Exposure time for Andromeda?

[Astrosurf]

What would be the best exposure time for Andromeda?

Alexxx

[RobH]

Depends on what you are imaging with Alexx.

What's your gear?

The general rule with M31 though is to not overexpose the core.

Some folk take short core exposures, maybe 30-60s for the core, and then longer for the rest and blend them, but I've recently been shooting M31 and using 10 minute subs with a fast scope and OSC CCD and don't have a burned out core, but it needs care when stretching the image in processing.

Not finished it yet though

Cheers

Rob

[pete_l]

Here's a link to a sample image

The parameters are a 20D modified DSLR, 60 seconds at ISO 200 and F/2.8

To know how yours would compare with this, you'll need to know the F-ratio of your 'scope/lens the ISO equivalent of your imager and how long you expose for.

So if your F-ratio is (say) 8, then you'd need to expose for 8 (i.e. 8-squared divided by 2.8-squared) times longer to get a comparable shot, at the same ISO. If you image at ISO 400, then halve that time.

After that, take as many images as you have time for - until the sun comes up, the clouds come out or ou run out of disk space

[Astrosurf]

Thanks guys. My scope is an F5, 8" reflector. The ISO of my Canon EOS 40D was 800.

My tracking was off (it's rarely good even with 3-star align), so couldn't take subs longer than 30 secs as I get trailing. I'm going to post a query about auto-guiding.

I got tired too so needed to go to bed after just a few shots, but then the Moon is nearly full so wasn't the best conditions. I'll try again another time and spend more time setting up the scope properly!

[JimStan]

Is your scope mounted on an Alt-Az or an EQ mount ? If it is an Alt-Az fork style, you are going to have tracking star images no matter how accurate your polar alignment or 3 star alignment is, due to field rotation.

Field rotation happens when the scope moves parallel and at right angles to the horizon. The stars actually move on a circular path through the sky, and the difference between the motion of the stars and that of the telescope will cause trails!

Two tools can be used to eliminate this field rotation. A German Equatorial Mount ( GEM ) is the most practical way, and a field rotator that fits between your camera and the optical output port of the scope is the other. Obviously, the field rotator is the more expensive way to solve the problem!

Hope this helps.

Jim S

[Astrosurf]

Thanks Jim. Sorry I didn't mention it, daft! My mount is an HEQ5 Pro equatorial.

[ollypenrice]

Like Rob I found that the core is surprisingly hard to cook! I used 5 minute subs at F3.9 and could extract nothing useful from the short core exposures I tried. In hindsight, or for anouther time, I'd take some longer ones to get the very outer reaches, maybe, but I was quite happy with what I got. What you really need, as ever, is A LOT!

Olly

[Earl]

Im working on my 5th batch of subs tonight im doing 300s with a OSC CCD, my last tinker left me quiet pleased with the core.

So far im up to 5 hours of data, im about half way.

[Astrosurf]

Well, I think I need to autoguide!

[ollypenrice]

Well, I think I need to autoguide!

...everyone needs to autoguide!

Olly

[Astrosurf]

Simple and to the point!