NGC281 and the Pacman nebula. Dobsonian image

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Joined: Mon Aug 26, 2019 5:50 pm

NGC281 and the Pacman nebula. Dobsonian image

Post by timh »

Hi folks,

I put this one up not because it is a really outstanding image but rather as an example of a deepsky image obtained with a goto Dobsonian and a ZWO ASI 294 PRO camera using very short exposures. There are all sorts of mini-projects that people have going on - my particular quest has been to learn how to optimise astrophotography for my goto Dobsonian (Goto Synscan 250 - now fitted with a simple electrically controlled flat-profile focuser and 0.9X SW coma corrector). The scope is controlled by WiFi using the SW WiFi dongle and the Synscan App plus SkySafari PRO on an android phone. Just thought that I would share where I am on this in case of interest to others.

Set up

0.9X reducer/ corrector
Astronomik UHC filter + ZWO IR cutoff filter
Artesky M48 manual field rotator
F= 4.16. f = 1050 mm

The drift graphs in Sharpcap livestack indicated that the Dob mount's background tracking drift rate (~12 pixel/ minute) was going to constrain exposure time more than the effect of field rotation (~0.25 deg/min ~ < 3 pixel/ minute within the central half of the field).

Bortle 6 sky, waning gibbous moon ca 0.5-0.6 e/ pixel/s with the UHC filter in place

Exposure time 3.0s with Sharpcap livestacking rejecting frames with low brightness and FWHM < 4.3. (most were 3.5-4)
Gain 200, offset 10
Sigma stacking with sigma value 3
Autostack and save every 10 minutes (200 frames)
Pause at 30 minutes to manually approximately re-adjust for field rotation.
Total exposure 60 min = 6 x 10 minute stacks.

6 x 32bit 10 minute stacks stacked in PixInsight -> ABE- MLT smoothing - COL and starmask. Tidied up a bit in PhotoScapeX freeware

The SC Brain calculation indicated that I should expect (I think) approximately 40% of the total noise to originate from read noise (the price for needing 1200 frames for an hour's exposure). That still seems not bad being less than half of the total noise. I messed up using the manual field rotator (initially going in the wrong direction :-) ) and needed to crop down a fair bit.

Overall though quite pleased with the result so far. A dob setup is never going to compete with an EQ mounted large scope but for a simple and relatively cheap set up it does seem to provide a signficantly more detailed view of the core of compact objects than from blowing up even the best images from my guided 4 inch EQ-mounted refractor. So an OK complementary instrument to keep and use for detail on more compact targets. Sharpcap has really been invaluable in providing 1) useful analytical tools to guide the set up and whole approach 2) a way to deal with, select and stack so many frames on the fly (bypassing the need to store and deal with thousands of frames later).

But not optimal yet. Two obvious satellite tracks across the image. So will next try changing the sigma stacking parameters and try using more of and shorter autostacks so that some can be rejected if need be. A 4 or 5s exposure time may prove to be slightly better? I'd also like to identify some way of reducing the size of the Goto tracking drift and/ or understand whether it is at a level that is about par for the course - drift rather than field rotation seems to be the limiting factor. ... 29&o=OneUp

best wishes
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Re: NGC281 and the Pacman nebula. Dobsonian image

Post by admin »

Hi Tim,

some really good images there - impressive work!

Cheers, Robin
Posts: 77
Joined: Mon Aug 26, 2019 5:50 pm

Re: NGC281 and the Pacman nebula. Dobsonian image

Post by timh »

Many thanks Robin,

I think that my quest to optimise for the Dobsonian is about done. Your online talks, Brain and this forum have provided some very useful insights and Sharpcap is working fine. Thankyou. Been busy in the gaps between clouds testing against more objects.

The simple bottom line for the dob was that erratic tracking movements meant that an exposure of about 3 or 4 s was the working maximum. This worked fine with a gain of about 200 (ASI 294 PRO) and offset to move the histogram just away from the zero point. Then using the SC autostack feature and Sigma clipping an hour or so of data from thousands of frames collected and stored economically in 2 or 4 minute 16 bit FTS files for later further stacking. A simple change from what I was doing before (10 and 15s frames at higher gain) but comparing the new pictures with the old I can now see significantly sharper stars, less bloating and better looking pictures with a more pleasing appearance with better dynamic range.

I always liked the Dob light bucket thing for visual and am now really enjoying the fact that the same dob now also works for not only big bright EEA but also for deep and quite detailed astrophotography of intriguing small deep sky objects.

best wishes Tim
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