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Sensible exposures times


johnb
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Hi

I’m currently unguided so assuming I have a good polar alignment and have setup my kit carefully and accurately what sort of exposure lengths should I be looking as with my DSLR ?

Regards

John Berman

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The EQ6 worm period is 480 seconds (or 8 minutes). The typical value of periodic error for EQ6 is +- 30 arc-seconds. If you don't do PEC and we assume that most of that +-30 swing happens over half the period, you are limited to 2-3 minutes (and even then a substantial fraction of the subs will be during the big PE swing). If you can't see the swing due to your very short focal length, you can go for much longer (limited by polar alignment accuracy). Then there's flexure to worry about...

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That gives you about 0.9 arc-seconds per pixel.

for small polar alignment errors, each arc-minute of error will give you a drift rate of 3.8 arc-seconds per minute (for stars in the equator).

It's tough at that focal length.

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John, even with 30 - 40 second exposures with an unmodded dSLR is able to capture Ha frequencies with enough subs... (I used 168 mind you...). The best bet is to try it and see how far you can go on exposure time before trailing becomes unacceptable. Start with 60 seconds and see what happens.

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With my EQ6 Pro and 250px I managed up to 90 seconds on objects nearer the pole (eg M81), however I could never get over 60 seconds nearer DEC 0. In contrast.. when imaging low in a Southerly direction 30 seconds was my realistic maximum. The further your target is from the pole, the more movement in RA you're mount has to deal with.

Using PEC is an option to improve on gear errors causing tracking problems. This did proove to add a little more precious time to my exposures. Of course now I have moved over to 'the dark side' namely guiding. This has opened up a whole new world to my imaging.

Matt

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Yes, perhaps the best advice is to see how long you can go with each target, rather than aim for a specific amount for every target, there are just so many variables involved there just isnt a definitive answer.

But whichever way you go, try to get as many good subs as you can, you want at least 25 really to start to get the noise down.

Btw, guiding is easy ;) and as Matt says, really opens up the skies.

Cheers

Tim

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