Theatre Hipparchus

CCD Imaging has a steep learning curve...

I can certainly understand the trials and tribulations that first time imagers go through. I had 18 months under the belt and then went back to take a look at the data I was capturing in the first weeks with the MX7C - a one shot CCD Imager from Starlight Xpress. One shot just means that all colour elements are captured at the same time. Others SX imagers use a filter wheel and capture Red, Blue, Green and Luminance and other filters like Ha as a series of different exposures. 

Here is the data from my second astro image, but first a few caveats. I knew this object C19 - also known as The Cocoon Nebula would be overhead, in a good position to capture. I didn't have an idea of exposure times or really know what to expect from the raw data (raws). I took the images binned 2X2 thinking that I would get better colour.

Here is the combined 30 images x 30 seconds - combined in Maxim. Needless to say I was quite disapointed. No nebula! The colour was horrifying. I checked against my planetarium program Starry Night and felt I was in the right area using the stars as a guide. So I relegated this set to the CD and forgot all about it.

A year and a half later.

Cold winter nights here in Ontario present a great chance to go back and apply some of the techniques you have picked up over the more active months. Here is the exact same data with a different approach to processing the image.

It's hard to believe that this much data was embedded in the image. This processing method relies on not eliminating any data until the final stages. The main themes of this method was presented by Paul LeFevre in an online tutorial. Here are the steps that I use for the MX7C:

• Inspect raws for poor focus, movement or other anamolies and delete these out of workflow
• With a one shot camera I split the raws into individual LRGB frames - Astroart Mike Smith plug-in 
• Splitting the raws allows a little more control with colour later on. I have never had success converting a full MX7C raw to colour using Maxim or any other program
• Set up your calibration frames (darks, flats and bias) - note that some SX imagers choose not to use this calibration step at all. The thinking is that the CCD produces very little current that affects the outcome. If I have reasonable calibration frames I will use them.
• Calibrate, align and combine each L R G B frames. At this step save only the end result and not the frames used to get there. Keep the originals (or duplicate them) in their uncalibrated, unaligned state in case you want to try new methods with the raw data in the future. Considering the cost in equipment and time to capture the images it makes sense to save the raws as FITS (flexible image transfer) to CD or to a plug in storage unit.
• When saving, make sure that you retain all of the data in the histogram. In some cases if the image contains white point data above 65,355 the program should "shrink" the data to fit within this range. My problem occurs at the other end, the left side of the histogram where most astronomical image data resides. Clipping (or cutting off) data on the left side of the histogram cuts into the background and in some cases parts of the mid range data as well. The point is to keep that precious data you have captured at this stage.

Here is another example with the very first image I tried to capture            

This is NGC 6946 and it was the first galaxy I observed with the LX200 so it was a natural target and favourite galaxy for me. This is about 38 x 1 minute of exposure and I was just happy to have something appear on the screen. I saved the data and hoped to improve at processing.

This is the same data with a little more patience attached. Secretly, I was pleased that the capture had stood up because I didn't really have a good feel at the telescope. 

Back to the Process

Now that we have our four Fits files L R G B, I open them in Maxim and do an align. Inspect them after the align to make sure nothing untoward has happened. I find the Green sometimes is upside down or in a crazy position. For align most times I use Auto star matching, but in cases that involve poor data a correlation is a last resort. There is also manual two stars and because you are only aligning four images it works as well. After the align click on colour and set up the respective files in their places. Check the preview window to see what kind of a result. Normally in Maxim I don't do any colour adjustment during the colour combine. If the data looks OK I will save the frame as a RGB 8 bit tiff and do the stretch as part of the save. I use Max pixel for this. The Lum is saved in the same way. 

Image manipulation   

I used to just hammer away at the data with a heavy deconvolution set at 80, toss in a couple of passes with this filter or that filter. Lately, I have been trying to get as much data as possible over to Photoshop and do the refining (read: subtraction) there.

Side trip: 

For these older captures I am finding Chris Icough's star bloat filter in Astroart on a mild setting to be quite effective. A heavy setting gets tiny stars but most of the nebula and midrange ends up mottled. I have not tried to seperate the stars and do a heavy pass. The conservative setting seems to preserve the data for finer tuning ahead. Note: from Astroart I will export the data as a 24 bit JPEG.

Photoshop  

Using version 8 and having difficulty with a 16 bit FITS image. Even using the Fits Liberator I find the levels adjustment to be extreme even taken in several small increments. The 8 bit TIFF and even the 24 bit JPEGs work fine at the moment. (I know I have to figure out something better here)

I think understanding Photoshop or like program is where the great imagers already are. Here all kinds of manipulation is possible. I'm currently studying Photoshop for masking, layers and plugins that aid astronomy images. Hoping to have that report for spring 2005.

By all means leave a comment or share an idea that others might benefit from. If you have specific Photoshop ideas please point us to them.

Best,

David