SharpCap Forums

Hi have watch the video and read some responses on this forum. still confused.
I have f6.4 10” ritchey chretien a zwo asi071 and asi2600mc. Want to use 3nm filters. In bortle 7am I right in thinking 3000sec is needed for sub length and will the high gain mode on the asi 2600 make a difference. Using 071?at 90 gain and 2600 at100.

John

Hi,

you do need to have longer exposures for narrowband filters, since they cut out most of the light (3nm filter cuts out 99% of incoming visible light), but I think in your circumstances 3000s is excessive.

Putting figures for your setup into https://tools.sharpcap.co.uk/, I get light pollution rates of 0.12 e/pix/second for the ASI071 and 0.08 e/pix/second for the ASI2600 (smaller pixels on the 2600). The rule of thumb is then 10 times read noise squared divided by light pollution rate, so

ASI2600 - gain 100 - read noise ~1.4e, exposure needed ~245s
ASI071 - gain 90 - read noise ~2.7, exposure needed ~600s

Your relatively high light pollution helps keep the required exposures down - if you moved to a darker site you would need longer exposures.

cheers,

Robin

Hi have updated info based on clearskies bortle and zwo asi2600mm

Your Sky Brightness
Sky Magnitude
19.80
magnitude per arcsec2
or Bortle Number
5.0
(Suburban sky)
or Naked Eye Limiting Magnitude
5.8
Your Telescope
F Ratio
6.4
Your Camera
Pixel Size
3.76
microns
Quantum Efficiency
90
%
Your Filter
Selected Filter
Custom
×
Bandwidth
3.5
nm
The Result
Sky Electron Rate
0.04 e/pixel/s
does this mean 1.4 x 10=14 14x14= 196/.04 =2600 ?

Hi,

no, it's 1.4 * 1.4 * 10 / 0.04 = 490.

Fortunately the factor of 10 only needs to be multiplied in once

Still a long exposure - longer than the ones I calculated since you have used a darker bortle number and obtained a lower light pollution rate.

cheers,

Robin

Thank you for you help sure I will need some more.
John

I can attest that you will get great results with that exposure since the parameters are very similar to mine. However, I'll add that you will need more integration time for SII and OIII since they are generally much weaker than Ha, that is if you are using a mono camera. (I'm not familiar with using a color camera and narrowband filters, just mono.)

Brian