Quote:
Originally Posted by LewisM
My testing with my SXVR-M25C and an NEQ6 have shown, as you have, that subs below 5 minutes are optimal - my optimal figure was around the 3.5 minute mark in nil wind - to avoid slight elongation.
The image below of M83 I did was similar to yours in many respects. You can notice very slight - yet to me unacceptable - star elongation as these are 5 minute frames.
Having switched to a Vixen GPD2, I will have to retest to achieve optimal, but with a PEC of apparently sub-15, I am anticipating at least 4 minutes per frame (fingers crossed). Also using a 50mm guidescope.
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Thanks for that info Lewis.
The “optimum” sub exposure that I use is based on maximising the signal to noise ratio – it is not dependent on the tracking. But of course there is no point in using long subs if the tracking goes to pieces.
The basis for the procedure is summarised very well on the Starizona website, but the underlying idea is that you can minimise the read noise contribution to the total noise by making subs as long as possible (to minimise the number of read events). The accepted rule of thumb is that subs are “long enough” when the read noise contribution is below ~ 5% of the total noise.
I ran your 102 system through my spreadsheet – purely on the basis of signal to noise ratio, you will do best with subs of about 20 minutes for the native focal length and a bit under 10 minutes with the focal reducer. You can use shorter subs if you increase the total imaging time. I have a QHY8 which has the same chip as your camera and with my f4 scope it works best with at least 5 minute subs - this is based on practice as well as theory.
Hope your new mount works out well - specs are good.
regards Ray
ref:
http://starizona.com/acb/ccd/advtheoryexp.aspx