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focal reducer & Barlow

Started by rodeorat, 04/08/2011 09:43PM
Posted 04/08/2011 09:43PM Opening Post
Hello all,

I'm not new to astronomy, but I am new to SCT's. Just picked up a mint C8SE and am wondering when you would want to use a focal reducer versus a barlow lense?
At a basic level, I understand that for astrophotography, you'd want a faster scope, so I presume you'd want to lower the focal ratio to f/6.3. Would it be worth it to go to a f/3.3 (Meade)?
What about reducers I see that are .5 reducers. Would I want to stick with a 6.3 or would a F/5 work as well?

Thanks in advance.
Brian
Posted 04/09/2011 01:15AM #1
Brian Schaefer said:

Hello all,

I'm not new to astronomy, but I am new to SCT's. Just picked up a mint C8SE and am wondering when you would want to use a focal reducer versus a barlow lense?
At a basic level, I understand that for astrophotography, you'd want a faster scope, so I presume you'd want to lower the focal ratio to f/6.3. Would it be worth it to go to a f/3.3 (Meade)?
What about reducers I see that are .5 reducers. Would I want to stick with a 6.3 or would a F/5 work as well?

Thanks in advance.
Brian

Brian:

Visually a focal reducer allows you to achieve a wider field of view with 1.25 inch eyepieces. The standard F/6.3 is also a corrector, it flattens the rather curved field of the SCT.

Photographically, the F/6.3 reducer/corrector was designed around the 35mm camera. A 35mm frame is actually 24mm x 34mm and the F/6.3 provides a fully illuminated 24mm circle. The image will vignette but only in the 34mm dimension. The F/5 and F/3.3 are designed for use with smaller chip cameras, some CCD cameras have a small chip, maybe only 7mm on the diagonal so a faster reducer is usable.

At the heart of the vignetting issue is the 1.5 inch (38mm)rear baffle of the C-8, the hole in the back, the rear port is only 1.5 inches in diameter, it acts as an out of focus field stop.

Visually, the action of a focal reducer can be thought of as increasing the field stop diameter of an eyepiece, an F/6.3 focal reducers increases the effective field stop diameter by the ratio 10/6.3 = 1.587. For example a 20mm field stop used with an F/6.3 focal reducer will be effectively 31.8mm, increasing the TFoV from about 0.56 degrees to about 0.90 degrees. The maximum possible field stop diameter for a 1.25 inch eyepiece is 28mm which will result in an effective field stop diameter of 44.4 mm, greater than the 38mm rear port diameter so some vignetting will occur but it is probably minor, the eye is insensitive to vignetting so it would be a nice 1.25degree TFoV, up considerably from the 0.79 degrees otherwise possible.

Visually, vignetting becomes a problem when the F/6.3 reducer corrector is used with longer focal length 2 inch eyepieces with larger field stops. A 35mm Panoptic has a field stop of 38.7mm, already slightly larger than the rear port, if you use it with a F/6.3 focal reducer, it is effectively 61.5mm and vignetting will be serious. You cannot just put a 61mm field stop up to a 38mm hole and expect to have anywhere near full illumination.

In general, the standard F/6.3 reducer/corrector is the one people use, faster ones may not be a corrector and further limit the eyepieces possible and are best suited for small chip astrophotography. Many of them are not full aperture, they are smaller than the 38mm rear port because the small chips just do not need it.

As far as using a Barlow with an SCT, it's the same as with any scope, the long focal length of the SCT means Barlows are not needs as frequently as with other designs.

I hope this helps....

Jon Isaacs