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How Long To Expose?

ahanmal

By ahanmal
in Getting Started With Imaging

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ahanmal

ahanmal

Posted
Posted

Hi,

I am preparing to receive my new Celestron C8 OTA and CGEM mount. I was wondering how long I should expose my Canon EOS Rebel T1i for deep-sky astrophotography? Is there a table or a calculation for each sub-frame. I plan on stacking the images as I live in a moderately light polluted area. The C8 has an aperture of 203.2 mm, focal length of 2032 mm, and the f-stop is f/10.

Please advise.

Thanks

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laser_jock99

laser_jock99

Posted
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Exposures will largely depend on the brightness of the object, how long you can reliably track for, how much 'noise' your camera builds up, how light polluted your sky is plus a number of other factors.

There's no fixed answer as each situation is different. Best to do some trials and see what works best with the gear you have.

F10 is optically quite slow and 2000mm focal length quite long so it would be best to start on bright objects like M31 galaxy or M42. Start with 60, 120, 240 & 300 seconds @ ISO1000 and see what you get?

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Tim

Tim

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Firstly you will need to get the mount set up as accurately as possible. This will help you take longer pictures with less star trailing.

If you use a separate guiding system then you can take longer exposures still.

Each part of your sky will present you with a different challenge and maximum exposure. Best thing to do, when starting on a target, is take a few exposures and then see how long you can reasonably go without star trailing and light pollution wrecking the picture.

Keep the Histogram spike in the left hand third of the graph for best results.

Don't forget to have a go at the moon. Very short exposures will suffice there :)

All the best, just shout if you need more help.

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laser_jock99

laser_jock99

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The longest single sub I've attempted to date was this Cone Nebula area of 20 minutes. As you can see there was a fair amount of 'drift' between what the guide scope was looking at and what the imaging scope was looking at! Now I keep to maximum of 10 minute exposures.

DSIR6408_noels_1024.jpg

With a bright object much shorter expoures will get some detail. With the same 12" scope I got this single sub of the bright core of the M31 galaxy in just 10 seconds.

DSIR5952_noels_002_.jpg

More usually I would make a stack of say 60 second exposures or longer from a dark sky site.

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todd8137

todd8137

Posted
Posted

Hi,

I am preparing to receive my new Celestron C8 OTA and CGEM mount. I was wondering how long I should expose my Canon EOS Rebel T1i for deep-sky astrophotography? Is there a table or a calculation for each sub-frame. I plan on stacking the images as I live in a moderately light polluted area. The C8 has an aperture of 203.2 mm, focal length of 2032 mm, and the f-stop is f/10.

Please advise.

TThanksgiving mount will track unguided for a round 45 secs unguided so go for a high iso maybe 800, I have this mount get a guide scope and cam to guide with even with good polar align you will still be pushed to get 45 secs

pat

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ollypenrice

ollypenrice

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You have a very long focal length (which means you need very accurate tracking) and you have a very slow focal ratio (which means you need very long exposures.) This means you need very very accurate tracking and that means autoguiding. I'd make reading up on this a priority.

In the mean time see how long you can go while keeping round stars.

Olly

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Tim

Tim

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A good investment for you would be a Focal reducer. This will give you a shorter focal length, bigger field of view, and enable you to take considerably shorter exposures and gather the same amount of info.

http://www.firstlightoptics.com/reducersflatteners/celestron-f63-focal-reducer.html

With a few accessories, like this reducer, your new telescope will be a very capable all round imaging scope.

Sounds like you have just started out on the learning ladder. Enjoy the climb :)

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ollypenrice

ollypenrice

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A good investment for you would be a Focal reducer. This will give you a shorter focal length, bigger field of view, and enable you to take considerably shorter exposures and gather the same amount of info.

http://www.firstligh...al-reducer.html

With a few accessories, like this reducer, your new telescope will be a very capable all round imaging scope.

