Optimum exposure results - can someone check these results?

[AndyM274]

Hi guys,
I’ve ran the sensor analysis earlier today & managed to get out tonight between the clouds to run the optimal exposure calculator.
I’ve ran it against 3 different types of filter:
L, RGB & 7Nm NB filters.
The camera is an Altair Astro 26M & SharpCap is saying that my optimum gain is 200 with 396 offset.
What I can’t believe is the results for the exposures:
8.5s for LRGB
147s for NB

Does this look correct?
See attached images of my R filter
Any advice gratefully received
Cheers
Andy

[oopfan]

Hi Andy,

I can't speak to the accuracy of the numbers but I can say that NB is always longer than LRGB. Why is that? The sky background level determines the exposure. For example, if you've got bad light pollution using a luminance filter, then you will saturate the frame in just a few seconds. However, with NB that same light pollution is barely seen by the sensor, therefore you can tolerate much longer exposures.

In my case with Bortle 5 suburban skies, I expose for 10 minutes whereas RGB exposures are limited to no more than 2 minutes. Actually SharpCap tells me that I can expose longer with NB, but my guiding is not good enough with my 50 year old mount.

For this reason, narrowband astrophotography is immensely popular with people having problems with light pollution.

Brian

[admin]

Hi,

the value for LRGB sounds about right - rule of thumb is

10 * read noise squared / (sky brightness rate in e/pix/s)

Your read noise is about 1.5 once you get to 200 gain and your sky background for LRGB looks to be 2.29, so that works out.

A colour (RGB) filter will pass about a 100nm band, a narrow band one about 3-5nm, so the sky background will be 20-30 times smaller, which means an exposure of 20-30 times longer will be needed, again tying in with about 150s or so.

Having a low read noise works wonders - if you had an older CCD with read noise of 7e, you would to take exposures about 20 times longer than for your 26C.

cheers,

Robin

[AndyM274]

Thanks for clarifying Robin,
Like a duffer, I used the native Altair driver which didn’t give me the option to engage ultra low noise mode.
So I re-ran the sensor analysis today after using the ASCOM driver that had that feature - or will that not make any difference due to how SharpCap averages out the values for gain/read noise?
Cheers
Andy

[admin]

Hi Andy,

The native driver should let you choose ULN mode - just set the colour space to 'RAW16 - Ultra Low Noise'.

If that doesn't show up then I will have to check with Altair, since it should.

cheers,

Robin

[AndyM274]

Hi Robin,
I downloaded the latest version of both SharpCap & the Altair drivers (incl the ASCOM driver) today - only option was the 8bit or 16bit
Hope this helps
Andy

[admin]

Hi,

Ok, let me dig a bit further - might be an SDK bug or maybe something I've done has stuffed it up

cheers,

Robin

[admin]

Ah, OK, it was my mistake. Back when I added ultra low noise support to SharpCap, it only existed in colour cameras, and I ended up writing the code to support it in such a way that the whole thing got ignored for mono cameras.

Two lines of code moved from one place to another and mono cameras can join the ultra low noise party - this change will be in next Monday's update to SharpCap 4.

cheers,

Robin

[AndyM274]

That’s amazing - thank you so much Robin