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Equipment Discussions >> ATM, Optics and DIY Forum

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sivers
journeyman


Reged: 12/07/12

Loc: Tomsk, Siberia
Focal reducer 0.33 new
      #5561821 - 12/09/12 02:10 AM

Hi all, just a question. Excuse my bad English. I'm bad at school.
Do not have extensive knowledge in optics. The essence of the issue:
I have mirror 1200/200 F6. The camera in an spider vanes. I want to reduce focus to get to F2. The options are:
1. Use the standard 1.25 focal reducer, but with it is a slight vignetting and apparently there will be aberrations inherent achromat with F3?
2. May be may apply ED doublet from what-ever monocular or binocular. What the?
3. A quality CCTV camera lens as an analogue(focus far after infinity)? None of them do not know.
4. CCTV lens and eyepiece to each other with a eyepiece focal three times greater than that of the CCTV lens?

4.8mm to 3.6mm sensor. Pixel 6.25 by 6.5 microns. The mirror can be taken as a perfect parabola. I want to get aberrations of the entire system less than a pixel in 70% of the field.
Please help.
Serg.


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ccs_hello
Postmaster
*****

Reged: 07/03/04

Re: Focal reducer 0.33 new [Re: sivers]
      #5561929 - 12/09/12 07:01 AM

Serg,

Welcome to Cloudy Nights!

1) re: camera in an spider vanes
pick a camera small enough to not obstruct incoming light too much
2) for F.R., use a 7x50 binocular's object lens (now you have two). See this DIY CN post.

Good luck tinkering!

Clear Skies!

ccs_hello


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sivers
journeyman


Reged: 12/07/12

Loc: Tomsk, Siberia
Re: Focal reducer 0.33 new [Re: ccs_hello]
      #5562087 - 12/09/12 09:50 AM

Thank you. Nice job. But.
Simple calculations for my telescope show that the reduction of 0.33, so that there was no vignetting even at the center, I need the lens F / 3. Besides doublet will work full aperture, so there will be spherical and chromatic aberration.


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ccs_hello
Postmaster
*****

Reged: 07/03/04

Re: Focal reducer 0.33 new [Re: sivers]
      #5562535 - 12/09/12 02:14 PM

Most commercial F.R. use acromat doublet.
Meade's (now discontinued) 0.33x SCT thread FR (for SCT) use 3 elements. Meade SCT thread 0.63x uses 4-elements and is mainly designed for SCT for field flattening.

F.R. is fairly close to the focal plane thus introduced CA is limited.

Regarding optical aberration, my take is FR is already cheating on OTA's native capability. If you don't like a commercial F.R. and want to have a "better" one, better design it from ground-up to fit your own OTA's characteristics. If not so picky, many are perfectly happy with doublet based commercial design anyway.

Clear Skies!

ccs_hello


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GlennLeDrew
Postmaster
*****

Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Focal reducer 0.33 new [Re: ccs_hello]
      #5562889 - 12/09/12 06:13 PM

Rock Mallin (of Mallincam fame) offers a 2" format doublet reducer which is something like f/2.7, with a clear aperture of about 38mm.

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sivers
journeyman


Reged: 12/07/12

Loc: Tomsk, Siberia
Re: Focal reducer 0.33 [Re: sivers]
      #5563816 - 12/10/12 09:33 AM

Quote:


4. CCTV lens and eyepiece to each other with a eyepiece focal three times greater than that of the CCTV lens.



Why not?
Qualitative 8mm F1.2 CCTV lens + FMC Super Plossl eyepieces 25mm? As in digiscoping?
Serg.

Edited by sivers (12/10/12 09:36 AM)


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ccs_hello
Postmaster
*****

Reged: 07/03/04

Re: Focal reducer 0.33 new [Re: sivers]
      #5564818 - 12/10/12 08:15 PM

Serg,

Afocal is not really optimal but is the only choice if the camera's lens is not removable. It has many lens elements (light attenuation and optical quality degradation), not easy to have a good mechanical coupling, and the exit pupil from EP and the entrance pupil from camera lens often can mismatch a lot.

Let's use a Point-n-Shoot zoom digicam as the example...

1) When in its max. tele setting and its native f.l. is greater than EP's, it's magnification.
2) Now gradually changing camera's zoom from tele to wide direction, at one point, there is no magnification (i.e., 1x). At this point, the EP's fl = digicam's fl.
3) Further moving toward digicam's wide side, the image scale reduces and appears brighter, i.e., focal length reducing. This accomplishes the same effect as a F.R.

Sounds magical. But is it really that good?
The answer is no. In many EP and digicam combinations, one found out that only in extreme tele zoom setting, vignetting would go away, while in the extreme wide mode, severe vignetting occurs (as if it's shooting through a straw.)

Clear Skies!

ccs_hello


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