I'm not sure if this image shows my concerns, but the stars when very blown up show distortions (all in one direction). My Polar alignment was excellent, guiding satisfactory (30 sec exposures / 120 gain). I was using a Hyperstar f/1.9 on my 8" SCT.
If this is a collimation issue (which I suspect), I'm wondering if anyone has tried to collimate a Hyperstar...it was shipped and calibrated by Starzonia to lie flat (no space)...any Hyperstar advice would be most appreciated.
Do you have a higher resolution version of your image? This is sadly too small to really see anything.
If the image is too large (1Mb or more), you can use a site like https://www.imgbb.com to share an image.
There are many errors possible, of which some of them are collimation errors. You can look here for example if anything looks like your error: https://www.limerickastronomyclub.com/c ... n_help.htm
You describe a distortion in one direction. That could be caused by a collimation fault but it has to be a rather big fault in my experience for that to happen.
The distortion in one direction can also be caused because the telescope is not cooled enough. If I start capturing with my SCT after 15 minutes, my stars are also "stretched". Most of the times I give it around 45 minutes to cool and adjust to the outside temperature.
Thank you Menno (I'm not much with computers, but I tried to do this in Photoshop). If you enlarge your screen you might be able to see that the stars in this image have a kind of bright oblong center and at eleven o'clock you can see a kind of round "halo" bulging out. I can try to find a better image if necessary.
I do appreciate you help...I've tried to collimate my HyperStar in the past, and if this is the problem I'll try again.
do they look like this all across the image, or does the amount/direction vary in different parts of the image?
To me that looks a bit like coma - on a Newtonian in correct collimation you would expect sharp stars in the middle of the image and coma around the edges, with the 'tail'- pointing away from the center. If you see them all pointing one way then it would quite likely be poor collimation. Not sure if that applies to SCT+Hyperstar too.
it certainly looks like it - the fact that they all point in the same direction is the key. To be honest though, you would be best checking with someone who has hyperstar experience to make sure our guesses are correct.
Questions:
1. Does EVERY frame look like this?
2. Did you try a shorter exposure?
My guess is it is a combination of poor focus and bad tracking/guiding. Last month when I processed your NA Nebula, I noticed two things:
1. Guiding was not the best, even with a short 30-second exposure. (Short for me, at least)
2. There was an unexpected "bump" that occurred 20 minutes or so into the sequence.
Is your telescope balanced? Try this indoors: unlock the RA and DEC clutch. Rotate the OTA so that it points skyward. Let go of the OTA (be careful!!) Does the OTA hold its position or does it want to rotate in one direction or the other or both? If it doesn't want to hold position, then you need to adjust your counterweights.
I’m not sure if every frame looks like this. Since the raw data is in fits files, I can’t open to look. I’ve learned to use APP, would that allow me to view the individual frames?
I do struggle with guiding, is it my AVX mount??? I’m on a waiting list for ASIAIR Plus, hopefully the guiding will be less arduous using the ASIAIR. I simply don’t know how to improve the guiding in Phd2.
I’d love to be able to take longer exposures, but with my guiding, guess (unfortunately) I should stick with 30 seconds.
No, I didn’t try shorter exposures.
I don’t know if you’ve ever used hyperstar Brian, but do you think I should mess with collimation? Someone suggested using three shims to raise the Hyperstar in order to allow more play in order to make the collimation adjustments…UGH! Guess I could do a star de-focus to see what’s what before doing anything.
As for balance, I'm pretty careful, however, my OTA was parallel to the ground when I tried the RA balance…I did make it slightly ‘east-heavy.’ Does it make a difference if it’s not pointed up?
Thanks so much for your suggestions Brian, the image of Andromeda was fantastic, in spite of the bad stars.
The NA nebula's guiding may not have been the best but at least the stars were well-formed. Do you remember if you captured the nebula before or after you removed the camera to take darks? If I remember correctly, you captured darks before the lights. So you didn't have collimation issues then. What is different today? Did you remove the camera for a second time and now you've lost collimation?
Regarding your question about using APP to zoom in on stars, sure it can do it. Click on the LOAD tab and then click the Lights button. Select a couple dozen light frames from that session with the suspected collimation issues. Then turn your attention to the panel at the bottom of the screen. Double click the first file in the list. The image appears in the large panel above it. Move your mouse on top of the image where you want to zoom. Double click several times until you can clearly see the shape of the stars. Finally, move your mouse back to the bottom panel with the list. Double click on the second file. Now, simply press the down arrow on your keyboard to sequence through the dozen files. When you get to the bottom of the list, then press the up arrow. APP should keep the same zoom level for each frame. Now, what your looking for is this: Is the shape of the stars consistently bad from one frame to the next, or do you get mostly good frames followed by a bad one?
If you want a quick way to view and zoom into for FITS files, I can recommend Irfanview. It's a free, all purpose image viewer but one with a lot of possibilities.
One of them is opening FITS files like a normal image. You will see it in black/white but in this case that doesn't matter.
Info and download on https://www.irfanview.com
