Short exposure F 4.2 Dobsonian pictures of M13 and M33

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timh
Posts: 69
Joined: Mon Aug 26, 2019 5:50 pm

Short exposure F 4.2 Dobsonian pictures of M13 and M33

Post by timh » Wed Sep 30, 2020 11:22 am

Herewith a couple of images captured using the Sharpcap autostack facility and a goto Dob.

Synscan 250 Dobsonian F4.7 f 1200mm.
Skywatcher 0.9X reducer/ coma corrector
ZWO ASI 294 PRO camera
4 minute autostacks of 80x3s exposures with Sigma stacking filtered by brightness and FWHM

Selected and then stacked those autostacks in DSS having FWHM (pixel) < 4.5 (corresponding to ~ 4 arcsec)
Processed in PixInsight (ABE, MLT noise reduction and PhotometricColour)
Adjusted depth/colour saturation in PhotoscapeX.

The M33 image was total exposure 72 min (here the dob has yielded a less sharp but brighter image than e.g. Menno's F10 image)

The M13 image was total exposure 30 min (a point of interest is the massive contrast in distances of the objects in field, the orange K2 star HIP81848 at about 1800 LY, M13 at 25K LY and the faint galaxy IC4617 at about 490M LY)

https://onedrive.live.com/?cid=330584F8 ... 29&o=OneUp

https://onedrive.live.com/?cid=330584F8 ... 29&o=OneUp

best wishes
Tim

umasscrew39
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Re: Short exposure F 4.2 Dobsonian pictures of M13 and M33

Post by umasscrew39 » Wed Sep 30, 2020 12:05 pm

Tim- these are both beautiful images. The photometric color tool in PI is amazing and way better than the standard ColorCalibration tool. I reprocessed some of my older images with it about a month ago and it made a big difference. The ASI294 continues to be an excellent camera. Congrats again with the amazing 3s exposure images.

Bruce

timh
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Re: Short exposure F 4.2 Dobsonian pictures of M13 and M33

Post by timh » Wed Sep 30, 2020 2:03 pm

Thanks very much Bruce,

I have got rather obsessed recently with exploring how to get the best out of a dob- the answer - at least as far as sharpness goes - seems to be to sum up lots of short exposures. However - longer term - I am sure that a big EQ mount and something like an Edge 11 will be the way to go. I see that you posted some excellent C11 images on Astrobin along with informative overlays that describe them. Will have to learn how to start using Astrobin I think..

best wishes
Tim

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Menno555
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Re: Short exposure F 4.2 Dobsonian pictures of M13 and M33

Post by Menno555 » Wed Sep 30, 2020 3:11 pm

Nice!!
Indeed way brighter then mine. But if you add more exposures, will it become a bit sharper / more details?

Menno

timh
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Joined: Mon Aug 26, 2019 5:50 pm

Re: Short exposure F 4.2 Dobsonian pictures of M13 and M33

Post by timh » Wed Sep 30, 2020 5:13 pm

Hi Menno,

Brightness and the contrast in larger scale features of the image improve with more frames - but I am pretty sure that it won't get any sharper. Sharpness was noticeably improved by excluding the more blurred (FWHM >4.5) autostacks. I think that it is fundamentally limited by seeing plus the amount of dob movement within an exposure.

3-5s worked pretty well for objects that don't need a filter and - in principle - using the field rotator - it should be possible to get >2h total exposure. However with a UHC filter 10s was about the minimum that gave an acceptable histogram and this at the cost of FWHM rising to about 5 or so.

Inspired by your very nice Pelican HH objects image I also had a go at that using a UHC filter and 10s exposures and could also see at least one of the HH objects - more blurred than your image but I was happy to see it at all.

But overall I am now probably pushing at the limits of what can be done with a Dob in terms of image detail/ sharpness.

best wishes
Tim

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turfpit
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Re: Short exposure F 4.2 Dobsonian pictures of M13 and M33

Post by turfpit » Thu Oct 08, 2020 11:18 am

2 very nice images there Tim.

Dave

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