First stacked shots with 20" dob w GOTO

[Ships and Stars]

Hi all,

I'm very much a visual observer but half-heartedly tried stacking some images the other night of M51 with shots taken using a 20" Stargate dob with GOTO and tracking. Also M97, M108 and tried a little around Markarian's Chain but those only have about 10 x 13" subs, no bias or flats.

Results are erm...not great, but sort of a record shot.

These are my first test shots with stacking and only M51 has many subs (115 13" subs for c.25 min integration under 20.3 SQM skies). No bias or flats. Just images+20 darks. Processing in DeepSkyStacker and just editing TIFF in LR 5.8, no editing in DSS.

Using an unmodded Nikon D810 at ISO 3200. 

My main concern at the moment is obviously focus and coma. I didn't use a coma corrector, have the ES HR CC but my nikon adaptor doesn't fit in the end well. However, I'd like to remove the 2" eyepiece holder and attach camera directly to CC.

Therefore wondering if: 

1.) a coma corrector would make a big difference, and has anyone used the ES HR CC for photography? I need to remove the 2" EP holder and attach camera directly to CC tube. 

2.) is there a better way to achieve focus? No focus mask for 20" dobs, so trying to use live view magnified to achieve best focus, but clearly not spot on. Wondering as well if diffraction spikes are blurring as the stars turn, since it's alt-az.

I have collimated scope as best as I can, but can perhaps squeeze a bit better collimation out.

I'll no doubt have more questions! Thanks in advance. 

Note - mods feel free to move if in wrong subforum.

 

Edited April 18, 2020 by Ships and Stars

[JeremyS]

Very impressive @Ships and Stars!

[Ships and Stars]

1 minute ago, JeremyS said:

Very impressive @Ships and Stars!

I have a long way to go! Haha. Will try and line up on Markarian's Chain tomorrow night with a coma corrector. There is also horrible vignetting with the full frame sensor, but this is just sort of for fun - nice to have some record shots of various objects.

[Hughsie]

20” dob? Now that is up close and personal!

The M51 is glorious and by far the largest rendition I have seen.

[Ships and Stars]

5 minutes ago, Hughsie said:

20” dob? Now that is up close and personal!

The M51 is glorious and by far the largest rendition I have seen.

Thanks, it's a 36mp sensor on the D810, so plenty of room to crop the horrible vignetting  

I was going to try it though my 2x powermate to get more detail, but would probably have to double ISO to 6400 or take an absolute ton of subs. Still might try it one of these days!

First port of call is to get the coma corrector working and I think I can get away with an even lower ISO of c.2000 or 2500 if I take plenty of subs.

Not sure how much flat frames will help, I have a lot of learning to do!

[Ships and Stars]

My brain has engaged momentarily and I looked in the ES coma corrector box. Lo and behold there is a t-mount adaptor which in turn threads onto my Nikon t-mount adaptor, so I now have a Nikon DSLR/coma corrector set up at last. Amazing 🤣

Moving on, I have set up interval shooting in the D810 menu using mirror-up mode for 10" sec exposures at ISO 2500 x 200 shots, but will do some test shots first before I let it do its thing. 

Incredibly, it is supposed to be clear here tonight so I will have another bash at Markarian's Chain, or maybe something up in UMa. 

And collimate, collimate, collimate (and fine focus as best as I can) 👍

Edited April 18, 2020 by Ships and Stars

[bottletopburly]

Remember the D810 is iso invariant ,recommended iso is 200 nothing gained cranking iso up, other than noise http://dslr-astrophotography.com/iso-values-nikon-cameras/ loveley images btw .

[The-MathMog]

I own the ES HR Coma Corrector, and it works great for my camera/telescope setup, when the sensor to objective distance is correct. I've read in the manual that the distance isn't AS crucial with DSLR's (for some reason unbeknownst to me), but if you are still struggling with coma, then the recommended distance is 55mm from sensor to objective.
And great images for such short exposures, but with a beast of a scope like that, no wonder.. 😅

[Knight of Clear Skies]

The M51 has come out really well given the short integration time of 25 minutes.

[ollypenrice]

A successful experiment. I'll admit to being pleasantly surprised by how well you've done here.

Olly

[Ships and Stars]

20 hours ago, bottletopburly said:

Remember the D810 is iso invariant ,recommended iso is 200 nothing gained cranking iso up, other than noise http://dslr-astrophotography.com/iso-values-nikon-cameras/ loveley images btw .

