upper barlow limit

Thanks John, Appreciate the information.

Clear skies
Mike

ok. that makes sense. so how do i work out the best suited powermate for my uses. i know magnification is scope focal length /focallength of the eyepiece. As im not using an eyepiece, but just a barlow and a webcam, how do i determine what magnification im getting :

1. from the scope on its own, assuming i was sticking the camera directly onto the back of the scope with no barlow?

2. from the scope including the barlow?

If its purely imaging you are doing, you don't need to worry about magnification as much as image scale.

Downoad Ron Wodaski's calulator and plug in all your equipment parameters. It will show you graphically what your images will look like.

www.newastro.com/newastro/book_new/camera_app.asp

whts the difference between a 4x barlow, and a 4x powermate? i have had a look online and can only see powermate's by televue. is it just a "fancy barlow" from televue that delivers great results ( at astronomical prices!)?

Televue PowerMates are indeed Barlow lenses and will work with any eyepiece from any manufacturer perfectly well. However, when you use a PowerMate in conjunction with almose any other TeleVue eyepiece you will get a better result! The reason is that Al Nagler optimised the PowerMate lenses specifically for use with his own range of eyepieces. Used together the PowerMate minimises residual aberrations and controls vignetting (light loss towards the edge of the field of view) better than with a standard Barlow. This is especially important with the super-wide field of view eyepieces like Panoptic, Nagler and his latest 100deg FOV Ethos monster which makes my insides curdle when I see the price!! :shock:

You don't need to worry about magnification if you are imaging at prime focus using just a Barlow. The Barlow will simply double the effective focal length of your scope. So your 8" f/10 (effective focal length=80") becomes an 8" f/20 with an effective focal length of 160". This will simply change the image scale as seen by your CCD or digital camera. It will effectively halve the amount of sky you can image per shot by doubling the image scale. A good book to check out is Michael Covington's "Astrophotography for the Amateur" which gives you all the formulae you need to work out image scale and just about everything you could possibly want to know!

Phil.

To add onto what Phil said, the other big difference between a barlow and a powermate is that a barlows multiplication is very dependent on how far the eyepiece is away form the barlow lens.
If you have say a 3x barlow and then put an extension tube between it and the eyepiece, the barlow is no longer 3x, it could be 3.5x or even 4x or 5x depending on how far the eyepiece is away form the barlow lens.
Nearly all of the powermartes have fixed powers, the powermate-eyepiece distance doesn't matter so much.

Thanks guys for the info,
As an update I have purchased a x5 PowerMate with an adapter that fits the DFK FireWire camera, just waiting for a clear night to target the Moon. A rough image scale works out ~0.15 arcsec per pixel with a FOV of ~2.6 x 3.5 arcmin using the above combo with an 8” SCT F10 fitted with a f6.3 FR.

Had some initial problems trying to get everything parfocal using the Meade #647 flip mirror system, I can get the camera focused OK but the reticule eyepiece I use for aligning is out to its last upper position on the flip mirror, it works but could be better. I haven’t even tried it with the PowerMate yet, I think the focal reducer wasn’t designed to be used with the #647 flip mirror and my reservations lie with the FR in causing this problem. Anyone else have similar issues with flip mirrors or recommendations?

Clear skies
Mike