Short exposures deep sky imaging with altazimutal mount (Dobsonian)

Topic starter

Jan 7, 2021

Thank you very much for your advice. I will try this way. At the beginning of my experience I would like to reduce the problems as much as I can so I don’t plan to use the derotator just now but it is in my plan to introduce this device when I will become familiar to the technique. The derotator is very important to try to use the majority of the field of the camera for long integration times.
Thank you very much, I will keep you updated.
Filippo

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Matthew Proulx

Jan 7, 2021

andrea tasselli:
Hi,

First off short exposures won't get you very far, although with 600mm your yardstick is far longer than mine. You can use Iris (Buil's own) to post-process and stack thousands of fits files ( used to do that way myself) in a reasonable time even on slower PC (like mine). I guess you can convert from .ser files to .fit files with some other utility. Use the highest bit depth you can record with because 8-bit won't take you very far either.

I would consider using a de-rotator as this will take the pain of stacking 1,000a of shots out of the way.

short exposures can get you very far depending on the target. I see you did 120” on the Eskimo, here’s one at 500ms
https://www.astrobin.com/rbty9m/?nc=user

Concise

IOTD Staff

Jan 7, 2021

short exposures can get you very far depending on the target. I see you did 120” on the Eskimo, here’s one at 500ms

*100% agreed. I use short exposures and it can get you very far. With a 600mm scope at f/3 threre are many objects to capture.

You Can also mix short and long exposures. I like this. Siril is a good software. Did you try the global registration mode ? JF

Jan 7, 2021 · Edited: Jan 7, 2021

H
If you are interested in experimenting with Short exposures with a Dobson, try Sharpcap. Im pretty new to astrophotography. I purchased a 4 year old 14 inch Skywatcher GOTO Dobson as my first scope in June. A few nights ago I tried using Sharpcap's livestacking function with an ASI 224. It repeatedly selected, aligned and stacked 75 800 ms exposures, subtracted dark frames before spitting out a beautiful tiff file of M42 Orion Nebula. All completed as a I was at the scope. I was amazed how easy it was. I had it set to save and process an image every minute. I collected several photos all of which I'm very happy with. I might play with the photos in Photoshop or Lightroom, but they are pretty good as is. Nice round stars, the brightest ones show 4-star-points. The neat thing is you can immediately see the impact of exposure time and gain on alignment and final image quality, as you are at the scope. Bottom line, short exposure with a Dobson, can really help image quality with DSOs. I'd previously tried imaging with long exposures (15 secs) and suffered from distorted star shapes.

I plan to try and use Sharpcap to image more deep sky images in the next few months, Stay tuned!

Hope this helps!
PS. New to this forum - just signed up 10 mins ago. I will upload photos in the next few days.

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andrea tasselli

Frequent contributor

Jan 7, 2021

andrea tasselli:
Hi,

First off short exposures won't get you very far, although with 600mm your yardstick is far longer than mine. You can use Iris (Buil's own) to post-process and stack thousands of fits files ( used to do that way myself) in a reasonable time even on slower PC (like mine). I guess you can convert from .ser files to .fit files with some other utility. Use the highest bit depth you can record with because 8-bit won't take you very far either.

I would consider using a de-rotator as this will take the pain of stacking 1,000a of shots out of the way.

short exposures can get you very far depending on the target. I see you did 120” on the Eskimo, here’s one at 500ms
https://www.astrobin.com/rbty9m/?nc=user

Impressive image. My hat off. Well deserved IOTD.

Sure they can, given large apertures and the right target and the right conditions. But not very far if your yardstick is the weak and tiny or the weak and diffuse. Try to do that with something like Abell 7 or even Abell 9. From my backyard. You won't go very far, even with with 600mm aperture at your disposal.

