If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below.

 Making a homemade focal reducer
 Author Name Remember Me? Password
 Site Map Home Authors List Search Today's Posts Mark Forums Read Web Partners

## Making a homemade focal reducer

 Thread Tools Display Modes
#1
November 29th 05, 05:23 AM posted to sci.astro.amateur
 [email protected] external usenet poster Posts: n/a
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
November 29th 05, 09:44 AM posted to sci.astro.amateur
 Roger Hamlett external usenet poster Posts: n/a
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=(f-s)/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=(208-69)/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
November 29th 05, 04:46 PM posted to sci.astro.amateur
 [email protected] external usenet poster Posts: n/a
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=(f-s)/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=(208-69)/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

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
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.

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts vB code is On Smilies are On [IMG] code is On HTML code is Off
 Forum Jump User Control Panel Private Messages Subscriptions Who's Online Search Forums Forums Home Space Science     Space Science Misc     News     Space Shuttle     Space Station     Science     Technology     Policy     History Astronomy and Astrophysics     Astronomy Misc     Amateur Astronomy     CCD Imaging     Research     FITS     Satellites     Hubble     SETI Others     Astro Pictures     Solar     UK Astronomy     Misc About SpaceBanter     About this forum

 Similar Threads Thread Thread Starter Forum Replies Last Post Focal reducer for LX90 & DSI Shim UK Astronomy 3 November 29th 05 07:35 PM Focal Reducer for the Sun? RS Amateur Astronomy 2 June 7th 04 06:32 PM SCT Focal Reducers and Vignetting (explained) Stephen Paul Amateur Astronomy 12 January 24th 04 05:04 PM Any 0.5x focal reducer suitable for Televue Ranger Chotechai Amateur Astronomy 0 November 15th 03 05:18 PM Starsweeper Focal Reducer Chuck Amateur Astronomy 0 November 2nd 03 06:21 PM

All times are GMT +1. The time now is 02:55 PM.

 - Contact Us - SpaceBanter Home - FAQ - Links - Privacy Statement - Top