Yes. Attaching (fits frame, debayered, resized and saved as jpg).
Pretty bad, huh? and overexposed again.
Thanks!

Max,

thanks for posting the picture. I suspect this is not an unprocessed, raw file. IMO it has (probably automatically) been stretched. An unprocessed image taken with my ASI 294 Pro looks like this (300s sub taken at an F/6,5 scope + flattener. -10 degrees C, gain 120, offset 8. Overall, very close to your setup).



Image processing software can apply an automatic stretch during the debayeringn process (for example 12bit -> 16bit like it is used for 12bit Canon DSLR shots). When I do that, the picture looks much more like yours:



You can download the original FITS file here

You can see lots of green, red and blue hot pixels in my file. I do not see any in your files which also suggests to me they must have been processed (not only stretched but calibrated, too?)

This is why I believe your "bright" background is not massive over-exposure but rather the effect of unintended stretching. Certainly, our signal-to-noise ratio suffers from a light polluted sky. But whose doesn't?

Regarding coma: I've done a quick search for pictures taken with your scope and one example that I found is this: Astrobin
An ASI 1600MM was used. This cam's sensor size is 17,7 x 13,4mm (diameter 22,2mm) which is comparable to our 19,1 x 13,0mm sensor (diameter 23,1mm). There is a fair amount of coma present in mielejr's picture, too. So either this is not unusual for this scope+flattener combination OR both your flatteners are not positioned at the correct distance to the focal plane. I suggest you contact your dealer or the manufacturer of your flattener and ask them what the correct distance focal plane - flattener is for _your_ scope. An answer like "It's 55mm for ALL scopes" would be considere unacceptable here. In any event, some research into this issue could be well worth the effort. Maybe other owners of the same scope and flattener have done their own test series and found the optimal position...

I wish you clear skies and good luck at the "dark site"!

Alfred,
Thanks for looking into this!
I'll talk to Orion or Astro-Tech about the right distance.
As to the image I posted. Of course it wasn't a raw image - it was debayered in Fitswork, resized and saved as jpeg.
How did you convert your fits image?
Thanks for the sendspace hint...
If this is still of interest to you, here's a raw fits file: www.sendspace.com/file/swygpa
Thanks!

I stand corrected, your background is too bright in the original fits files. Our setups are not that much different. Your 240s vs. my 300s roughly compensate for the F6/F6,5 difference. What remains is gain 200 vs. 120 (and sky quality, of course). Anyway, the [Regim V3.8] debayered images...





They look 100% the same in FITS Viewer when automatic stretch is disabled. Looking at these results, I still can hardly believe the only difference is gain and sky. I'm curious to see your "dark site" results. Good Luck!

I am astounded to see what a difference gain makes. I noticed the last file you linked is a 60s exposure only, but at gain 300. This one looks even brighter! Again Regim V3.8 debayered...



Lowering your gain setting would give you a (much) darker background. Since your stars and the cores of both galaxies have hit the "full well barrier" in the 60s exposure already, I guess your SNR would benefit from it, too. Maybe Ron can chime in here since this is not exactly my field of expertise.

Yeah at 200 Gain - at that LP level - 10-20 or 30 seconds.

did you make a typo above?

Your camera has 6.5 mm (plus 11mn? oh I see you're saying 17.5 is 11mm+6.5 okay) neve rmind.

Someone else suggested shooting with no FF I recommend the same. (focus is off which doesn't help the coma) but it's still to much. If you have a way to add 2mm more - it would be a nice test also.

So my FF I had to return to stellarvue - (recall) was because it was being sold for F6 scopes when it didn't work with them. It worked with F7-8 scopes. Mine is a F6. Since the dealer didn't know they're taking it back and swapping it out. (I've suffered with it for 2 years) with never a flat frame. gmmm and eded up buying the Hotech (They're pretty reasonably priced) . I also liked that the Hotech didn't reduce my focal length. When using a small scope for galaxies we need all the FL we can get.

Ron/All,
OK. Just returned from a supposedly Bortle 2 site (Mojave National Preserve). 8-hour drive.
It was dark alright (new Moon too!) but there was some light pollution (Las Vegas was glowing 50 mi away and some other light sources).
Shot the Markarian's. Much better of course. This time I might've underexposed a little.
120 sec x 60. AT72EDII, ASI294MC, -10C, 200 gain. Stacked in DSS. Processed in Photoshop. Cropped.



Single fits file. Debayered, resized and saved as jpeg.



Location. Las Vegas is on the left (50 mi). I-40 on the right (12 mi). My telescope in the center.



Thanks!

Max,

I still think the raw image is too bright, particularly for a dark site. It should be almost all black, with no galaxies visible, just a few, very bright stars. Maybe Andres is on to something? Do you see 12bit in your Indi settings where I see 14bit?

OK guys - thanks for helping! The main thing I got out from all this - it's hard to get good results @ Bortle 8 site. Interestingly enough, yesterday, I got a decent picture of the Owl nebula with just 45 x 60 sec (it's high up in the sky - maybe less pollution reaches there). Leo Triplet was OK too two weeks ago. Just Markarian's Chain was awful (shot next day after the Triplet).
I'll play with gain, offset, exposure time to get better results.
Thanks!

Sorry. Just one more. Here's one of my fits (from the dark place) processed in Regim. It's pretty dark.
Different softwares handle fits files differently. Apparently. The files themselves are the same just representation is different.

It looks like a linear (i.e. unstretched) image to me and it is supposed to be so dark. What happens if you stretch it to a non-linear image? I don't know of any free tools to do that. I always use either PixInsight or AstroPixelProcessor.


Wouter