There are two schools of thought here: using eyepiece projection, and using
one or more Barlow lenses. Either will get the job done. In both cases, once
you get up around f/20 or f/30 (those are good focal ratios for planetary
imaging), however, you will find it is much harder to put objects on the
chip, and seeing will be your limiting factor.
If you use an eyepiece to center an object, and then insert the camera,
you'll get by a little easier. This is also true when using a flip mirror,
however if the Barlow affects both eyepiece and camera, you may be at a
disadvantage with the flip mirror as your field of view will be small for
both. I like to attach the Barlow to the camera, and simply slip the
eyepiece out and the camera in. This does require slight refocusing each
time, but the ease of putting the planet in the FOV makes up for that.
If you DO use a flip mirror, you can put a 2X diagonal ahead of the flip
mirror, and get roughly 3x instead of 2x as a result of putting more
distance between the Barlow and the camera/eyepiece. If you do not use a
flip mirror, you can get the same result by putting an extension tube
between Barlow and camera/eyepiece, or using your diagonal between them.
In my experience, Barlows are easier to work with than eyepiece projection.
If you need a LOT of magnification, you can either go with a TeleVue 4X or
5X PowerMate, or go with eyepiece projection.
After you've done this for a while, you should be able to correlate the
visible seeing with the photographic results. It takes time and effort to
set up and execute for planetary imaging, and it's really best to do
planetary imaging when the seeing is much better than average. Under most
conditions, what you get in your images will be less sharp than what you see
through the eyepiece. This is due to seeing -- even for very short
exposures, there will be some atmospheric disruption of the image. As you go
to higher f-ratios, your image gets less bright, meaning that you will have
to go to longer exposures. It's a basic tradeoff, and nothing helps the
planetary imager more than superb seeing.
Post-processing can bring out some detail, but the better your image is to
start with, the better it will wind up. They key technique for
post-processing is to combine large numbers of images by averaging, plus
some deconvolution, unsharp masking, and or simple sharpening.
I've imaged at as low as .25 arc-seconds per pixel, and Maurizio at
Excelsior optics has imaged at as low as .1 arc-seconds per pixel -- but he
has the seeing conditions in south Florida to support that high of a
resolution. For most nights, getting to f/20 will be about right, and you
can take whatever resolution your camera provides at that focal ratio. On
really good nights, you may find it useful to go to f/30, but those are
The New Astronomy Book Site - http://www.newastro.com
From: john_strachan [mailto:john_strachan@...
Sent: Monday, September 04, 2000 11:31 PM
Subject: [ccd-newastro] Increasing FL for planets
I have a TV101 and an MX5c camera. I would like to be able to be able to
increase the focal length of the system so that I can attempt to take
pictures of Saturn and Jupiter. I understand that I could use either
eyepiece projection or barlow(s) to increase FL. What would be the
reccomendation of the group. I have a decent selection of EP's and a single
x2 Meade 140 barlow. Using a flip mirror, how would I arrange the various
elements? Would the same reccomendations hold to simply increase the FL by
x2 to get the Nyquist sampling right for deep space imaging?
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