6x 180 sec @ -12 for each of LRGB Tak FS102 with QSI 583
If you’ve been reading my blogs you will know that I’ve been struggling with tracking. It’s been months now and last night the problem finally became clear. I’m just a very slow learner.
My ST 402, is a great little guide camera, hanging off my MOAG I never fail to find a star. The problem was that I was getting pixel movements during guiding of up to 1.8 pixels in X and Y. Obviously the resulting images where poor. I was reading again about sampling and came across the old 2 arcsec per pixel rule. I run my FLI Proline 16803 at 2x binning to get a ratio of about 1.7 arcsec per pixel (as reported by Maxim). The ST 402 has pixels of 6.9 by 4.3 so the ratio when hanging off the back of my MOAG is about 0.36 (an average of the two dimensions). I changed the binning to 3×3 giving a ratio of about 1.1, and surprisingly the tracking variation is now reduced to a max of 0.4 pixels in either direction, average deviation is less than 0.15. The pictures look heaps better as well.
Now I have to go back and fix my auto focus, FocusMax is causing me grief.
26/01/2013 – I’ve worked out FocusMax and what a wonderful solution it is. Perfect focus every time! The trick is choosing a dim enough star (in my case around 7 is perfect) and following the instructions. They one major factor in this was counting the circles on the profile where the line began to diverge from straight, to determine the Near Focus HDF. After I got this right it worked like a charm. See the pictures below.
Sydney has been cloud bound for months. Looking at my observing log there has been 15 nights of observing since August last year. In the mean time I’ve been attempting to work on my configuration and processing.
During the processing I’ve noticed real differences in the relative brightness of the images between R, G and B frames. Combining the aligned frames in a 1:1:1 ratio yields strange results. I found on Don Goldmans imaging site a great tutorial for measuring the filter ratios for my system. I use an LX 200 with a QSI583 and Astronomiks filters (2c).
The process I used for measuring the relative intensities was as follows: download the G2V database off Don’s site and add to the libraries of The Sky. Slew to a G2V star somewhere high in the sky and take a series of images with short exposure times. The short exposure times are needed to ensure the star does not max out the pixel intensities, in my case < 55,000. For the star I chose it was 2 seconds. I use Maxim DL so after taking the image I opened the info window and clicked on my selected star, this gave me the intensity values. I did this for a series of frames made with each filter, recorded the average values and used these averages in my calculations.
The QI583 has a KAF 8300 sensor which is most sensitive in the green so using green as the reference filter I took my average intensitiy values from the previous paragraph and used them to calculate the ratios. I divided the intesity value for green by the value for green (suprise it was 1), red/green and blue/green.
I was very surprised by the results the where for R:G:B 1.02:1:0.60. Now I could live with the R:G ratios perhaps just a small error (BTW it wasn’t an error I’ve retested it on a series of nights and got identical values), but the G:B? I tried it, I took a 300 second image in green and then a 500 second blue and the looked at the results. Smack on
A final note focus is very important, get it right first, it might well be, like mine, be different for each filter.
After weeks of mucking around I’ve finally achieved reasonable tracking. The stars are round – most of the time. Tests with 100 and 300 second subs yields nice round stars, I’ve yet to try 600 seconds.
First step was polar alignment, I used the PHD graphing technique. This doesn’t take very long and yields good results. Check out ftp://sarcasmogerdes.dyndns.org/download/Drift-alignment-using-PHD-graphs.pptx
Post alignment shots of 30 seconds still showed star elongation in only one direction, vertically in the image. Checking out the Maxim guiding graphs the Y axis (dec) had RMS of less than 0.1 but the X axis (RA) was around 2. 2 pixels!
I’m using a Megrez 88 as a guide scope. It has a focal length of 498 mm. Is this to short for the 3m focal length of the LX200? I tried a barlow. This only made it much worse.
So back to basics what could possibly be wrong? Backlash!! What is backlash? I had no idea. Don from Bintel sorted me out. The RA axis on my G11 moved when I wobbled the scope. It moved only a little but it was enough – backlash. I have the losmandy one piece worm gear installed so I refitted it, did everything up again and remeasured. It was better but far from perfect.
Down to this evening. I read somewhere last night on the Net that exposure time of the guider could contribute to guiding errors. So I went back to my 1 second exposures. I had used 3 seconds because I was attemping to avoid seeing issues with stars. Guess what perfection! The stars are not as round as my old refractor but the are very close. Tracking errors in RA are now RMS about 0.4. Much better.
PEC training time.
So tonight was one of those charmed evenings when all the configuration came together. Amazing what a bit of reading can do.
I started with collimation. After trying to complete it manually for a couple of nights and doing battle with the eyepiece, I gave up and downloaded CCD Inspector from CCD Ware. Installed and operational within a few minutes, it linked to Maxim smart quick and the realtime collimation, single star started. Initially the collimation showed numbers greater than thirty – great with the eyepiece eh? . The direction and magnitude was always consistent, same direction, same size.
To figure out which way to move the knobs I initially tried turning them about 1/8 of a turn to see which way the star moved. I did this for each of the knobs and after writing down the direction I could then move the star to follow the line pretty simply.
I got to numbers of around 7 and the line direction started jumping around, something fierce. I noticed that every 2 out of 3 refreshes the line still pointed in one direction but with varying magnitudes. After many small adjustments the line direction started pointing in any direction almost randomly but with all the magnitudes less than 7, with common occurances of less than 3. Pictures below. The larger numbers where form earlier in the process.
The resultant stars in Maxium showed showed elongation of 0.080 or less. Nice!
Holidays! So I’ve had a few hours to practice with the solar scope. I tried it on my camera tripod. A major disaster. It was to heavy and the tripod was just not able to cope. Down to the store again to find a decent tripod. I just brought the Bintel Skyview Alt Az mount. It seems to work very nicely and is sufficently stable to support the scope and my 5D camera with ease.
First real views of the Sun clearly showed detail on the surface, a Sun spot and many prominences. I am constantly amazed at being able to see these things in real life. It makes me stop and wonder…
If nothing else astronomy teaches me patience. I was waiting for clouds to move today and was just thinking how nice it is to learn to be still and wait instead of the crazy, frenetic pace I normally live at.
Hopefully first photos tomorrow.