Thread: Grus - Quartet
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Old 06-10-2014, 09:59 PM
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PRejto (Peter)
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Join Date: Jan 2011
Location: Rylstone, NSW, Australia
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Quote:
Originally Posted by Shiraz View Post
hi Peter. looking pretty impressive overall, with only a bit of noise appearing to be the main remaining problem.

As I understand it, apart from spectral filtering, the two approaches that work in pollution are:
1. optimise the sub length for the conditions (if it varies, err on the longer side)
2. use some very high quality flats (also need high quality darks (and possibly bias) to calibrate them). What sort of flat procedure do you use?

Once you have done that, there is nothing but extra total exposure time - each extra mag in sky brightness requires very roughly double the total exposure time - but more total time will certainly do the job if your flats are good enough. Longer subs will not help though.

Cheers, Ray
Hi Ray,

I appreciate your comments as always.

Firstly, I have not figured out how to dither two scopes at the same time especially since I'm often taking different length exposures. But, even with the same exposure I can't quite see how to do it given the differing download times of the two cameras (the Trius is fast and the G2 slow). But, I know dithering would really help the noise so I'd like to attempt some sort of solution.

As for darks, bias. flats. Currently I'm taking 40 bias, 20 darks, and 50 flats. The flats have a mean adu of 33,900 with a min= 9,600 and a max = 52,158. Should I be looking for an mean that is lower given that max value? I've generated all of my calibration frames in CCDStack. For rejection during stacking I use the Standard Sigma Reject for luminance when I have a lot of subs, and the Poisson reject for RGB. If you have another suggestion please let me know.

So, if I've got this right the issue in LP is the decreasing (with time) of the dynamic range of the camera. And, it's the dynamic range that allows the real signal to be seen over the noise. But, I think I can safely say that 10 1 min subs will not give the depth of a single 10 min sub. Getting the balance just right between those two extremes is the tricky bit.

Many thanks,
Peter
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