Recommended exposure change/concern

[TonyD]

Hello everyone,

I use an ASI533MC Pro and ASI2600MC Pro. I started using an Optolong L-Ultimate narrowband filter awhile ago and the smart histogram would be in the range of 400 seconds to 600 seconds. Recently, it’s been upwards of 1100-1200 seconds with the 2600MC. I even went from an F5 to an F4 scope with no change in recommended exposure time. I know there’s a lot of variables that affect exposure times, but this seems very excessive and trying to diagnose if this is an equipment issue or normal.

[admin]

Hi Tony,

the recommendation is sensitive to the measured sky brightness value, so it will vary based on the amount of light pollution in the area you are imaging. While you haven't changed the filter or the camera, you may now be pointing to targets higher in the sky (where the background can be much darker), or just have had some nights with better transparency.

Another thing to watch out for is if you have changed which colour channel SharpCap is measuring in the sky background - if you have (for instance) set the blue channel but the filter you are using does not transmit much in the blue then you will get very long suggested times.

cheers,

Robin

[TonyD]

I apologize for the delay, thank you! That might be the answer because I have always measured in the green and this is a dual narrowband filter for HA and Oiii

[TonyD]

Good afternoon,

I tried the different color channels with no luck. I understand sky conditions with affect this, but a jump from the 400-600s range up to 1100s-1200s range seems a bit excessive and seems like something is off. None of my settings have changed since this. Even running broadband filter and no filter at all results in an excessively high recommendation across all channels.

[admin]

Hi Tony,

one more thing that comes to mind is to ask if you have vignetting (darkening) at the corners of the frame? If you do then you should tick the box to avoid edges/corners when measuring, otherwise SharpCap will try to optimise for the darkened corners and over-estimate the exposure.

If its not that then we can try to estimate what your sky brightness should be from

* F-ratio of telescope
* Pixel size of camera
* QE of camera
* Bortle classification (or equivalent) of your sky
* filter in use

We can then see how that estimate compares to the measurement that SharpCap is making.

I have created an online calculator to do the sky brightness estimate, which you can find here : https://tools.sharpcap.co.uk/

cheers,

Robin

[TonyD]

Thank you! It’s showing 7.13 e/pixel/s, but it won’t allow me to enter filter information.

Bortle 6
3.76 pixel size
80% qe
Optolong L-Ultimate filter

When I ran the sensor analysis, if I understood correctly, I do not need to have the filter installed, correct?

[admin]

Hi,

the tool was designed before dual/tri band filters for colour cameras became popular - you have to choose 'Monochrome' camera mode to enter a filter, but the results will be fine if you do.

Working backwards, it looks like you must have a f/4 telescope to get the ~7 e/pix/s value for a colour camera. If I then change to monochrome and 3nm bandwidth to match your filter, the value goes down to 0.21 e/pix/s.

Working with that approximate value of 0.2 e/pix/s, for the ASI2600MC, you should end up with a suggested exposure of about 500s at gain 0 and about 100s at gain 100. The longer suggested values indicate that most likely the measured background is darker than we are estimating here.

Probably the best thing to do would be to grab a screenshot of the 'Brain' suggestion next time you try it - that will show the measured sky background and all the settings you are using, hopefully revealing something about what is going on and why.

thanks,

Robin