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FOCAL REDUCERS



The F/6.3 reducer-corrector delivers a LOT of field for the $$ vs 2" accessories

Schmidt-Cassegrain catadioptric telescopes are a very popular hardware choice in amateur astronomy, for a number of reasons. They are good for visual observing, astrophotography, and CCD imaging. They are compact and deliver a lot of aperture in relatively small, portable packages.

One thing they are not. They are not short focus instruments. Typically, their focal lengths are in the f/10 or f/11 range. Not overly long, but not ideal for wide visual or photographic fields, or for short photographic or imaging exposures. For visual observing, this means you must invest in large, expensive 2" eyepieces & 2" star diagonals in order to obtain wide field views. And for photography & imaging, it requires long exposures. ...or DOES it?

Enter the reducer/corrector. They come in several versions, and are available from Meade, Celestron, Lumicon, and others. I'm going to talk exclusively about the Celestron & Meade versions here, because these are the units I've used extensively.

What do they do?

Several things, actually. First and foremost, they reduce an f/10 SCT to f/6.3. For the CCD imager or astrophotographer this actually means two things; dramatically shorter exposure times, and a distinctly flatter field. Said differently, they add corrections to put much more of the field in critical focus at the same time than would normally be the case with a SCT without a reducer/corrector.

Simply, they take a lot of tedium out of imaging or astrophotography, and improve overall image quality at the same time, while delivering a larger field of view. At around $110 to $130, that's a lot of performance for less than the cost many single eyepieces!

For the visual observer, it may be even better, since it will allow you to see a much larger field of view with all your 1.25" eyepieces from about 32mm and down, because of the dramatic effective reduction in telescope focal length.

For example, with my 10" SCT, a 26mm Meade Super Plossl will deliver a magnification of about 98 X, and a field of about .53 degrees. With the f/6.3 focal reducer/corrector screwed onto the back of the same scope, the same eyepiece delivers 61 X with a field of .84 degrees. A 32mm Plossl, delivers a well corrected 1 degree field of view with the 10", at 50 X. Perfect for one of my favorite objects; the Double Cluster in Perseus.

CAUTION

Many folks seem to fail to do their homework before buying accessories. A common mistake is to buy one or more expensive long focal length highly corrected 2" wide field eyepieces, and then attempt to use them in a two inch diagonal behind an f/6.3 reducer/corrector, thinking they'll get a wide enough field to see BEHIND them... But all they really see is the secondary mirror shadow obscuring their field of view and then get mad at the focal reducer.

Sorry!!! For those of you looking for an ultra-wide visual system, this isn't it. You'll suffer severe vignetting if you use an eyepiece longer than about 32mm with the r/c. But the good news is that with this attachment, you can get about as wide an unvignetted field as is possible with any 2" eyepiece without having to resort to 2" eyepieces and diagonal to get it!!!

ANOTHER CAUTION:

For best results get your scope in critical collimation (I don't mean close; I mean dead on) before screwing on the reducer/corrector. My personal preference is not to collimate with the reducer/corrector installed.

A FINAL NOTE:

Since the reducer corrector also makes a really good dust seal at the back end of your scope, and delivers well corrected wide field views & shorter exposure times, why not just screw it on there and forget it except when you want to really dial the power up???

So do I recommend this accessory to the owners of SCT's? You bet I do!!! And... I haven't seen any significant differences between the Meade or Celestron units, visually, so don't be shy! Get the one you can cut the best deal on!!!

04/19/00

  • Frank Loflin, wongtheo, elrico and 3 others like this


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