best equipment can produce crappy images if not tweaked to accurate calibrations and focus.

Finally, processing all of those raw red, green, blue and luminance images took a while to get right.  Well, actually I don' t think that I have them quite right yet.   still working on that part....

As in any complicated process, the basics have to be right before the final result is right.   While there is always bigger and better, my current mix of equipment is satisfactory in its ability to produce fine images of the cosmos.

The Equipment

Losmandy G11mount with Gemini, on a concrete pier
6" Astro-Phyiscs Starfire refractor telescope
2.8" Televue Pronto refractor telescope
SBIG ST2000XM CCD camera
SBIG ST5C CCD camera for autoguiding
RoboFocus on the Starfire
Kendrick dew heaters on the scopes
An old PC or two
Too many cables and wires

The Software

CCDops - camera control & imaging software, usualy used for the autoguider
CCDSoft - camera & robofocus control and imaging software, usualy for primary imaging camera
RoboFocus - smart hardware, pc control for focusing
TheSky - map of the sky, can control scope position
Gemini Control Lite - excellent utility for running the Gemini

Telescope, Mount and Camera Setup Steps

The following setup areas are all important to taking good images, and all must be done as accurately as is reasonably possible.   A well polar aligned scope will produce images with round stars for up to a few minutes without guiding.:

- Polar Alignment - without it, your stars will be worms.   Close isn't good enough for imaging, although it is fine for visual observing.  Do a good drift alignment.   When done well it allows pictures of one to a few minutes without autoguiding.  With the addition of autoguiding,the telescope can follow the target almost exactly, and for hours.

- Build a good object model in the Gemini - The mount's controller can build a 3D image of the pointing characteristics of the mount by accurately centering on one object at a time and calibrating an additional point on it. The more objects you add to the model, the more accurate telescope slewing becomes.  When it's set up well, the scope will point just about anywhere and put the target in the center the field of view.   Put 8 or a dozen eastern objects in it and another set to the west.

- Target Selection - choose a target that fits your telescope and camera's field of view.    Many targets will be either too large to fit in the image, too small to be more than noticable, or too faint to get much detail in a short exposure.  TheSky can show your FOV relative the part of the sky being imaged.

- Autoguiding - Use a seperate guide scope and camera and rigidly attach it to the main telescope to avoid flexture.  Using the second CCD chip on the ST2K camera works fine as a guider, but 1) has a small FOV that might not contain a good star, and 2) you will often loose the guide star when the blue filter is in place.    I have read a recommendation that at least 1/2 of the mm focal length of the main scope should used as a guide scope for accuracy.   I don't disagree.

- Focus - the word says it all, it's criticaly important and can be quite a challenge to zero in on.  Small stars should resolve to a pinpoint, it's easy to get close, but close isn't good enough.  The RoboFocus absolutely helps to tweak it up.  CCDSoft's @focus routine often does a good job of finding where it is, but I often tweak it by eye anyway.

- Imaging - finaly, shoot some images!  Take a number of the same shot for each color component you take.  They are: red, green, blue, luminance, hydrogen alpha.  The exposure length of each image depends on the target.  Planets and moon are very bright and a short exposure time is used, everything else is faint and exposures of 5 or 10 minutes are typical.

- Dark Images - a dark represents the electronic noise in the camera.  They should be done with the same temperature and duration as the light exposures they will fix.   Take 9 darks and average them into a master dark, then bulid a library of tempature and time combinations.

- Flat Field Images - should be taken after each imaging session but are optional.  Experience suggests that they're more apt to be needed if the moon is bright and/or it's a little hazy out.  They will eliminate the little dark dust donuts that show up in your shots and reduce vignetting.   Lazy people like me don't take them as often as we should.   To make a master flat, average 9 flats after dark subtracting them.  The flat is valid for all light images until the camera is loosened.  This is because the flat corresponds to dust or other stuff in the optical path.

Processing all the raw images is discussed on the next page.