A friday night Markarian's Chain

[Art Gecko]

I had a visit from a photographer friend of mine on friday night, who has taken an interest in my AP from the start and came to see how it's done, and have a go at it with his own camera... Now I fully admit I'm no expert, and I've only just started using an autoguider, but we had a lot of fun and came up against some interesting problems to solve.

Firstly, once he'd arrived and we'd got the scope balanced with all the equipment loaded on it, we started a 2 star alignment and used the bright stars in the process to focus the camera... upon taking the first picute I realised that his Nikon D750 had a full frame sensor and my inch and a quarter focus drawtube was inhibitting the view, producing a black ring all around the image... much like when you point your smartphone camera into an eyepiece and take a shot... But since this session was all about using HIS camera we agreed we could just crop out the dark parts and we carried on.... besides, his camera works with BYNikon, which mine doesn't!

Now, he really wanted a shot of the Pleiades but by the time we'd got everything up and running it was far too low in the sky, so I suggested a galaxy cluster rather than a star cluster and he seemed quite happy with that, so we set the synscan to find M84 and then set about framing the shot and when we were happy with it, set BYNikon to take 24 x 300 second subs at ISO 800 + dithering and finally settled back with a beer

At 1am it finished it's run and we were both getting tired so we took the equipment down and went to bed... First thing in the morning we managed to get some flats, flat-darks and bias frames... that done, we sat down and started going through the shots in DSS, but when we loaded the flat-darks and bias frames they seemed to be 16x16 pixels smaller than the lights and flats... completely perplexed at how this may have happened, we simply applied the flats and carried on without the other calibration frames... I then gave him a quick tutorial in photoshop development and sent him on his way with all the files he needed to have a go himself at home.... At which point I realised if he had the same trouble with the calibration frames at home, he could at least take another run with just the lens cap on and see if he could get some bias and flat-darks that work.

So this morning I thought I'd take another look at his data. I ran 2 stacks, one with flats and one without, this is because without bias frames, the flats seemed to over compensate and the black corners turned white giving the picture a sort of negative gradient... by combining the 2 stacks I was able to smooth the background gradient and then set about processing, and here is the result:

Art

[Handy_Andy]

I've found that the disparity in image sizes when using my canon happens when I've switched between Raw files as the output to jpegs. The raw files being those few pixels larger. Great shot too

[Art Gecko]

9 minutes ago, Handy_Andy said:

I've found that the disparity in image sizes when using my canon happens when I've switched between Raw files as the output to jpegs. The raw files being those few pixels larger. Great shot too

Thanks Andy, now I think of it... we did export the flats from the SD card directly to their own folder, but the bias and flat-darks were exported all together then transferred into their own folders respectively... could this have something to do with it I wander? I guess these things can happen when passing a laptop between two people and not doing everything yourself

[Handy_Andy]

When I've done it in the past I've switched the file output from raw to regular images for daytime work. This is an easy setting to switch on canon cameras not sure about Nikon.

You can have a single set of bias frames and reuse them for different pictures. You only really need to do flats each time to get rid of any dust bunnies in the imaging chain and differing illumination levels over the sensor.

[Art Gecko]

I do have a back catalogue of calibration frames for my own camera... just a shame they won't work for his! 

[Art Gecko]

When my friend took the files home and tried them on his computer, the calibration frames were apparently the right size and he got DSS to work the subs properly... Here is the result: