SharpCap Forums

if it's 14 bit then everything will be a multiple of 4 and so on.

absolutely! that's the detail I missed! that's obvious! Thanks!

Just one note. Some dslr doesn't scale (multiply) values. I don't know if some dedicated camera behaves the same. How could you sort it out?

Hi,

SharpCap normally deals with the image scaling if necessary - for instance it can usually find out the max pixel value from the camera SDK or ASCOM driver and then bitshifts the data to use the full 16 bit range. This is actually done mainly to avoid an enormous number of people asking 'why is my image so dark'

cheers,

Robin

great Robin, thanks a lot for the replies! Completely clear

Have a nice day.

Miguel

Hi,
I have a question about optimal exposure length. I have SharpCap pro. I have done the sensor analysis and have watched the YouTube lecture about optimal exposure length, gain, etc. I understand that with darker skies the calculated exposure times can be quite lengthy. I ran the exposure calculator plugin in NINA and linked to the SharpCap sensor analysis results. I'm in Bortle 2 (maybe Bortle 1.5) skies so I'm lucky. So the results that I obtain for my system after measuring sky background are:
Lum 841 s
R 2,103 s
G 2,403 s
B 3,364 s
NB 3 nm
Ha 16,823 s
OIII 16,823 s
SII infinity.
What would be some practical subexposure lengths for the various filters. I have been using 300 s or 600 sec subexposures. No issues with tracking or guiding. Plenty of satellites these days with Starlink though.

Sincerely,
Allen

Hi Allen,

Just curious, what scope do you have? From the numbers shown you could really benefit from a faster scope given your dark skies.

Brian

Hi Allen,

wow - nice dark skies! Given those figures the only thing to do is to take the longest exposures that your mount will reliably cope with. What model camera are you using? If it has high read noise then there may be a benefit to looking at models that have lower read noise (ie going from a CCD with read noise 8e to a CMOS camera with read noise 1.5e would reduce the calculated exposure times by a factor of ~28)

cheers,

Robin

I'm using QHY600 mono with chroma LRGB and 3nm NB filters.
F 7.2
No problems with mount or guiding.

Hi,

I just had a look at the way the read noise varies for that model (look at the readout noise graph on https://www.qhyccd.com/astronomical-camera-qhy600/)

If you run in either of the 'extended fullwell' modes then the read noise is up in the 5.5e to 7.5e range and doesn't drop much with increased gain. However in the 'High Gain' mode, gain 56 gets a read noise of 1.6e, which may give you significantly reduced recommended exposure times and be more appropriate for the narrowband filters.

cheers,

Robin

Robin,

Full Well at Gain 56:
21.6ke- using High Gain Mode
40.0ke- using Extended Full Well Mode

Read Noise at Gain 56:
1.7e- using High Gain Mode
7.1e- using Extended Full Well Mode

I absolutely agree! That's a huge improvement. I'd even suggest using High Gain Mode with all filters, not just NB.

EDIT: Do we know what mode Allen used for his exposure analysis?

Brian

Hello,
A question on the magnitude of light pollution.
The formula for determining the optimum exposure includes the amount of light pollution. If I understand correctly, this value can be obtained with bias frame and light frame. To do this, take the median light and subtract the median bias, multiply the result by the GAIN of the camera, divide by the exposure time and multiply by the quantum efficiency of the camera. You get the result in e-/pixel/s.

My camera is Player One Poseidon-C Pro. Scope F ratio=10
Gain 130 is ~0.25 e-/ADU and ReadNoise ~1.35e-
Temp -15C
Offset 3
Exposure light 100s
Qe ~ 80%
Binning 2 - pixel size=3.76*2=7.52 mkm
At these parameters median Bias ~ 24, median Light ~ 204
From these data we get the value of light pollution: 0.8*(204-24)*0.25/100= 0.36 e-/px/s.
I was shooting in the suburbs. SQM estimates are 19-20, i.e. Bortle ~5
I was shooting at zenith with the Moon at 0.65 phase.
If you input this data into the SharpCap online calculator, you get a sky background value several times higher about 1.48 e-/px/s for Bortle 4.8.
Where am I wrong?

Alex