Long /short exposures

[Fieldsy]

Hi all 

I use well will use when we get a clr night a   130pds, uncooled mono cmos

if you had 30 mins of data by way of 10 x 3 min expos or 90 x 20 secs expo would the results be similar or not.? Or could you increase the amount say (180) for the same result as the longer exposure.?

prob a silly question.

thanks

[Adam J]

32 minutes ago, Fieldsy said:

Hi all 

I use well will use when we get a clr night a   130pds, uncooled mono cmos

if you had 30 mins of data by way of 10 x 3 min expos or 90 x 20 secs expo would the results be similar or not.? Or could you increase the amount say (180) for the same result as the longer exposure.?

prob a silly question.

thanks

It would depend greatly on ambient temperature. Read noise is fixed irrespective of exposure length but dark current builds as exposure length is increased. Once thermal noise exceeds the read noise then there is little point in increasing exposure further. It all also depends on read noise Vs sky brightness and the filter used. I'll see if I can dig out an old spreadsheet to help you calculate optimal exposure. 

[Demonperformer]

The formula in my signature addresses this very question. It is condensed from an article on the Sharpcap forum.

A is the clear area of the scope objective
mu is the pixel size of the camera
sigma is the read noise of the camera
T is the total time
n is the number of subs
t is the individual sub length

Edited June 23, 2019 by Demonperformer

[Demonperformer]

Oh, and Q is the quantum efficiency of the chip

[MarsG76]

Short and simple answer is no... longer sub exposures will always capture the fainter matter of a object... the 3 minutes subs will definitely have more fainter material visible at the expense of being noisier than the 20 sec subs... I personally do a variey of sub lengths in my imaging, depending on the object, I can capture sub length from 15 sec, 30, 60 120, 180, 300, 600 for RGB OSC images.... and when I do Narrowband, I'll capture 600,900,1200,1800 and 2400 sec subs..... I find that this is what I need for the variance in dynamic range.

[ollypenrice]

2 hours ago, MarsG76 said:

Short and simple answer is no... longer sub exposures will always capture the fainter matter of a object... the 3 minutes subs will definitely have more fainter material visible at the expense of being noisier than the 20 sec subs... I personally do a variey of sub lengths in my imaging, depending on the object, I can capture sub length from 15 sec, 30, 60 120, 180, 300, 600 for RGB OSC images.... and when I do Narrowband, I'll capture 600,900,1200,1800 and 2400 sec subs..... I find that this is what I need for the variance in dynamic range.

I agree with you on longer going deeper, up to the point where thermal noise gets too high. However, I'm surprised by your need for varied sub lengths. Of course this may be down to well depth but if you examine your longer subs do you actually find that they are significantly saturated in the linear state? If they are not, careful stretching can avoid introducing any new saturation.

Olly

[MarsG76]

8 minutes ago, ollypenrice said:

I agree with you on longer going deeper, up to the point where thermal noise gets too high. However, I'm surprised by your need for varied sub lengths. Of course this may be down to well depth but if you examine your longer subs do you actually find that they are significantly saturated in the linear state? If they are not, careful stretching can avoid introducing any new saturation.

Olly

Yeah Olly.. not always but most of the time, when I do, for example 1800s ISO1600 subs through a HAlpha filter, the brighter parts are blown out....