A poor subject title, sorry, but after last night's session with the gear in the backyard I've got a few questions to ask. Since I'm a beginner I'm posting here, but if the topic belongs elsewhere, feel free to shift this thread. If the wall of text is too much, jump to the end for the "TL;DR, questions listed" bit.
Background: My current main goal is to take some good DSO images using a DSLR from my backyard. My "sub-goals" towards this include learning to do good polar alignment; focusing; understanding and recognizing errors and limitations as they appear on the images.
1. Polar Alignment. This was interesting as I can't see SCP from my setup location. It's blocked by the house. I used the G11's PA assist routine (
http://gemini-2.com/hc-English/E026.php) and while that was good, I found that when it slew to the star it was so far off that I could barely see it in the finder scope (sometimes it wasn't even in the field of view). To be thorough: I'm fairly certain I've set lat/long correctly. Time set is probably within 1 minute - does this need to be more accurate?
I did note that I was "generous" with my mount level when I set it up. I did note a significant improvement in the PA when I adjusted things further to a more rigorous level.
From this point I started taking longer images (about 30 sec to 1 minute) to check the alignment and attempt to correct errors found. I had issues with focus (I'll detail below), but by iteration the gross trailing in the images was soon (relatively) gone.
2. Focus. I had a lot of issues with this. I did the G11 PA assist with the eye piece in, so focus wasn't an issue. I then switched in the camera to check the results of the PA as I don't believe I have any other way to do so via the eye piece. I found it difficult to focus with the live view either on the back of the camera or on the computer screen. I think this was a combination of issues - it's quite a precise focus required and small movements either way could throw it out. The small screen on the back of the camera, even at 5x or 10x wasn't easy to see the differences. It was slightly easier on the computer screen - but a current limitation is the computer is inside, not conveniently near the scope. Then there was the to-ing and fro-ing between screen and focus adjuster. In the end, the best result I got was pointing to the moon and focusing on that before heading elsewhere.
I did some research and note the use of Bahtinov Mask to help focus. Does a Bahtinov Mask work well with the live view mode of the DSLR?
Although I may get a laptop in the near future to be able to bring closer to the scope, I think my manual fiddling with the focuser is error prone. What alternatives to manual focusing can I look into? Any recommendations for my setup (see sig)?
With the DSLR, I noticed that the focus got softer the more I zoomed in on the captured image, so while it might look ok at certain resolution/zoom levels, looking too "closely" is not good. I've read that this is something to expect with DSLRs, so I'm not overly concerned. For my Canon EOS 6D, how can I determine the quality of my focus for an image and/or how much zoom can I expect to be able to do and still have reasonable focus?
3. Errors and Limitations. After a couple hours of the above last night, I got to the point where I thought I could grab some images to process. I wasn't concerned at this point to be accurately picking a target - I just wanted to grab some data and see what errors popped up. I went to ISO 800 and 1 min exposures, and I was pointing somewhere around Eta Carina (ok, I was greedy and wanted some pretty colours
). Because the images were downloading to my computer, I could see what was happening after each shot. And the results were interesting and not consistent. I took 33 images in total over a few sets. Ignoring some soft focus issues, about 5 to 8 (I think) were acceptable to DSS to use. I think I know why most images were rubbish (e.g. needed to improve PA further for example) and I hope to post example problems in follow up posts to get specific feedback. Ignoring my skill level in the equation (as hopefully that will change
), how can I determine the maximum exposure length I can effectively do given the conditions to minimise problems? Given 5 or so images "worked" while others didn't, there's some variables I have not accounted for at this time.
Finally, how far can I possibly go for exposures without using guiding?
Thanks for any advice/assistance!
TL;DR, questions listed:
1. Time set is probably within 1 minute - does this need to be more accurate for PA and/or pointing to a location?
2. Does a Bahtinov Mask work well with the live view mode of the DSLR?
3. What alternatives to manual focusing can I look into? Any recommendations for my setup (see sig)?
4. For my Canon EOS 6D, how can I determine the quality of my focus for an image and/or how much zoom can I expect to be able to do and still have reasonable focus?
5. Ignoring my skill level in the equation (as hopefully that will change
), how can I determine the maximum exposure length I can effectively do given the conditions to minimise problems?
6. Finally, how far can I possibly go for exposures without using guiding?