Messier 7

[Luke.]

I had a few minutes of clear sky last night so I decided to have a quick go at M7 before the wind picked up. It's only my second try at a DSO so I am pretty happy with it.

I used my Nikon D3000. 300mm focal length.
~50 lights
5 darks
5 bias

I could only get 1 second exposures but there was nothing in the viewfinder when I was trying to find m7 so I was pretty happy that I found it.
Stacked in DSS and processed in Photoshop.

[raymo]

Your stars are elongated, which they shouldn't be at that focal length and
declination and exposure time, so either it was windy, or perhaps your
camera is not perpendicular to the light path. Do you usually get round
stars? It's very noisy, so five darks were not enough. If your darks
match your lights, bias frames are totally unnecessary, as darks contain
bias data.
raymo

[Luke.]

That is probably as good as I have got stars to look, wind or no wind, but I am sure I was probably doing something wrong.
How should the camera be to be perpendicular to the light path?
How many darks do you recommend?
I have been doing the bias because all the video's I have watched about DSS said to get bias shots. Do I not need these at all?

[raymo]

I just finished a detailed reply and my comp crashed.:

If your darks match your lights[exposure, ISO etc:] you don't need biases,
as most quality DSLR astro imaging books will explain. If they don't match
then you need biases.
The front face [and therefore the sensor] of the camera must be
perpendicular to the light path, or you won't get round stars. The most
common cause of this problem is focuser droop/sag, caused by either overloading the focuser, or by cheap, sloppily fitting extension tubes, or
adaptors, or T-rings. Everything should fit snugly.
DSLRs vary widely in the amount of noise they produce, so the number of darks you need to take varies with the model of camera you have, and the
ambient temp when you were imaging. The higher the temp, the more darks you need. I'd use a starting point of at least half as many darks as lights, and see how you go from there.
raymo

[Luke.]

Cheers Raymo. They always seem to pick the most inconvenient time to crash!
Well I was using a Nikon lens so I imagine that there shouldn't be any problems there? It fits on properly. Maybe its the mirror? My camera doesn't have a mirror lock feature.

Should I be taking flats along with the darks?

[raymo]

Sorry, what I was saying about focuser sag is not relevant, I had forgotten
that you weren't shooting through a scope, but through a lens.
That puts a whole new slant on it. I can't imagine your camera's optical
train is faulty. That leaves a camera that is not mounted solidly enough,
or a million to one shot of a faulty lens[virtually no chance],
raymo

[Mickoid (Michael)]

Luke I assume you just had your camera on a tripod. With that focal length lens 1 sec is probably the maximum you can go without your stars looking like eggs. There is somewhere on the net I remember reading which gave maximum times for each particular focal length lens before stars were no longer round. Obviously the shorter/wide angled lenses gave greater exposure times than the longer focal length ones. You've probably done the best you could have achieved with your lens at 300mm fl on a tripod.

[Luke.]

Originally Posted by raymo

Sorry, what I was saying about focuser sag is not relevant, I had forgotten
that you weren't shooting through a scope, but through a lens.
That puts a whole new slant on it. I can't imagine your camera's optical
train is faulty. That leaves a camera that is not mounted solidly enough,
or a million to one shot of a faulty lens[virtually no chance],
raymo

Originally Posted by Mickoid

Luke I assume you just had your camera on a tripod. With that focal length lens 1 sec is probably the maximum you can go without your stars looking like eggs. There is somewhere on the net I remember reading which gave maximum times for each particular focal length lens before stars were no longer round. Obviously the shorter/wide angled lenses gave greater exposure times than the longer focal length ones. You've probably done the best you could have achieved with your lens at 300mm fl on a tripod.

Yeah on a tripod. It is not flimsy but it could certainly be more sturdy which would probably help a fair bit.
If I went to 1.3" the stars looked a lot worse. The rule I was using was focal length / 500 which I had to reduce anyway.

[raymo]

Max exposure for round stars @ 300mm is 1 sec @ 0 degrees Declination
1.3 @ 40 " "
2.0 @ 60 " "
6.0 @ 80 " "
I wouldn't worry about flats until you have sorted this problem out,
and worked out your darks.
raymo