Which focal ratio for minor planet detection?

[vincentnm]

Dear All,

Now that I have a second EQ6, I am thinking of dedicating one to minor planet search. Have all the software sorted out - TheSky6 Mosaic/Orchestrator, etc. Not sure which of my scopes to use though.

In order to detect Mag 18 objects, which is the major factor - focal-ratio or high magnification ? I understand that a combination of low focal-ratio and high focal length is ideal, thus pushing up the aperture requirement. But I have a limted choice with the below scopes.

C9.25

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Long focal length at 2350mm and hence finer sky coverage per pixel. But light intensity at the detector is not great due to F10.

C8 - Hyperstar

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At 400mm focal length, a minor planet would be rather small. But at F2, better light concentrating power.

ED80

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600mm focal length and F7.5. Aperture is only 80mm. Hence not ideal for this work.

Considering all other factors the same (sky, ccd, exposure etc), which scope would be better at detecting mag 18 objects?

Thanks,

Vincent.

[Merlin66]

I'd put my money on the C8 Hyperstar option.

Big enough aperture and fast.

You might also want to consider doing some filtered magnitude estimates on minor Planets.

Also, check out the MPO stuff by Brian Warner

Minor Planet Observer / Palmer Divide Observatory - MPO Software

[dph1nm]

In order to detect Mag 18 objects, which is the major factor - focal-ratio or high magnification ?

Neither really - aperture is what matters. You also you want to make sure your pixel scale neither over- or under-samples the seeing, as both will reduced your limiting magnitude.

NigelM

[FraserClarke]

Normally one would say focal-ratio; but in this case you need to keep an eye on plate scale (which is equivalent to magnification) so that you can measure positions (hence orbits) accurately enough.

What camera will you be using? The pixel size will affect your choice of scope.

The C9.25 with 2350mm fl will give you a plate scale of 88"/mm. So for a 10micron pixel, that's 0.9"/pixel -- a pretty good match to UK seeing (2-3").

At 400mm, you'd need a camera with 2micron pixels to get the same sampling.

Are you planning to follow-up known/recently discovered objects (a very worthy effort) or make a blind survey for new objects?? If the later, you'll also want to consider field of view to increase your survey speed.

[vincentnm]

Thanks for your suggestions guys. Expect more posts when I start getting some results.