Optimum barlow power for a 5"/127 SCT

[McMoon]

Hi,

The title says it all really. I have a Skymax 127 and an Orion SSCI IV webcam but I have not used the cam yet.....it is still boxed and wrapped until a certain date arrives and my long suffering will give it to me....but I see some imagers out there with variable or varied barlow mags. What would be optimum for my set up on planetary and what should stop me using a x3 or x 5? Currently I have a televue 2x.

Thanks

Andy

[Cornelius Varley]

Your 2x barlow would give you the nearest optimum focal length for imaging with the Orion.

[Cornelius Varley]

Having changed the batteries in my calculator and read the manual for the camera I can give you a bit more info.

Using the camera at full resolution (1280 x 960 ) the optimum focal length for 2 pixel resolution is 1626mm, which is nearly the same as the telescopes' native focal length. For 3 pixel resolution the focal length is 2464mm (f19) which would require a barlow of 1.6x. A 2x barlow would slightly oversample the image, but not by much, so you could use that. The full resolution would work best for lunar images.

At 640 x 480 resolution (2x binned mode) a 2x barlow would give you 3251mm (f25 ish) and a 3x barlow @5000mm ( f38 ). For planetary imaging I would use the 640x480 mode with a 2x barlow.

[McMoon]

Thats great...thanks

[JamesF]

Yes, I agree with Peter. f/20 is about right for unbinned images with this camera so 2x is probably best. On a really bright target (Jupiter at opposition, perhaps) I might push for a bit more if the seeing was exceptional, but I'm not sure there's much to be gained by it.

As it happens, I've just come in from a session imaging Jupiter with my new-ish ASI120MC. That has 3.75um pixels as opposed to the 3.6um (I think) of yours, but optimum focal ratio is still close enough to f/20. I used to use a 2.5x barlow and 40mm extension with my SPC900 and 127 Mak (giving about f/33) and had dropped the extension when I moved to the new camera. It's been hard work. Dropping down to the 2x tonight really felt much easier and there shouldn't be any significant loss of detail. It also meant I could push the frame rate up much higher to make the best of the seeing. The proof will be once I've processed the data of course...

If you're interested I wrote up some stuff about optimal focal ratios for planetary imaging here when I was trying to get my head around it last year:

http://www.tanstaafl.co.uk/2012/03/focal-ratio-capture-length-planetary-imaging/

James

[McMoon]

All great stuff add added to my links for reading another evening.