I want to try webcam imaging of planets. How do I figure the F ratio w/ a barlow or Televue powermate?

I have a 10'; 2500mm f10 Lx200 and will be using a Toucam. I have a 2x barlow but am thinking about getting one of the Televue powermates for additional magnification.



*How do I determine the focal ratio by using a barlow and/or Powermate.

*How does this change if I also use an extension tube?

*Any suggestions on which Powermate, ie 2x-5x?



Thanks!I want to try webcam imaging of planets. How do I figure the F ratio w/ a barlow or Televue powermate?
First, telescope's focal ratio is determined by dividing it's focal length by it's aperture, therefore your telescopes focal ratio is f/10:

2500mm / 254mm ~= 10



Using a Barlow or extension tube will multiply your telescope's focal length, so the focal ratio with a 2x Barlow is f/20:

2*2500mm / 254mm ~= 20



The same goes with a Powermate, it multiplies the telescope's focal length and a 5x Powermate gives you a focal ratio f/50. And the same formula applies to an extension tube also, it gives you more focal legth and less f-ratio.



The focal length and focal ratio determines both your magnification and how much light is projected on to your web-cam's CCD-chip, or how bright the image is. The magnification is directly multiplied by the focal length. When using a 4x Powermate you'll have 4x the focal length and 4x the magnification.



On the other hand the necessary exposure time grows as the square of focal ratio: the lower your focal ratio is the longer exposures you'll have to get - in square! So, when going from f/10 to f/40 with a 4x Powermate you'll need 4^2 = 16x the exposure time to get the same brightness.



So here's the trade-off:

Planets are bright objects in a telescope, but when using low f -ratios you'll have to use much longer exposure times. A longer exposure time means you will have more atmospheric distortions in *each* of your images. So a bigger magnification does not necessarily mean you'll have better planetary images. It depends a lot on seeing and atmospheric conditions. Generally stacking smaller but crisper images will give you better results. But only generally. It really depends.



Therefore my suggestion is to try first with the 2x barlow, and based on those results to try with a bigger magnification later. A 5x Powermate is a good piece of equipment, it gives you a lot more magnification. But keep in mind that bigger magnification doesn't necessarily mean you'll get better planetary images. On a good night it will, on a bad night you'll only get less images to stack.



I suggest you to visit Cloudynights, IMO it's the best forum for these things. Here's one good discussion on the subject: http://www.cloudynights.com/ubbthreads/s



Clear skies!I want to try webcam imaging of planets. How do I figure the F ratio w/ a barlow or Televue powermate?
By coincidence, I have the same telescope (though not an LX200, I have a more venerable LX50 10';SCT).

Your OTA (optical tube assembly) has an f/ ratio of f/10. Using a 2X Barlow lens will effectively double your f/ ratio, making it f/20. A 3X Barlow would make it f/30, and so on.

If you're planning on doing eyepiece projection astrophotography, the same formula would apply.

If doing afocal astrophotography, it gets a little trickier, because the camera's focal plane is not held rigid to the eyepiece, introducing a variable.

Check out this link:

http://www.astro.shoregalaxy.com/webcam_

This should help a good deal.



Good Luck, and Have Fun!

Clear Skies!

Bob
First of all, this is a terrible place to ask a technical question like this. 99% of the people here wouldn't know which end of a telescope to look through, let alone what a Barlow or a Powermate is. Try one of the Yahoo technical forums instead for reliable information.



The information the others have given you is accurate. I'd only add one point: there is a significant difference between how a Barlow and a Powermate operates. With a Barlow, if you add an extension tube between Barlow and telescope, you increase the magnification. Not so with a Powermate. The magnification stays the same or even goes down. Take a look at the graphs on the Tele Vue web site. My personal preference is for the Powermate, as it is a modern design and controls aberrations much better than a Barlow.