Andromeda with a dslr help please

[superjody]

Can anyone give me tips on how to get imaging right for andromeda please??

My equipment is....

skywatcher 250 on an eq6

unmodded canon 550D mounted to the focuser with a remote shutter cable.

Do i need to use mirror lock?? i seem to get a little bit of wobble when i take an image

How long do the exposures have to be and how many before i stack them.

What does it mean when people say they use darks and flats when stacking images??

thanks

Jody

[carastro]

I can't answer the bit about wobble and Mirror lock.

I use a DSLR (450D) and connect to my laptop with a cable which plugs into the same socket as your remote and into the laptop at the other end.

I currently use Canon Utility software for capture, but APT is good too.

I use Manual setting, 800 ISO, Bulb and for Andromeda M31 you will need 30sec exposures (about 15 - 20) to use for the core which is bright and the longest exposures you can get up to say 5min exposures, for the main part of the galaxy, I say this because as you seem new to imaging so I assume you are not guiding as yet. Without guiding it really depends on how good your tracking and alignment is on how long a sub you will be able to get without trailing, you'll just have to try it and see. But the longer the better. (Try to take about 1 - 1 1/2 hours worth of images to start with if at all possible, but for a first stab at this half an hour might be acceptable).

When you come to stack the images you should use the 30sec stack for the core and combine this with the longer subs in layers in photoshop (too long to explain here) but google it and you'll find some explanations.

Flats are used to subtract vignetting in the light path and dust particles etc from the image. These are done using a diffuse light source, (I use 100 ISO AV mode), DSS will subtract these in the stacking procedure.

Darks are images taken exactly the same as the light images at the same temperature but with the lens cap on, this captures the electrical noise and when you stack them the idea is that they subtract noise from the image.

Hope this helps.

carole

[ollypenrice]

I don't use a DSLR but I can help with darks and flats.

Darks; cover your scope obective and take a five minute exposure. You should get a jet black image but you won't, it will be speckled with noise and amp glow, etc. This dark noise needs to be subtracted from your lights, your real images.

Flats; take a picture of an even, pure white surface and your picture should now be a perfect even white, but it won't be. The edges may be dark due to vignetting, there will be doughnut-like shadows cast by dust, etc. Again the real image needs to be corrected to allow for these artefacts. You must not disturb the imaging train between real images and flats. Don't rotate the camera, don't touch the focus.

Darks are just taken at the same exposure settings as the real images - same time, same temperature (as nearly as poss.) same ISO etc etc but with the obective totally blacked out. Take at least 9 and give them to DSS in the format offered by the software.

Flats are a bit harder. You need an even light source and for your big scope that probably means a white T shirt stretched over the top and pointing at an even sky.

Aim to expose so that the peak of the histogram (shown in your camera screen perhaps?) is about half way to maximum. Again take a good few, not less than 9 and 30 would be better. These flats need to be dark subtracted themselves or they'll add noise, so just take a good set of bias frames - ie a set of dark frames at the shortest exposure your camera can take, again with the obective blacked out.

Deep Sky Stacker or any other astro imaging programme (I generally use AstroArt out of habit) will know what to do with each set. You just put them in the appropriate boxes.

Olly

PS While I take Carole's point about short subs for the core I found I never used mine and 5 minute subs were fine in a CCD at F3.9. Apologies for saying the same things but we crossed in the post.

[superjody]

cheers guys, some good info there, il get it into practice

[dph1nm]

I wouldn't have thought you will get the whole of M31 in the field of view at that focal length, by the way.

To be honest, I never use darks with the Canon 1000D, but you will need flats for M31, as it is such a large object - they make the final image look so much better. I do always use mirror lock.

NigelM