Reduced Effective Focal Ratio by binning - possible?

[cfpendock]

I would like to try to photograph some smaller objects using my de-forked CPC1100 Edge which has a focal length of 2800mm and normally operates at f10. 

I will mount it on my Temma 2 mount which is claimed to be good for 18kg.  All up weight of of the OTA, focuser, cameras and filter wheel etc is a tad over 14kg. 

I am aware that guiding and PA should be smack on for the focal length, but I am confident I can achieve this.  However, exposure times for faint small galaxies is likely to be relatively long at f10.  Using a reducer will of course reduce the effective focal length which I don’t want to do.

 

Using my Atik 4000 with the CPC1100 will give a resolution of around half a second of arc, which for UK skies is meaningless.  Therefore I will be binning.  2x2 binning will give me a resolution of around one second which is still beyond our sky.  What about 3x3?  This should still give around 1.5 seconds which is getting closer to reality.

 

I would like to know if by binning, the effective focal ratio of my CPC 1100 will be reduced.  It seems to me that if binning 2x2 provides 4 times the sensitivity, then I only need a quarter of the exposure time, whereas 3x3 would reduce it by a factor of 9.  Does this mean that the effective focal ratio is reduced by an equivalent, i.e. two stops?  In this case, the effective focal ratio would be reduced from about f10 to about f5.4 in the case of 2x2, or to slightly less than f4 at 3x3 binning.

 

Sound too good to be true - 2800mm focal length at f4!

 

Normally, “too good to be true” is exactly that.  Where exactly is my error?

 

Chris

[dph1nm]

Yes, this is exactly what binning (or using a CCD with large pixels) does (and one of the reasons why using f-ratio as  the indicator of speed is a somewhat flawed concept). You lose field of view of course - nothing is free!

NigelM

[cfpendock]

Thanks, Nigel.  I'm not quite sure that I understand why I will lose field of view, however.  The sensor size will stay the same, only the effective pixel size will change.  Just now the pixel size is 7.4 x 7.4 microns.  Binned 2x2 this becomes 14.8 x 14.8, twice as big, but only one quarter in number as they are still spread evenly over the area of the chip?????    

Chris

[Martin Meredith]

Hi Chris 

This is quite good: http://www.skyandtelescope.com/astronomy-resources/astrophotography-tips/of-pixel-size-and-focal-reducers/

I don't think you lose any FOV (well, a tiny bit on the outer row of pixels…).

cheers

Martin

[cfpendock]

This is quite good: http://www.skyandtelescope.com/astronomy-resources/astrophotography-tips/of-pixel-size-and-focal-reducers/

Thank you Martin.  That is an extremely informative article, despite being eight years out of date.  It's surprising how the technology of cameras has moved on.  But it confirms what I believed to be the case, i.e. for what I want to do, a reducer is not going to help.  Especially since some of the targets I am interested in are only a few minutes across.  Of course the detail will be grainy, but that is a consequence of not living in space.....

Chris

[dph1nm]

 I'm not quite sure that I understand why I will lose field of view, however.

Oh sorry - no, not in your case. What I meant was if you put your camera on scope which really had half the focal length you would cover twice the area of sky, so binning isn't entirely the same as having a shorter focal length scope.

NigelM