FOCAL REDUCER ?
Posted 03 April 2013 - 01:42 PM
Posted 03 April 2013 - 03:50 PM
What is the brand and model of you scope?
Posted 03 April 2013 - 04:54 PM
Indeed, a focal reducer rated at 0.5X effectively reduces the scope's focal length by half, thus reducing the f/ratio by half. BUT! Only when the distance between the reducer and the eyepiece field stop is correct. Changing this distance alters the reduction factor.
A reducer is no 'magic bullet.' Often, use of a long focal length or otherwise wide field eyepiece results in disappointing results, such as edge darkening and bad off-axis aberrations and de-focus.
If used within certain constraints, it *can* be useful visually. Especially if your scope has a 1.25" focuser. For 2" focuser-equipped scopes, a reducer is almost certainly not as good as using even a so-so wide field 2" eyepiece.
Finally, be aware that a reducer requires extra inward focuser travel, perhaps on the order of between 1-2.5" (25-65mm), depending on its own intrinsic size and focal length.
Posted 03 April 2013 - 05:40 PM
To add to Glenn's comments,
MY SCOPE HAS A FOCAL LENGTH OF 2670MM . IF I WERE TO PURCHASE A FOCAL REDUCER .5 AS I SEE ADVERTISED , IS THAT THE SAME AS 1/2 THE MAGNIFICATION AT TWICE THE FIELD OF VIEW FOR VISUAL USE. !?? WILL THE VIEW BE BRIGHTER AND SHARPER? IF SO THEN, THIS IS LIKE HAVING TWO TELESCOPES FOR THE PRICE OF ONE 1335MM AND 2670MM ?? AND A WHOLE OTHER SET OF EYEPIECES ?? IS IT THAT SIMPLE??
The opening in the back of the scope illuminates a field of a certain size. You probably have a Schmidt-Cassegrain or some other form of cassegrain telescope.
A 0.5X focal reducer compresses that opening illumination into a circle with 1/2 the diameter. it does not magically provide illumination for the outer parts of the field that were formerly outside the field of view of the scope.
Therefore, if you tried to use eyepieces with field stops larger than the now-smaller illuminated field of the scope, the result will be serious loss of light in the outer field.
With a 2670mm focal length and a typical 32mm Plossl, the magnification would be 83.4X and the true field would be 0.58 degrees. With the focal reducer in place, the magnification would be 41.7X and, theoretically, the true field would be 1.16 degrees. But that presumes the telescope will illuminate the entire field when the focal length is halved and the field size of the scope is shrunken.
Chances are likely it will with that combination.
But, the odds are not good as you get larger field stops in the eyepiece (as with 2" long focal-length eyepieces.
In my 8" SCT, I found I could see serious vignetting with a field stop over 30mm when the scope was reduced by 0.63X. A 0.5X reduction is more severe.
Last, the commercial focal reducers are aimed at particular scopes. The earlier Meade and Celestron 0.63X focal reducers worked on SCTs, but NOT the new generation of ACF and Edge designs from those companies. There are focal reducers aimed at those specific scopes. Likewise, Vixen's 8" Cassegrain scopes require yet another type of focal reducer specific to those scopes. So, whereas the presence of a focal reducer does multiply your telescope and eyepiece collection in a way, you need to be very careful to use the right focal reducer for your scope and to keep the field stop size of your eyepiece within the illuminated field size of the reducer. The manufacturer of the reducer can tell you the field size illuminated, and my 2013 Eyepiece Buyer's Guide on the top of the Eyepieces Forum can give you information about the field stop sizes of your eyepieces.
Posted 03 April 2013 - 06:10 PM
Posted 03 April 2013 - 06:24 PM
Posted 03 April 2013 - 07:52 PM
Instead of chasing each other around this hypothetical May Pole, why don't you tell us what you are trying to achieve and why...
Posted 03 April 2013 - 08:13 PM
Try measuring the opening in the back of the scope. Whatever it is, you can go a couple millimaters larger than that in the field stop of an eyepiece.
Hi don, the scope is a meade 7"mak...if i used a 20 or 25mm 52degree plossi eyepiece ,do you think the f/r either at .5 or.63 would work well for visual use. With the 25mm i`m only getting .49 tfov .. going with a 30mm it will only be .58 ? . is this just trial and error ..my fields are small,thanks
Let's say it's 38mm. An f/6.3 focal reducer means the same illumination is found in a field 24mm wide. An f/.5 focal reducer means the illumination is narrowed to 19mm.
You can go a couple millimeters larger than that and still not see serious vignetting.
So the gist of it is the maximum field stop you can use is a couple/few millimeters larger than the opening in the back of the scope.
You can use the formula I mentioned to figure out what the largest possible field of view is.
I assume this is a Meade 7" f/15 Mak, and, if so, the largest possible field in this scope is with 2" eyepieces of about the field stop size of the 35 Panoptic.
That's a 38.7mm field stop, yielding a true field of 0.83 degrees.
That's it. This is not a wide field instrument.
What it is is a high-resolution, high-power instrument that is perfect for planets, Moon, double stars, globular clusters, most smaller star clusters, nebulae, etc. The star images are excellent and the scope had excellent contrast.
What it is not is a wide field instrument for large nebulae, large star clusters, and asterisms.
But I'll mention that I do nearly all my observing with eyepieces that have fields of view of 42' and smaller (that's 0.7 degrees), so your scope is definitely capable of seeing thousands of deep sky objects.
If you supplement the scope with a short f/ratio scope with a wide field, you'll be capable of viewing anything. I view with a 5" narrow-field Maksutov, a wider field f/5 reflector, and a super widefield 4" refractor.
I cover the bases.
In your case, you have a "collector's item" of great quality and the inability to achieve super wide fields of view is not really a loss. When a 7" Mak is really cooled down and the star images get good, it is the equivalent of a 7" apo refractor for $15K or more, not counting the mount. So exult in the superb instrument you have, and try where you can to maximize true field. I recommend the conversion to 2" visual back instead of the focal reducer, as it is more versatile for eyepiece purchases.
By the way, one of the finest views of Saturn i ever got was through a 7" Mak. It was as if I was in orbit. Over 700X magnification...and sharp!
Posted 04 April 2013 - 12:10 PM
Posted 05 April 2013 - 10:00 AM