Calculating Exposure Times

[SAW]

Hi,

I've been watching Dr Robin Glovers YT vid on calculating exposure times and trying to work out what mine should be with my 183MM with Optolong LRGB and Ha, Siii & Oii filters but not sure I'm doing the equations right 😆

https://tools.sharpcap.co.uk

My bortle area is 4, F Ratio is 4.8, Pixel Size 2.40, Quantum Efficiency 84%.

I'm not sure what value I need to put in for the LRGB filters bandwidth ? For the narrowband do I use 7nm for the equation or do I use 656.3nm ?

Using either of those numbers I get,

10x1.6 squared/0.02 = 1280 pretty sure that's not my minimum exposure time !

10x1.6 squared/2.01 = 12.7s seems too short ?

What am I doing wrong ?

 

[The Lazy Astronomer]

17 hours ago, SAW said:

What am I doing wrong ?

Nothing! You are applying the equation correctly to come to 1280s. The reason it's so high is due to your tiny pixels and relatively dark sky. There are a few things you can do to lower the time:

1. Accept a slightly increased level of noise and use a lower C factor.

2. [Removed as not correct]

3. Consider moving to London for higher light pollution levels 😁

If you can't (or don't want to) expose this long, then don't worry about it too much - experimentation is very much encouraged in this hobby, and you may find that you're happy with the images you produce using exposure times lower than what is suggested as your minimum.

17 hours ago, SAW said:

I'm not sure what value I need to put in for the LRGB filters bandwidth ?

For L, select 'none' from the filter drop down list, and for RGB, select 'red/green/blue' (or divide the L electron rate by 3, whatever's easiest).

And, on the bright side, the long exposures will mean less hard drive space used up 😁

Edited March 8, 2022 by The Lazy Astronomer

[symmetal]

The bandwidth is the difference between the highest and lowest wavelengths that the filter passes. 

From the chart read the values at around 50% transmittance. For example, the red filter bandwidth looks to be around 685nm - 580nm = 105nm.

For narrowband, the bandwidth is stated in its description, so a 7nm Ha filter has a bandwidth of 7nm. In reality, optimum narrowband exposure durations are as long as you can get away with before you run the risk of mount tracking errors, or aircraft etc, spoiling the image.  I usually use 10mins exposure for NB, even though the calculated optimum values are over an hour for my Bortle 3 skies. 😉

If you have heavy light pollution, then the calculated NB exposures may be more useful values to use.

Alan

[vlaiv]

12 hours ago, The Lazy Astronomer said:

2. Bin your pixels. A 2x2 bin would lower the time by a factor of approx. 3.6.

That only works for CCD sensors where binning happens prior to read noise.

With 183mm - it will make no difference and time needs to be calculated for single pixel even if you bin (binning effectively adds read noise from all binned pixels so camera has higher read noise per binned pixel and LP goes up - but so does read noise - relevant ratio remains the same)

 

[The Lazy Astronomer]

1 hour ago, vlaiv said:

That only works for CCD sensors where binning happens prior to read noise.

With 183mm - it will make no difference and time needs to be calculated for single pixel even if you bin (binning effectively adds read noise from all binned pixels so camera has higher read noise per binned pixel and LP goes up - but so does read noise - relevant ratio remains the same)

 

Oh, I'd been lead to believe that for CMOS, the read noise was not just added together, but added and then divided by sqrt of the number of pixels binned, so read noise doubles for a 2x2 bin. Is this not right? 

Edit: I've just realised my mistake - I didn't use the increased read noise figure in the calculation 😬

Edited March 8, 2022 by The Lazy Astronomer
Stupid, stupid, stupid!!

[vlaiv]

15 minutes ago, The Lazy Astronomer said:

Oh, I'd been lead to believe that for CMOS, the read noise was not just added together, but added and then divided by sqrt of the number of pixels binned, so read noise doubles for a 2x2 bin. Is this not right? 

Quite right - it increases like that due to way noise adds, and it is precisely what ensures that ratio of LP noise to read noise remains the same when binning.

LP noise is square root of LP signal

LP signal is x4 when you bin 2x2 (add 4 pixels together), hence LP noise goes up by x2 - square root of 4.

Read noise goes up by x2 when you bin 2x2.

Ratio of LP noise and Read noise remains the same as both go up by same factor.