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Old 04-12-2016, 11:33 AM
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Shiraz (Ray)
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Shiraz is offline
Join Date: Apr 2010
Location: ardrossan south australia
Posts: 4,810
Originally Posted by SamD View Post
Thanks Ray, the spreadsheet is nicely laid out. I've had a chance to compare it with some similar things I've done.

I think the read noise effect shown on row 48 is really useful. I’d often choose a 10% contribution, rather than 5%, so that I could use shorter sub lengths. I’d only have to increase the total number of subs a little to get the same stack SNR at the end.

Without trying to complicate things anymore, a few points that might be relevant:

1) Measuring Sky Brightness
Rather than enter the sky brightness in cell F7, I think it's more useful to derive it using the background sky level in each sub (net of bias). In fact, you can do this on your spreadsheet as it is by changing the Sky Brightness cell F7 so that the value in row 53 is equal to the background ADU count in subs (at the relevant sub length column).

2) Sky transparency - Celestial altitude of target
It might be helpful to change the Sky transparency in E10 to account for the celestial altitude of the target. Roughly:

Transparency = POWER(0.85, 1/COS(Target degrees from zenith) )

i.e. A star at the zenith has 0.85 its top of atmosphere intensity (at 525nm and at sea level), but the same star is only 0.72 its top of atmosphere intensity at an altitude of 30 degrees.

3) Stack SNR – Impact of resampling during alignment
In calculating final stack SNR, I use (roughly):

Total stack SNR = Sub SNR x SQRT(No of subs) x 1.5

The 1.5 is an extra factor to account for noise reduction during resampling (that happens when subs are aligned). I'll make a separate post to see what people's thoughts are on this. I haven't seen it mentioned elsewhere so I could be wrong - but it does seem to account for measured SNR in my stacks.

4) Mirror reflectivity – variation with colour
I measured my RC8 mirrors combined reflectivity to be 0.87 x 0.87 in green, pretty close to your typical 0.85 per mirror. There’s a significant variation with colour: 0.90 x 0.90 for blue; dropping to 0.82 x 0.82 in red; and around 0.80 x 0.80 for the Ha line). This means that with two mirrors, reflectors often only reflect 0.64 of the incident Ha, compared to refractors which generally transmit well at all wavelengths. Refractors beat reflectors in Ha !
thanks Sam.

by point:
1. I wanted a way to design a system before building something and having measured data - which is why it is that way around. Putting in measured data allows the model to be validated, but it requires that the physical system exist. However, comparing measured and calculated signal does provide a way to estimate sky brightness - its a pretty expensive SQM, but will add a comment to that effect.
2. agree in principle, but most imaging is done higher up and transparency is a guess anyway. seeing also has a similar falloff. Will add a comment pointing out that the model applies best high up
3. Great insight. the spreadsheet still provides valid SNR in the subs, but I will add a comment to the effect that what happens next (stacking and filtering etc.) can influence the final SNR and in particular that interpolation for stacking can significantly increase the SNR.
4. I deliberately kept the whole thing as a band-average model. Most of the calculations could be spectral, but my experience has been that band-average is good enough for this type of modelling. Thanks for the data on mirror reflectivity - hard to come by for real systems.

thanks for the thoughtful post - appreciated. regards Ray

Last edited by Shiraz; 04-12-2016 at 08:31 PM.
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