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M33 - can't see it, might as well image it

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#1 Peregrinatum

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Posted 07 December 2019 - 11:17 PM

Can't see M33 in my Bortle 6/7 skies with my 12" Dob or C925, so it was on my imaging to do list.

 

C925HD Edge at F7, ASI071MC UVIR cut filter, EQ6-R, OAG/PHD2 guided
953 OSC frames, 9.25 hours total, average ~ 35s per frame (changed exp during session so RN was swamped at 95%), dithered, 80 gain, moon < 25% over 5 sessions
Processed in Pixinsight: PhotometricColorCalibration, LocalNormalization, DrizzleIntegration, among many other things

 

I have difficulty with LP gradients so what I did differently in my processing this time was to use a "Noise Floor" to cancel out some mottling left behind from DBE, this is a cool way to help deal with background noise when flattening the field... if you want to read about it here are the particulars:

 

https://www.cloudyni...ling/?p=8322649

 

Jn2jegFl.jpg


Edited by Peregrinatum, 07 December 2019 - 11:21 PM.

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#2 zxx

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Posted 08 December 2019 - 12:09 AM

Why not use your 115mm refractor?



#3 Peregrinatum

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Posted 08 December 2019 - 12:40 AM

Why not use your 115mm refractor?

For viewing, or imaging?



#4 zxx

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Posted 08 December 2019 - 12:50 AM

For viewing, or imaging?

I believe 9+ hours on M33 would have gave a much better result with the 115 using longer exposures. And would have framed nicer.


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#5 17.5Dob

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Posted 08 December 2019 - 01:04 AM

Have to agree, the 115mm would be perfect....

FWIW, I've never had a problem seeing it visually in any "suitable" OTA, in any conditions all the way up to a Bortle 8 site.

From my dark site it's an easy naked eye object...

This is 1 hr from my old Bortle 8 backyard. One of my first images 5 years ago. Shot with the worst AP dSLR I've used and the worst OTA I've "tried" to use....


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#6 F.Meiresonne

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Posted 08 December 2019 - 08:34 AM

It is a nice picture though. I would be king if i could that.

"

M33 otherwise a tough target visually. You can see a hint of it in big binoculars in my skies if it is really clear.In my 18 " not so well actually, probably due to it's big size. I am under Bortle 5 skies, but in practice, transperency is in many cases an issue. Too much particals in the air most of the time which is not very favorable for such targets...


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#7 Peregrinatum

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Posted 08 December 2019 - 08:41 AM

Go figure!  Nobody in my local astronomy club of approximately 250 members can view M33 in town, have to drive an hour away to visually observe it.



#8 Peregrinatum

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Posted 08 December 2019 - 10:29 AM

I believe 9+ hours on M33 would have gave a much better result with the 115 using longer exposures. And would have framed nicer.

Framing is a matter of taste, I liked how the core details can be seen that the C925 provided and would have been lost with the smaller aperture of the 115mm.

 

As far as longer exposure is concerned, its always a Kabuki dance between swamping RN and clipping stars.  I think M33 is a difficult target to image well, it has a low surface brightness yet has numerous bright stars, faint limbs, and is surrounded by a dim hydrogen cloud.  I think it is difficult to get it all right, and I am not saying that I did here.  I didn't want the chalky/grainy look from the hydrogen cloud that you see in 17.5Dob's image yet I wanted it bright enough to see the limbs and core.  I do not believe longer exposures would have given increased SNR for the same overall imaging time, maximizing the number of frames in the same overall time increases SNR, so I chose to optimize on 10*RN^2 to determine optimal exposure.

 

Overall I learned a lot shooting this target, and had a lot of fun capturing and processing it.


Edited by Peregrinatum, 08 December 2019 - 12:23 PM.

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#9 RedLionNJ

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Posted 08 December 2019 - 10:49 AM

"raising the noise floor"  -  this sounds awfully like black-clipping to me. A very bad practice.

 

In situations like these, dithering and just simply a lot more data is the correct solution.


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#10 Peregrinatum

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Posted 08 December 2019 - 11:07 AM

"raising the noise floor"  -  this sounds awfully like black-clipping to me. A very bad practice.

 

In situations like these, dithering and just simply a lot more data is the correct solution.

Did you go to the link and read about the method, is this black clipping, and if so why is it a bad practice?



#11 elmiko

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Posted 08 December 2019 - 01:12 PM

Looks pretty good to me! I think if you took longer subs, you would get more data which would make it easier to stretch brighter.... Which would bring out more details. You were guiding right? Try 3 to 5 minute subs. And Dither!


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