IOTD Staff

Jan 7, 2021 · Edited: Jan 7, 2021

If interested in short exposures images, I created a group here many months ago :

https://www.astrobin.com/groups/92/

You Can find some very good examples

JF

Stefano Ciapetti

Jan 7, 2021

I have taken several images with my 30 cm SW goto dobson, using 5 to max 30 secs exposures. I use to stack Astroart which is very, very fast. To manage the dobson I use Astroart as well. As the SW dobson CANNOT be guided, I have setup a small script with Astroart which does a platesolve on every frame (Astroart plate solve engine is very fast) and when the object is getting too far from the center, automatically recenter the scope. If the scope is correctly balanced it works quite well and can track an object almost the full night. But the object must be carefully choosen because SW dobson doesn't track correctly near the zenith (at least mine).

Helpful Concise

Topic starter

Jan 7, 2021 · Edited: Jan 7, 2021

andrea tasselli:
Hi,

First off short exposures won't get you very far, although with 600mm your yardstick is far longer than mine. You can use Iris (Buil's own) to post-process and stack thousands of fits files ( used to do that way myself) in a reasonable time even on slower PC (like mine). I guess you can convert from .ser files to .fit files with some other utility. Use the highest bit depth you can record with because 8-bit won't take you very far either.

I would consider using a de-rotator as this will take the pain of stacking 1,000a of shots out of the way.

short exposures can get you very far depending on the target. I see you did 120” on the Eskimo, here’s one at 500ms
https://www.astrobin.com/rbty9m/?nc=user

This is simply amzing. I know him and I tried to ask for some help. He was very nice to me but my main problem is that I do not know how to stack so many frames in a reasonable time. He does it with Siril from a video .ser. It was not possible for me to allign the frames.

Topic starter

Jan 7, 2021

Jeffbax Velocicaptor:

short exposures can get you very far depending on the target. I see you did 120” on the Eskimo, here’s one at 500ms

*100% agreed. I use short exposures and it can get you very far. With a 600mm scope at f/3 threre are many objects to capture.

You Can also mix short and long exposures. I like this. Siril is a good software. Did you try the global registration mode ? JF

Thank you! I will try it. Unfortunately I have no experience at all regarding deep sky imaging si everything about this topic for is difficult. I just started from the stack menu so I did not try the registration manu before. I will try tonight.

Topic starter

Jan 7, 2021

Jeffbax Velocicaptor:
If interested in short exposures images, I created a group here many months ago :

https://www.astrobin.com/groups/92/

You Can find some very good examples

JF

I immediately joyned this amazing group! I hope I will contribute with some pictures.

Topic starter

Jan 7, 2021

Stefano Ciapetti:
I have taken several images with my 30 cm SW goto dobson, using 5 to max 30 secs exposures. I use to stack Astroart which is very, very fast. To manage the dobson I use Astroart as well. As the SW dobson CANNOT be guided, I have setup a small script with Astroart which does a platesolve on every frame (Astroart plate solve engine is very fast) and when the object is getting too far from the center, automatically recenter the scope. If the scope is correctly balanced it works quite well and can track an object almost the full night. But the object must be carefully choosen because SW dobson doesn't track correctly near the zenith (at least mine).

The plate solving would be the solution because my Dob is very precise but obviously after 3-5 minutes the target is not in the center anymore so I have to spend the night in front of the PS to correct the tracking. Do you think I can manage my StellarCAT + Nexus DSC system? It is quite difficult because this system is not intended to interface with plate solving softwere functions.

Stefano Ciapetti

Jan 8, 2021

If stellarCAt has an ascom driver it will work for sure. For the plate solve, Astroart needs a few parameters. Ccd size for sure. If you send me a fits image and the size of the ccd + the focal length of your telescope I can do a test with Astroart.

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https://www.astrobin.com/72om2h/0/

https://www.astrobin...i6d5/D/?nc=user

https://www.astrobin...kqgq/B/?nc=user

https://www.astrobin...page=2&nc=&nce=

https://www.astrobin...page=2&nc=&nce=

Topic starter

Jan 8, 2021

Stefano Ciapetti:
If stellarCAt has an ascom driver it will work for sure. For the plate solve, Astroart needs a few parameters. Ccd size for sure. If you send me a fits image and the size of the ccd + the focal length of your telescope I can do a test with Astroart.

