Making The Best of Budget - Imaging

Discussion of using SharpCap for Deep Sky Imaging
Kopa
Posts: 21
Joined: Wed Jul 08, 2020 6:26 pm

Re: Making The Best of Budget - Imaging

#11

Post by Kopa »

Hi Dave and Brian,

Thank you for your suggestions, pitfalls warnings and guidance.

From the SV305 photos link I think it will do for me for the time being as a beginner Camera.
I had actually also tried SVBony 0.5 reducer route, but the stars had increasing tails pointing to the centre the further away they were. The tails made them difficult to be detected as stars for Live stacking I assumed.


Yesterday was predicted an excellent stargazing night by Nightshift and Clear Outside apps but it turned out that only the brightest stars were visible with Sirus twinkling.
With alignment complete and Regulus centred in sharpcap reticule, it took about 45min with Regulus still within FOV. The strange thing to me was that it started drifting in an orbit path that rose towards NW then dropped to the west and crossed heading southwards about 3/4 from exiting the FOV in the west.

I shall try setting up new alignment stars profiles as suggested when a chance comes.

Sharpcap Brain used to recommend 3 to 4sec for my Bortle 8 but last night was 75sec and I had tried a CLS filter from SVBony. Will have to wait for better skies to try again. Maybe the snow around London made skies too bright.

Kind regards

Vince
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oopfan
Posts: 1321
Joined: Sat Jul 08, 2017 2:37 pm
Location: New York
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Re: Making The Best of Budget - Imaging

#12

Post by oopfan »

Vince,

Keep pursuing tracking. With some luck you can find a configuration that can give you a 20-second exposure.

There is another possible cause of tracking problems: Periodic Error. This is something that I have to deal with in an equatorial mount. The gearing is not perfectly machined, meaning that the teeth are not separated by a consistently exact distance. Even very expensive mounts have some amount of PE. Without correction my PE is 70 arc-seconds peak-to-peak. What that means is that if I drive the gearing at a constant sidereal rate, then stars will appear to oscillate back and forth over one revolution of the worm gear, about 8.5 minutes. With correction I can get PE down to 10 arc-seconds. That's good, but even 10 arc-seconds will create a star trail. The solution is active guiding via PHD2, but I would need to write an ASCOM driver for my DIY Raspberry Pi/stepper motor solution. I'm not keen on doing that. At that point I would rather buy a new mount. Thankfully with 10 arc-seconds of PE, I can take 90-second exposures while rejecting only one out of four frames. That is plenty long for my Bortle 5 skies. The only time I would like to have active guiding is when I do narrowband where 10-minute exposures are common. In your case having an alt-azimuth mount, tracking requires running the altitude axis and the azimuth axis concurrently. If both gears have PE then that would explain the strange path that Regulus takes. Like I suggested, keep on trying, but you will probably end up having to instruct LiveStack to reject frames that don't meet your standards. That means that it might take you two hours to capture only one hour of good quality data.

Brian
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