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Old 03-02-2014, 12:50 PM
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Shiraz (Ray)
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Shiraz is offline
Join Date: Apr 2010
Location: ardrossan south australia
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Originally Posted by alistairsam View Post
Hi Ray,

Can you help with data foe the kaf8300 that a lot of us use.
Once calculated, you mention we should aim at exposures that give us a background with this value. But that implies a few trial subs.
Isnt it easier to take on 3 min sub and use the online calculators.

I get 1800 in 3 minute L even with an LP filter, thats how bad my LP is and the online calculators always given me figures like 90 or 120secs.
However ive seen a marked difference with 5, 10 min subs.
So if I go by your calc or the online calc values, get lots of smaller subs, will that be better in terms of getting details

Originally Posted by multiweb View Post
+1. What's the ideal time with a QHY9 mono? Not that I can guide that long but here's wishful thinking.
The calculations depend on the parameters of the specific camera, not the chip.

You can easily measure bias by taking a short exposure dark - you need only do it once. Bias is likely set by a potentiometer or similar in the camera - yours will be different to mine.

Read noise and gain can be measured reasonably easily, but if you wish to use manufacturer's typical data, you should not be too far wrong. For example, SBIG quote 9.3 e read noise and 0.37 gain for their STF8300. QSI quote 8 electrons read and 0.5 or 1.1 gain (user can set) for their 583. If you have an SBIG, use (Bias + 2300). If you have a QSI583 use (Bias + 1300) at 0.5 gain.

If you have an 8300 camera for which you cannot find/measure gain or read noise data it would probably be reasonable to assume a read noise of 9 and gain of 0.4, so use (Bias + 2000).

Alistair, the online calculators have some possible pitfalls - for example you need to remove the bias from the test sub - it is not clear how this is done in some. With your camera, you will need longer subs if you are only getting 1800 for the sky (does this include bias or is it calibrated out?). As shown above, you need about (2300 + Bias), so longer subs might be more effective, if your tracking is OK,

Marc, the QHY9 is unique in that the user can set both the gain and bias. Suggest that you use a gain of 0, set the offset to get a bias around 1000ADU, measure the actual Bias and set your sub exposures so that you get somewhere near a background sky ADU of (Bias + 2000).

The real advantage of this approach is that you need do it once for your camera. Thereafter, just adjust your sub exposure times on the fly so that you get somewhere near the desired ADU and that's all you need to do. One sub should be enough to tell you if you need to adjust. It doesn't matter what scope you are using, which filter, what the sky is like or anything else - just adjust the sub exposure times so that you get in the ballpark of the calculated ADU on the sky background. Of course, getting it a bit wrong is not the end of the world; this process (and the online calculators) aim to optimise your overall imaging efficiency and you can still get good results if you get it a bit wrong - it will take a bit longer to get there if you underexpose or you will lose a bit of dynamic range if you expose for too long.

regards ray

Last edited by Shiraz; 03-02-2014 at 04:47 PM.
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