Amp glow and image shift

[JohnH]

I am taking longer exposures with my DSLR now that I have autoguiding working (mostly). A couple of questions arise, attached is a 16*300s stack of iso1600 images of NGC253, darks, flats and bias applied in IP2.8 and then I selected digital development, auto.

It appears I do not have anything like enough signal yet - this is shown by the amp glow? I thought darks would do a better job of getting rid of that but maybe not when you push this hard? So what is better - more subs or longer subs? What is a reasonable amount of exposure time for this object (200mm scope)?

The other observation I have is there is image shift from frame to frame (the black bar at bottom of frame shows area covered by only some of the frames) but - the system was autoguided and the guide s'ware reported no such shift - I do not think this is field rotation - the shift is in RA and consistant across the frame. Is this the dreaded differential flexure?

[Striker (Tony)]

Yep looks like Flexure to me.

Something that a lot people dont take into account...I was getting a fair bit of flexure and have improved the system via some tube rings on my OTA but I still have a little.

The flex can be coming from anywhere....Focuser, mount plates, guide rings, camera connection.I wouldn't be too concerned for 16 images thats not that bad.

Regarding exposure time 5minutes is ample..I would take more exposures and lower the ISO to reduce noise.

yes your dark frames should have reduced the amp glow more then it did.

[Garyh]

Hi John,
I think that 5 min exposures at 1600 iso would be enough for this galaxy and you have stacked quiet a few so should be a good signal to noise ratio. I find that the darks never seem to remove all the amp glow. I usually crop that bit off, mine is in the lower left corner. Maybe its in the way you have processed the picture?
As for the flexture. I am getting the same thing with my new newtonian, its driving me nuts... . I know its not the mount or my guidescope and always a steady image shift the same way as gravity pulls on the scope, I thought it was my guiding to start but I imaged 4 different things and still the same shift and the stars are oval usually in the same direction as well so things are moving as the image is being taken while the star does not move in the guidescope..maybe the primary cell holder or tube rings or dovetail bar??? made the newt very strong so am very baffled???
Anyone have any suggestions for me and John? or we will end up bald...
Cheers Gary

[Lester]

Good question John.

Another way of elliminating star trail is to use an off-axis guider, so the scope that is doing the main imaging is also used to pick up a guide star.
Yes there main down fall is the difficulty of finding a bright enough guide star without moving the image too far off of central in the camera.

Good luck.

[EzyStyles (Eric)]

same as gary. taking darks only subtract sa fraction of amp glow. easiest way is to crop it out. how many flat frames are you taking btw?

[[1ponders] (Paul)]

John, what f/ratio is your scope working at? Native or with FR? 16 shots should have done a way better job than that. Have you tried adjusting the sliders a little bit after doing the Auto on Digital Development? I've also found that if I DD in IP first then PS doesn't do a very good job of the post processing as too much info has already been discarded.

Maybe try either dropping your ISO or your exposure time and tweeking the DD a bit so it isn't as harsh.

This image is one I did on the 20-09-06 with my Orion @ f7.5. 13 x ISO1600 @ 180 sec. The first image has no DD in IP, just curves and levels in PS. The second image is pure auto DD.

[JohnH]

The image was at prime (F9) on the VC200L is full frame no cropping and just binned 4x4 and jpeged to get the size down after auto DD'd to show the glow in the raw as it were. I can get a better result with more careful processing but wanted to know if I was doing something wrong as I had not seem amp glow jump up at me like that when imaging other objects (brighter ones) so I went back and did 16*5 mins but saw the same effect...

I took 16 darks...1/4 moon does not help...

I'll have to find that flexure issue though I have an idea that it is probably droop on the main imaging train (astronomers droop?), my T-ring is quite sloppy but mostly I suspect the the flip mirror assembly which is a push fit. I will try again with the FR in place and the camera close coupled to the focuser - no flip mirror.

[gbeal]

At what age do us astro folk get astronomers droop, I'll need to be careful obviously.
One thing you may like to check is cables, as these can sometimes put extra strain and stress on the train as well. Most of my DSLR cables (USB & remote) are light and thin, but some CCD (SBIG and SX) cables are THICK, and HEAVY. Just a thought.

[matt]

Originally Posted by gbeal

At what age do us astro folk get astronomers droop

Around the same time our hard drives crash!!!

[JohnG (John)]

Hi John

I have been following this thread with interest.

I have attached for your info, my 300 second shots of the same object.

The 1st shot is of my Master Dark, 10 x 300 seconds

2nd shot is as developed within IP 2.80.

3rd shot is the final outcome.

I don't know if you are a member of the IP Yahoo Group, they have had a fair discussion on Amp Glow in 5 minutes shots lately.

Anyway, attached for your info.

Cheers

JohnG

EDIT: These are taken at Prime Focus (f/8) of the Tak FS-102 at ISO 800.