Feedback please, not sure what to do next. HCG-44 in Leo

[SteveBz]

Hi All,

The last few weeks have been great.  I've really been able to concentrate on technique.  I've spent the last few nights on Hickson Compact Group 44 in Leo.  Here it is with Over a hundred frames,cut down to 92, with flats, darks and biases.  My main problems are, in spite of the flats, which I took three times, there is still a lot of vignetting after stretching.  Also the colour variations in the stars seems to disappeared.  I'd appreciate any ideas or feedback.  Obviously cropping helps, but I thought I'd give you the un-cropped image to see.

Process:

lights 92 x 240 s

master bias (from 100 frames)

master dark (from 15 frames)

master flat (from 98 frames)

All converted from Nikon RAW (ie.NEF to .TIFF using DCRAW)

Deepskystacker with align colours in final image

Then GIMP v2.10 Stretch histogram using levels.

Setup:

Focus with Bahtinov mask,

PHD2 Guiding with separate guidescope and QHY5 (heritage version😀)

200 mm Newtonian with Skywatcher light polution filter and Baader coma correction

Nikon D5000 DSLR

EQ5 mount with Synscan goto.

Think that's it.

I'd love your thoughts.

Steve.

 

[Star101]

Moisture in the air. High cloud!!

Oh and Light pollution too.

Masks may help. Cover the galaxies and darken the surround.

I have had the same problem the last few weeks. Yet, last night was as clear as a bell. So, not the equipment or processing.

Edited April 16, 2020 by Star101

[tomato]

Great image, I have also been imaging this group over the last two nights, and conditions last night from the West Midlands were not great, based on the PHD2 SNR values. 
 

A processing gradient removal tool would help, not sure if GIMP has one available?

[SteveBz]

18 minutes ago, Star101 said:

Masks may help. Cover the galaxies and darken the surround.

OK, I've not tried masks before I'll look into it.

S

[SteveBz]

13 minutes ago, tomato said:

A processing gradient removal tool would help, not sure if GIMP has one available?

Thanks for the compliment. But I think you're right about GIMP not having a gradient removal tool, at least not a free one. You may be able to get a paid-for plug in.

[Tommohawk]

Thats a nice image!. I've also been working on this group and I found it quite challenging - the galaxies are relatively small but the surrounding stars are quite bright. 

I'm not sure about the colour issues, but I'm pretty sure your flats must be off. Can I ask how you're taking them?  

Also - and I may be wrong here - I thought you could load NEF files directly into DSS. Are you converting to TIFFS first?

[SteveBz]

11 minutes ago, Tommohawk said:

Thats a nice image!

Thanks.

11 minutes ago, Tommohawk said:

I'm not sure about the colour issues, but I'm pretty sure your flats must be off. Can I ask how you're taking them? 

A white plastic film (from dry-cleaning packaging) held over my scope with a stretchy luggage tie and pointed at the cloudless sky away from the sun (in the afternoon).  100 shots, converted to tiff, converted to monochrome, then combined with DSS.

They're clearly giving me the pincushion effect that flats should fix.  I wonder if I haven't been precise enough about the timing.  Maybe the photos were too saturated or not saturated enough so the divide-by-flat-intensity wasn't enough at the edges, or was too much in the middle.

I could do some tests.

14 minutes ago, Tommohawk said:

Are you converting to TIFFS first?

Yes. 

I tried .NEF files directly at the beginning, but they seemed to come out monochrome.  Don't know why.  I could try again.

S.

[Tommohawk]

2 minutes ago, SteveBz said:

A white plastic film (from dry-cleaning packaging) held over my scope with a stretchy luggage tie and pointed at the cloudless sky away from the sun (in the afternoon).  100 shots, converted to tiff, converted to monochrome, then combined with DSS.

Maybe this is too shiny and doing something weird? Maybe try white T shirt - seems to be the preferred thing - and then compare your master flat with the original. Re exposure sort of mid grey is what youre after. Dont worry about converting to mono - colour flats work OK. The fewer conversions the better.

Re colour - just thinking... I used a Nikon briefly but cant recall how you set white balance? Maybe that has a bearing on it?

Looks like you have quite good data, so some scope for improvement with  processing. 

[SteveBz]

56 minutes ago, Tommohawk said:

Looks like you have quite good data, so some scope for improvement with  processing. 

That was my thought too.

I haven't reset the white balance for a long time. You set it by taking a photo of a white surface and then say make it white.

Maybe I can edit the master flat. If I subtract a constant layer from it, it would amplify the edges more than the middle. I'll try.

S.