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 Post subject: Eyepiece projection for planetary imaging
PostPosted: Sat Mar 31, 2012 8:54 am 
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Now that a number of our Members are experimenting with planetary imaging (and with Jupiter, Venus, Mars and Saturn all on view, this is the ideal time! 8-) ), it's worth mentioning this technique which uses an eyepiece (or Barlow-lens) to project a magnified image of the planet onto the webcam's sensor.
But special adaptor-tubes are required to mount the eyepiece and camera onto the back of the telescope.

Since the planets appear so small (only 10-50 arc-seconds in diameter), we need to use a very long effective focal length to get a reasonably-sized image - for example my recent images of Jupiter and Mars have used an effective focal length of around 7m! Of course my telescope is not really this long (it's an 8" SCT with a focal length of 2m, but a tube less than 1/3 of this length), so I use the projection method to obtain the extra magnification.

There's a very handy free utility from the Astronomical Society of Southern New England: http://www.assne.org/ccd.zip which calculates the image size for a combination of CCD, telescope and eyepiece.

Let's take the example of imaging Saturn using a webcam through our new 80mm ED refractor.
Now the 'scope has a focal length of 555mm, so it forms a focal-plane image of Saturn around 0.1mm diameter, which is only around 20 pixels across on our sensor. However, if we place our webcam sensor 50mm behind 10mm eyepiece, we should be able to project an image around four times larger (as long as there is sufficient travel in the focusser) - so the telescope has an effective focal length of over 2.2m.
But since we've reduced the working focal ratio from f/6.9 to f/27, the image will be 16 times (i.e. four squared) fainter - so we'll need 16x longer exposure.

That's a rather extreme case - in practice it's best to start with a long focal-length 'scope, so that less extra magnification is needed. So using our 1300mm Maksutov we'd need less than twice the magnification to get the same-sized image - and we'd then be operating at f/22 so the image would be 1.5x as bright.

But in either case, it's no gain without pain - the extra magnification also means that it's more difficult to aim the 'scope to get (and keep) the image on the tiny CCD senor, and to focus it.

My own planetary imaging camera is a (re-boxed) Philips SPC880 webcam mounted on the back of a flip-mirror, so that I can quickly switch to a (parfocal) 25mm cross-hair eyepiece for aiming.
Attachment:
File comment: Ross's planetary imaging camera
planetarycamera.jpg
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And this assembly also provides the required extension for image magnification (here using an APO Barlow-lens).


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 Post subject: Re: Eyepiece projection for planetary imaging
PostPosted: Thu Apr 19, 2012 1:19 pm 
Thanks for this Ross but can I ask what might be a stupid question. You appear to be using a 25mm eyepiece to centre the object for the camera. My question is how do you achieve parfocality if you are looking through an eyepiece and a barlow and the camera is imaging through the barlow only. Sorry if this is a bit obvious but it seems to me that you still need to adjust the focus

Regards


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 Post subject: Re: Eyepiece projection for planetary imaging
PostPosted: Thu Apr 19, 2012 2:29 pm 
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I've adjusted the relative positions of the eyepiece and the camera so that with the mirror "down" the virtual image from the eyepiece appears in focus and with the mirror "up" the focused real image is projected onto the sensor.
First I used a 5mm "T" extension-ring to move the camera back, and then I could make fine adjustments by sliding the eyepiece slightly out of its holder (if you look carefully, you'll see that it's sitting about 2mm out from its proper position) and locking it there with the grub-screw.

Now this is set, I can get the image approximately in focus in the eyepiece whilst aiming, and then do the critical fine adjustments with the Moto-Focus whilst looking at the video stream from the camera.


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 Post subject: Re: Eyepiece projection for planetary imaging
PostPosted: Thu Apr 19, 2012 3:37 pm 
Thanks Ross


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