Best configuration for Planetary Imaging

[Moonhawk]

I was wondering if there was an idea configuration of optical elements/camera etc for planetary imaging.

I tend to use a 2x barlow nosepiece screwed onto the end of my webcam 1.25" adapter and then put this straight into the optical path (i.e. no diagonal).

Is this the best way (i.e. minimising optical components)?  Should I be using an eyepiece to project the image or using the barlow tube as well as the nosepiece?

[Earl]

yep id stick with that.

[Space Cowboy]

What scale of image are you getting?

[Laurie61]

What you are doing is fine, the only point I would make is a  barlow's magnification factor varies with distance from the ccd/ eyepiece so you may find you are not working at 2X but perhaps 1.5 - 2.5 X. Increasing the distance between barlow lens and camera will increase mag/scale.     

[JamesF]

It's often considered optimal for planetary imaging when the size of the pixels on the camera sensor match the smallest detail your OTA can resolve.  It's not particularly difficult maths to follow through, but a useful rule of thumb is that you want the effective focal ratio to be between five and six times the size of a camera pixel in thousandths of a millimetre.

When I was using my 127 Mak for planetary imaging with an SPC900 I'd achieve that by using a 2.5x Revelation barlow with a further 40mm extension (in fact a kit Skywatcher barlow with the lenses removed) between it and the camera.  Now I've moved on to a C9.,25 and ASI120 the much smaller pixel size means I'm working at a much lower focal ratio.

My preference is not to use any components that don't need to be there because they're only a potential source of error/trouble, so the diagonal never gets used.

Laurie's point about barlow multipliers is a good one.  Once you have an image it's worth working backwards to calculate the actual focal length/focal ratio at the time just to confirm that you've got things right, or so that you know what changes might need to be made.

James

[JamesF]

Perhaps an example would clarify that first paragraph...

The pixel size of an ASI120 is 3.75um, so the focal ratio should ideally be between 5 x 3.75 or 18.75 and 6 x 3.75 or 22.5.  By comparison the ASI174 that arrived on my desk this week has a pixel size of 5.86um, so the desirable focal ratio for that will be between 29.3 and 35.16.  Those are getting into territory that can be awkward to work in, so as the camera is often used for solar and lunar imaging I'd not be surprised if many people accept a lower resolution image in exchange for the other benefits the camera brings.

James

[Andy Milner]

James this is interesting..

I have an ASI120 3.75 that I have been using rather successfully on my 200mm 1000mm newtonian ( F5) with a 2.5x Powermate at F12.5 however when I use my 5x powermate giving me F25 I think I'm pushing the scope beyond its limit this losing detail.

I've been looking at the ASI174 as a potential replacement for my ASI120 but I'm worried, would this mean I would need to really push my scope to far at an even higher focal length? Would it be a good purchase for lunar / planets at 5x powermate ? (Slightly confused) and in need of a quick decision before zwo put their price back up later today.

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[JamesF]

James this is interesting..

I have an ASI120 3.75 that I have been using rather successfully on my 200mm 1000mm newtonian ( F5) with a 2.5x Powermate at F12.5 however when I use my 5x powermate giving me F25 I think I'm pushing the scope beyond its limit this losing detail.

I've been looking at the ASI174 as a potential replacement for my ASI120 but I'm worried, would this mean I would need to really push my scope to far at an even higher focal length? Would it be a good purchase for lunar / planets at 5x powermate ? (Slightly confused) and in need of a quick decision before zwo put their price back up later today.

It's a question I've raised elsewhere today myself.  In the absence of much comparative evidence it certainly seems to me as though the ASI174 could present quite a challenge for planetary imaging with fast reflectors.  Certainly far more than it might for someone using an SCT or Mak, say.  It may well turn out easier than I expect; I just don't know.

On the other hand, selling a used camera bought now surely won't represent much of a loss of cash once the prices have gone back up?

James

[Andy Milner]

It's a fair point about the price and resell value. I think honestly though it looks great for lunar imaging and for planetary imaging on my scope I know I'm limited so I may just go for it and see how it performs and compares.

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[Moonhawk]

Thanks for the replies - at least i'm along the right lines.  I might try experimenting with a few variations around my current setup (i.e. with and without the barlow tube) and see what difference it makes.