Thank you very much, that’s great. Would you like to send me your email so it is easier for me to send you the fits?

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Stefano Ciapetti

Jan 8, 2021

stefano@ciapettiagency.com

enricobonaventura

Jan 8, 2021

Hi, I’m interested too, with Maxim 5 and Maxim 6 plate solving works very well with my StellarCat and Ascom Driver, you can plate solve every X exposure, with Voyager you can realign to target every X minute

The_Exoplanets_Channel

Apr 4, 2021 · Edited: Apr 4, 2021

Bogdan Borz

Apr 4, 2021

Filippo Scopelliti:
Dear all,
My name is Filippo Scopelliti. I started to image Solar System a few months ago with my Dobsonian telescope. Some images of Mars taken during this opposition are presented in my gallery.
Several people are having very good results in imaging deep sky with Dobsonians. My very first attempts have been acceptable so I decided to continue the acquisitions.
My instrument is a large well corrected Dobsonian with short focal length (600 mm aperture f 3,8) and a very good tracking that allowed me to image Mars at about 21 meters focal length.
People doing this kind of technique are used to produce .ser files which are much easier to manage because if I make 3 hours of integration (for example) at 1,5-2 seconds exposition I obtain about 7-8000 frames, quite a lot of frames to stack. A video should be easier to stack instead of the same amount of standard .fit images. The stack is usually done with Siril because the planetary softweres as AS3, for example, are not intended for this purpose. In fact in my experience (very short to be onest) in was not possible to correctly allign the frames so the images I made have been done stacking some thousand of regular .fit files. Honestly it was possible, PI managed it quite nicely, but the process lasts hours.
Does anybody know how to manage .ser file with Siril or how to allign deep sky videos?
Thank you very much for your help.
Filippo Scopelliti

Hi Filippo

You should use align on a star. I tried some stacking with my Newtonian and the ASI224mc. Not very impressive for now : )). I believe however that you must be having field rotation with the dobsonian in Alt-Az tracking. Maybe that's the problem with Siril and the alignment. Siril can also export your frames from ser, but I am not a very experienced user of Siril. JeffBax Velocicaptor and Exaxe are the experts of this technique. But I believe both use an Equatorial mount compared with your Dobsonian.

Clear skies

Bogdan

Concise Supportive

Aug 5, 2021

Hi Fillipo

I see from your images, that you have made great progress in you short exposure imaging. You call it 'Lucky Imaging' which would mean that you are selecting the best frames for stacking - what is the main software that you are using to select the best frames? I see you have used several different packages (AS3, PI and Siril), I just wondered which you found best for DSO frame selection for your DSO short exposures?

Best

Mike

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matthew.maclean

Aug 6, 2021

SharpCap does have a "Live Stacking" feature ; I do not know if it would be helpful to you, but it may be worth looking at. I believe the freely downloadable version has the basic Live Stacking feature so it wouldn't hurt to try it out and see. If it works well enough for you, it would eliminate the need to manage the very large files entirely.

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IOTD Staff

Aug 6, 2021

Hi, SIRIL is very good for frame selection and sorting thousands of images. You can select the FWHM criteria or/and roudness.

CS

JF

Aug 6, 2021 · Edited: Aug 6, 2021

Hi, SIRIL is very good for frame selection and sorting thousands of images. You can select the FWHM criteria or/and roudness."

Great - thanks - I've been capturing and livestacking in SharpCap for the last few months, do you find this even stricter than the FWHM filter in Sharpcap? Does SIRIL have any other criteria that might put it above Sharpcap for frame selection?

Mike

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IOTD Staff

Aug 6, 2021 · Edited: Aug 6, 2021

In SIRIL you select the percentage you want to keep. For example the best 60 %. This corresponds to a value regarding your data. You can do this on FWHM criteria and/or (both available and mixable) roundness.

JF