Last night I decided to push the G-11 a bit more than I had, with 2min exposures, at NGC1365. Initially tried the Helix Neb, but couldn't get it to appear at all!
This image is 10x2min exposures. The black trailing dots appeared upon the final stretches. I am assuming this is likely from dark frame temp mismatch. The darks I took last week I'm sure were 2 degrees warmer than what last night was (30 degC vs 28deg C CCD temp).
Is this the likely culprit?? The trail of the dots would be indicative of how far the image drifted on the CCD over the 20 min.
The first thing I would try would be to stack and process just the lights, ie don't use the darks at all. If the artifacts still show up, then you know that they are in the image. If not, then the darks are responsible.
Drift is not an issue. Darks just remove dark current at particular positions on your camera sensor. It doesn't matter what is being imaged or where it is on the sensor.
Geoff
It is likely that is is a temperature thing. As Geoff has suggested stack the lights without dark subtraction first and see what happens.
As for the trailing of the dark spots, if it is a dark problem then when your program subtracted a master dark from each individual light and then stacked them, if each light is slightly offset from the previous then you will get the trail.
Drift is not an issue. Darks just remove dark current at particular positions on your camera sensor. It doesn't matter what is being imaged or where it is on the sensor.
Geoff
I've given it some more thought. I can see that if subtracting the darks is giving some crap and if you are stacking the images, then the crap will drift if the images drift.
Geoff
From my experience your hunch is correct. I sometimes have this problem and it's always because my dark frames don't match the light frames accurately enough. In the 'old days' it was because I re-used dark frames from weeks/months ago. Same temperature and all that, just things change slightly over time. That wasn't as bad as what you have, but it's the same deal.
You just need to take some new dark frames at the same temperature as the light frames, asap, and you should be fine. Otherwise it comes down to playing with the processing to minimise the visibility of them (perhaps only partially subtracting the dark frames).
Yep, I'm with all the others. I like the concept of a library of darks, and when you are shooting 20 minute or thirty minute exposures, this can be a great idea.
In your case though shooting 2 minute exposures, I would do them at the same time, and in fact do them part way through the sequence.
I am surprised you couldn't see the Helix, is it possible you were not "on the spot"? Why I ask is that I tried it last night, as it has been one of those subjects I have never really sorted. With a simple 5 second exposure binned 3x3 it was visible, (I did this to ensure the goto was accurate, and to "frame" it centrally). Even 7 seconds at 1x1 showed it. (ST2000XCM and 8" f4 newt).
From my experience your hunch is correct. I sometimes have this problem and it's always because my dark frames don't match the light frames accurately enough. In the 'old days' it was because I re-used dark frames from weeks/months ago. Same temperature and all that, just things change slightly over time. That wasn't as bad as what you have, but it's the same deal.
You just need to take some new dark frames at the same temperature as the light frames, asap, and you should be fine. Otherwise it comes down to playing with the processing to minimise the visibility of them (perhaps only partially subtracting the dark frames).
Roger.
Thanks Roger - cloudy night tonight - will set it up to take a heap, hopefully I'll get a group closer to the right temp!
In your case though shooting 2 minute exposures, I would do them at the same time, and in fact do them part way through the sequence.
I had the same thoughts - given when I'm imaging the temp is always dropping minute by minute.....
Quote:
Originally Posted by gbeal
I am surprised you couldn't see the Helix, is it possible you were not "on the spot"? Why I ask is that I tried it last night, as it has been one of those subjects I have never really sorted. With a simple 5 second exposure binned 3x3 it was visible, (I did this to ensure the goto was accurate, and to "frame" it centrally). Even 7 seconds at 1x1 showed it. (ST2000XCM and 8" f4 newt).
Don't know about not being on the spot - G-11 Gemini is pretty good, and its a big target! Put NGC1365 on the chip without realigning after the Helix search - the field looked right.... see what you think.... Don't know if we can compare - f/7.5 vs f/4 - SBIG vs DSI.... still thought I would see something.....
This is the scout frame - I estimate 50'x35' or so DSI frame.... I can't match it to anything near the Helix! maybe I did miss by a long way! Will try again when it is clearer..... If you can tell where I am please let me know
I've had a look at a few pics of the Helix - there is a nice pair of bright stars on its edge - and these aren't in my scout shot. Will have to aim better next time!
I tried, blinking yours then mine, but man it ain't easy. Differing orientations, and scales. Try again with the Helix though, it is well placed.
Took your lead though, and tried that 1365 last night. Neat looking huh?
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