 

9 hours ago, The-MathMog said:

I own the ES HR Coma Corrector, and it works great for my camera/telescope setup, when the sensor to objective distance is correct. I've read in the manual that the distance isn't AS crucial with DSLR's (for some reason unbeknownst to me), but if you are still struggling with coma, then the recommended distance is 55mm from sensor to objective.
And great images for such short exposures, but with a beast of a scope like that, no wonder.. 😅

 

4 hours ago, Knight of Clear Skies said:

The M51 has come out really well given the short integration time of 25 minutes.

 

2 hours ago, ollypenrice said:

A successful experiment. I'll admit to being pleasantly surprised by how well you've done here.

Olly

Thanks everyone, I didn't put a lot of thought into this, just downloaded DS Stacker and went for it and then added some dark frames taken at the end of the session. I'll look into the ISO 200 setting, that's very interesting! It makes sense though. I would love to shoot at ISO 200!

Thanks for the 55mm setting - I was wondering about that but haven't had a chance to read up. I'll go that route tonight, weather permitting.

25 minutes isn't bad I guess, wish I could take longer subs, but alt-az limitations w/o an EQ platform, so pretty happy with that.

And finally, thanks Olly! I tried it again last night but the mount wasn't playing along well near zenith (M101), I'll try it again tonight on a lower object after I inspect the altitude runners etc. Still tracking fine for visual. 

I'll see what else I can squeeze out of this set-up  👍

Edited April 19, 2020 by Ships and Stars

[Paul M]

Very impressive images indeed. I remember a time when M51 was a rather difficult subject for amateur equipment. You have done it great justice with apparent ease!

I had a session the other night, my first real attempt at imaging and the images were plagued with vignetting and dust bunnies. I didn't do any Flats initially but went back and did some next day with reasonable confidence that everything was in the same place. Mustn't have used a good enough T Shirt. The resulting flats made little improvement. Not too worried, that session was never about the end images but about getting hardware and software working together. Which they did, brilliantly.

Baby steps and all that.

Like you, now I have good proof of method, the sky is the limit!!

 

 

[ollypenrice]

5 hours ago, Ships and Stars said:

 

And finally, thanks Olly! I tried it again last night but the mount wasn't playing along well near zenith (M101),

 

The 'Dobsonian hole!'

😁lly

[Chris]

I'm pretty amazed at these, nice one!

[Ships and Stars]

4 hours ago, Paul M said:

Very impressive images indeed. I remember a time when M51 was a rather difficult subject for amateur equipment. You have done it great justice with apparent ease!

 

38 minutes ago, ollypenrice said:

The 'Dobsonian hole!'

😁lly

 

27 minutes ago, Lockie said:

I'm pretty amazed at these, nice one!

Thanks all! 👍 

Paul - I have an absolute ton of vignetting with my camera as well, full-frame dslr, so I just switch to aps-c mode generally. It's all just practice I believe, and I have lots to learn here.

Olly - now you mention it I have heard of the Dobsonian hole and fully understand it after last night! I was waiting for M101 to drop back down last night but cloud moved in about 2:30am. I was dog tired anyway.

I've set up again tonight and have taken extra care to collimate and level the scope. I also checked all the bearings for smoothness and everything seems ok.

Shall try for M101 again tonight, got a very rough photo last night but too embarrassed to share, haha. If it goes in the dobsonian hole, I'll drop down to M82.

Good luck everyone, hope the weather is clear for you all. 

 

[Greymouser]

Well done, excellent pics there, with a Dob of all things!

I have a question though, assuming you can answer it: Just how large a Dob is feasible in the UK, considering our skies, do you think? I ask because I was told not to bother with such a large scope as our skies are just not good enough, to justify one. Yet here we are you proving otherwise.

Edited April 19, 2020 by Greymouser

[Paul M]

1 hour ago, Ships and Stars said:

Paul - I have an absolute ton of vignetting with my camera as well, full-frame dslr, so I just switch to aps-c mode generally. It's all just practice I believe, and I have lots to learn here.

I just had to Google APS-C, it turns out that my Canon 1200D has such a sensor so I guess it's APS-C by default. Doesn't seem to have helped me. Maybe I need to bang a 1.5x Barlow in there and narrow the field down without throwing photons away.

I redid my flats this afternoon. There aren't a lot of white T-shirts in our house but I made a better job than my first attempt. Still didn't fully resolve the vignetting and dust bunnies.