Sounds like you have just started out on the learning ladder. Enjoy the climb :)

Good advice. Coming down to around F6.3 and reducing the focal length changes everything.

Olly

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ahanmal

ahanmal

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Hi Everyone - Thanks for your responses, they've been really helpful. I will look into the focal reducer, but $144 seems very high. How long do you expect that I can track without an autoguider for the CGEM? I also live in a light-polluted area and short exposures will probably be best. Please give your opinions.

Thanks!

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RobH

RobH

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I will look into the focal reducer, but $144 seems very high.

That's not high at all for a focal reducer that is of decent quality. The Celestron reducer also works as a field flattener with the SCT's.

Check the price of the reducers for the Edge series of scopes......you'll fall off your chair!

It'll be money well spent. As has been said, imaging at F10 will be time consuming and tricky, and with the sort of exposure lengths you can reasonably expect without autoguiding (about 1-1.5 minutes max), you'll struggle to image anything but bright objects well.

You could go half way and get a 1.25 inch 0.5x reducer...I think Antares make one. They are quite cheap and work quite well, but they're not field flatteners, so the edge of any image will have elongated stars, but if you're shooting smaller objects, like the crab nebula for example, this won't be a problem.

Cheers

Rob

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Chris Rowland

Chris Rowland

Posted
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There's an Antares SCT focal reducer that's said to be optically Identical to the Celestron one and less expensive.

Otherwise get an 80mm scope - an ED one ideally - and fit your DSLR to that. The shorter focal length will make starting with astro imaging a lot easier.

The C8 will almost certainly need guiding and the 80mm can be used as a guide scope.

Someone else will be along soon with another way of spending your money :-)

Chris

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Tim

Tim

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The 0.63 reducers can be used at anything from f4.7 or so by altering the spacing.

In answer to your question, try for 30 seconds and see how it looks. If the stars look ok, go a bit longer. Use a high iso. Get your mount polar aligned to within an inch of its life, and try for targets closer to the pole star.

Nobody can really answer your question apart from you yourself when you start imaging, as your sky and the way you setup is unique to you.

To be honest, exposure length is probably the least of your worries right now, best to concentrate on accurate setting up and focusing, balancing etc, and then the longer exposures will be easier in the long run.

Probably 30 seconds unguided at F10, up to maybe 90 seconds unguided at f6 is a reasonable goal to work towards, and build on that.

Cheers

Tim

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tingting44

tingting44

Posted
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I've done a section on processing on my website and gone into the histogram in some detail......have a look and ask away :-)

It's the section entitled 'primers and tutorials'

Cheers

Rob

very nice read!! im just trying to get my head around stretching/curves/levels etc lol so yes a real nice ready buddy :)

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ahanmal

ahanmal

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I've done a section on processing on my website and gone into the histogram in some detail......have a look and ask away :-)

It's the section entitled 'primers and tutorials'

Cheers

Rob

Is this comparable to DPP(Digital Development Processing) in Nebulosity??

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ollypenrice

ollypenrice

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Hi All! So, the CGEM features something called All Star Polar Alignment. Looks simple: Align using 2 Stars, Run All Star Polar Alignment, move scope manually, and then re-align. I found this guide online: http://www.digitalru...nstructions.pdf

Does All Star Polar Alignment work??

Thanks!

I don't know, but it uses what you might call 'secondary sources' whereas the drift method works on primary sources of information. The drift method, in a nutshell, cannot fail.

Olly

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RobH

RobH

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I don't know, but it uses what you might call 'secondary sources' whereas the drift method works on primary sources of information. The drift method, in a nutshell, cannot fail.

Olly

I 100% agree....once you've done it a few times, it becomes second nature too.

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Chris Rowland

Chris Rowland

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All Star Polar Alignment works.

The alignment you get will affected by your initial alignment but if that's good you can be polar aligned to within 5 to 10 arc minutes in 5 minutes. Use a reticle EP.

You can do better than that with drift aligning but it will take a long time.

Chris

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