Is 60s good enough?

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umasscrew39
Posts: 38
Joined: 20 Feb 2019, 17:47

Is 60s good enough?

Post by umasscrew39 » 22 Jun 2019, 16:28

Hello

I am going to try once again to see if I can do AP. Several months ago I bought AA7 and several of you were very helpful. I was having issues with the software on my windows 10 laptop but those are resolved. I have also upgraded my mount in my observatory (AP1100) and I have doing live viewing with 60s stacks using SharpCap. I use a C11" SCT both at f/7 and f/2, rarely at f/10. I have serious CMOS and CDD cameras. Before I give it a go, is doing 60s exposures enough to get a good stack for AP quality or do I need to guide? If guiding is needed, then I need to pause again and learn how to guide. I do have an OAG setup (Starlight Express filter wheel with built-in OAG that I can use my UltraStar) and I downloaded PHD2 but have never tried it. So, should I learn guiding first to extend my exposures out to like 5 mins or so or just try it with 60s exposures?

Thank you,
Bruce

fabdev
Posts: 109
Joined: 03 Dec 2018, 21:43

Re: Is 60s good enough?

Post by fabdev » 22 Jun 2019, 18:46

Hello, in my opinion, a system which is able to track well 60 seconds exposures (95% of them) is an excellent one and does not need guiding.
Just take 60-seconds sequences and automatically stack them. My opinion is driven by productivity (loss of time for setting up the autoguide vs loss of time/space for setting up and storing the sequence).
For myself, I consider the "threshold" for autoguide vs sequences at 30 seconds.

umasscrew39
Posts: 38
Joined: 20 Feb 2019, 17:47

Re: Is 60s good enough?

Post by umasscrew39 » 22 Jun 2019, 19:19

Very good to know- thanks much. There is so much conflicting info out there, it was hard to determine the correct course to take. I will give it a try without guiding and see how it goes.

einari
Posts: 17
Joined: 28 Jan 2019, 10:46

Re: Is 60s good enough?

Post by einari » 24 Jun 2019, 05:23

There are so many variables in AP so no wonder there's 'conflicting info'.
If you are going to image moderately bright targets then 60s might be enough - and you only need to stack less than 100 of them.
But if you are going NB and image faint targets then you need hundreds (even thousands) of images to stack.

Dunmunro
Posts: 58
Joined: 10 Jan 2019, 01:11

Re: Is 60s good enough?

Post by Dunmunro » 24 Jun 2019, 16:15

umasscrew39 wrote:
22 Jun 2019, 16:28
Hello

I am going to try once again to see if I can do AP. Several months ago I bought AA7 and several of you were very helpful. I was having issues with the software on my windows 10 laptop but those are resolved. I have also upgraded my mount in my observatory (AP1100) and I have doing live viewing with 60s stacks using SharpCap. I use a C11" SCT both at f/7 and f/2, rarely at f/10. I have serious CMOS and CDD cameras. Before I give it a go, is doing 60s exposures enough to get a good stack for AP quality or do I need to guide? If guiding is needed, then I need to pause again and learn how to guide. I do have an OAG setup (Starlight Express filter wheel with built-in OAG that I can use my UltraStar) and I downloaded PHD2 but have never tried it. So, should I learn guiding first to extend my exposures out to like 5 mins or so or just try it with 60s exposures?

Thank you,
Bruce
Your AP1100 should have factory trained Periodic Error Correction (PEC) that should bring total periodic error down to ~1 arc second. With PEC engaged you shouldn't have to guide for short exposures. I would suggest that you do test exposures of 1, 2, 5 and 10 minutes and then examine them for trailing. This will give you an idea of the maximum exposure time that you can use.

However, guiding is only one area where you will have problems during long total integrations. Another area is focus, because as the temperature falls SCT focus will drift considerably and you will need to refocus on a regular basis which will correspond to changes in temp causing the metal SCT tube to expand or contract. Additionally some slight changes will occur as the optics change temp, mainly the primary mirror. You should monitor the ambient and OTA tube temperature constantly.

Finally, many SCTs have serious mirror slop, where the mirror will tilt slightly during long exposures, causing trailing. The use of mirror locks on the EDGE version of the C-11 can help reduce this, but use of an OAG (and ideally an external focuser that is autofocused between images) is really the only cure. Stacking many short exposures alleviates the need for guiding but not for refocusing.

You can use a short focal length (~50mm F4) guidescope which would resemble a finderscope, for guiding and/or to train your PEC system if that is needed, in lieu of an OAG.

umasscrew39
Posts: 38
Joined: 20 Feb 2019, 17:47

Re: Is 60s good enough?

Post by umasscrew39 » 24 Jun 2019, 19:34

Thanks much- I am aware of the other issues but always good to be reminded of the SCT flaws for "long" exposures. I have the Celestron autofocuser but it does not allow lock down of the primary mirror. I also have a MoonLite stepper motofocuser to lock down the primary but I want the option to use my hyperstar lens, at least for now, so I have to use the Celestron autofocuser- which also does not have a temperature probe on it. So, I realize it is not an ideal (maybe not even a practical setup for long exposure AP) thus my original question on the 60s and having to use an OAG which I have but have never tried as I have only been doing short, unguided live viewing to date. The PEC is enabled and I confirmed that with AP so no concern about that. Good idea of simply testing various exposure times as I was just assuming that 60s is the limit without guiding.

Thanks again

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