focal reducers on fast scopes

[jamieren]

Are they a good idea? I've seen them around at prices that can easily be afforded (does this mean they're garbage, though?) and understand they transform a fast say f5-ish scope like mine into a f6.5 or so. This would do what, exactly? Does it improve image quality somehow (which is a bit of a surprise as one would be adding glass, right?) or does it adjust magnification slightly as I seem to remember reading somewhere?

[The Warthog]

Focal reducers are usually used in SCTs to reduce the f/10 ratio to f/6.3 or f/5. In an f/5 scope, they would turn your scope into an f/3.2 or f/2.5. I have never heard of anyone using a reducer in a Newt, but it's a big, strange world, so maybe someone has.

[winfij]

A focal (length) reducer actually makes the scope "faster", so a 0.5x reducer halves the effective focal length, transforming your f5.0 scope into a f2.5 scope.

This reduces the image scale by a factor of 2, so if you previously had a plate scale of 1 arcsecs per pixel, it would now be 2 arcsecs per pixel, i.e. a lower resolution.

It's a trade off of resolution v's sensitivity, you really need to look at the whole scope/camera setup to decide whether it's a worthwhyile thing to do.

Going to extremes also can increase the distortions over the image field, so again you have to choose the reducer to use carefully.

Generally, anything over 0.5x is likely to cause trouble, especially if you have a large imaging chip in the camera.

Also, as with everything in life, you get what you pay for.

John

[narrowbandpaul]

unless you have a high quality, plenty of infocus newtonian, you will not achieve focus.

If you have a good amount of infocus available, and you have a newt, I would suggest the ASA 2" 0.73x reducer...its probably the most you will get away with...flat field over 22mm

but it costs 600euros...

i wouldnt expect a 0.5x to work...and you get what you pay for...if you are serious, the ASA is probably the best you can get...and designed for a newt.

[Tiny]

I have read of people in the US with big newts actually cutting down tubes to move the secondary mirror focuser down the tube to make use of focal reducers in video astronomy. This on multi thousand dollar scopes!i