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#1




Making a homemade focal reducer
I want to make my own focal reducer for my 11" SCT. Can anyone tell me
how to figure the actual focal reduction based on the f.l. of the achromat lens? There are several 48mm achromats available online with focal lengths from 208mm to 360mm (I think). Any help is appreciated. Thanks. 
#2




Making a homemade focal reducer
wrote in message oups.com... I want to make my own focal reducer for my 11" SCT. Can anyone tell me how to figure the actual focal reduction based on the f.l. of the achromat lens? There are several 48mm achromats available online with focal lengths from 208mm to 360mm (I think). Any help is appreciated. Thanks. The formula for reduction is: Rf=(fs)/f Where 'Rf', is the reduction/magnification factor (the same formula works for Barlows as well), 'f' is the focal length of the lens assembly concerned, and 's' is the seperation between the optical centre of the lens, and the focal 'plane' So if you have a 208mm lens, and space it 69mm from the focal plane, you get: Rf=(20869)/208 = 0.668* However there is a large 'caveat' with what you are talking about. The actual focal field of an SCT,is significantly curved. On your 11" unit, probably with a radius of about 12" (it depends on the focal lengths of the two mirrors). When you apply a focal reducer, this curvature is made worse, reducing the field diameter at the CCD/film etc., that can be used before the defocus produced by field curvature, becomes unacceptable. This is why the commercial SCT reducers, are 'reducer/correctors'. They normally have plano convex lenses, in at least one element, to produce a field curvature (the other way)themseles, which at least partially corrects for this problem. Many people have used lenses from binoculars or similar sources to make reducers like you describe, and for small fields (a small CCD etc.), the results can be acceptable. However one of the big advantages of the commercial reducers, is the field flattening effect, which is why the view through (for example), the Celestron/Meade F*.63 unit, combined with perhaps a 26mm eyepiece, can in some cases be more useable than a 2" 40mm eyepiece. Though the human eye accomodates quite well for the field curvature, the effect of seeing well focussed stars across the larger field, can be impressive. Best Wishes 
#3




Making a homemade focal reducer
Roger Hamlett wrote: wrote in message oups.com... I want to make my own focal reducer for my 11" SCT. Can anyone tell me how to figure the actual focal reduction based on the f.l. of the achromat lens? There are several 48mm achromats available online with focal lengths from 208mm to 360mm (I think). Any help is appreciated. Thanks. The formula for reduction is: Rf=(fs)/f Where 'Rf', is the reduction/magnification factor (the same formula works for Barlows as well), 'f' is the focal length of the lens assembly concerned, and 's' is the seperation between the optical centre of the lens, and the focal 'plane' So if you have a 208mm lens, and space it 69mm from the focal plane, you get: Rf=(20869)/208 = 0.668* However there is a large 'caveat' with what you are talking about. The actual focal field of an SCT,is significantly curved. On your 11" unit, probably with a radius of about 12" (it depends on the focal lengths of the two mirrors). When you apply a focal reducer, this curvature is made worse, reducing the field diameter at the CCD/film etc., that can be used before the defocus produced by field curvature, becomes unacceptable. This is why the commercial SCT reducers, are 'reducer/correctors'. They normally have plano convex lenses, in at least one element, to produce a field curvature (the other way)themseles, which at least partially corrects for this problem. Many people have used lenses from binoculars or similar sources to make reducers like you describe, and for small fields (a small CCD etc.), the results can be acceptable. However one of the big advantages of the commercial reducers, is the field flattening effect, which is why the view through (for example), the Celestron/Meade F*.63 unit, combined with perhaps a 26mm eyepiece, can in some cases be more useable than a 2" 40mm eyepiece. Though the human eye accomodates quite well for the field curvature, the effect of seeing well focussed stars across the larger field, can be impressive. Best Wishes Thanks for the reply and useful info. If I may, could I ask your advice, then? My plan is to use an SCT rear cell that I acquired (used to be a lumicon filter, I believe  but the filter was broken and removed) Should I then add a PC lense along with the Achromat? I am strictly a visual observer, using this setup with a Nexstar11GPS and a Denk binoviewer. I've purchased a 2" Starsweeper reducer, so I really don't need an additional one. But I decided it would be an interesting project as I already have the rear cell. I'm thinking of sourcing the lenses from anchor optical, but am open to other suggestions. 
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