| serambin |
| (sage) |
| 02/09/08 10:47 PM |
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Tell me about Focal Reducers / Field Flatteners
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Please tell me about using a FR /FF on a C-8 for CCD imaging. How does it work and what impact does it have on focusing, point of focus ( will I have trouble with focal length using a CCD ), can it be used for visual imaging and will it help, and finally, where does it go?
Thanks,
Stan
| Patrick |
| (Postmaster) |
| 02/10/08 09:19 AM |
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Re: Tell me about Focal Reducers / Field Flattener
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Hi Stan,
The focal reducer does just what the name implies...it reduces the focal length of the scope. Since focal ratio is focal length divided by aperture, the focal ratio is also reduced. A 'faster' focal ratio allows more light to populate a CCD chip faster than a slower f/ratio. It also means that the FOV will be wider.
As far as focusing goes, in practice it will essentially be the same. For visual use the focal reducer will give a wider field of view and flatter field...that's about it. The focal reducer is screwed on the rear of the OTA in front of the diagonal on the the SCT threads.
Patrick
| edl |
| (super member) |
| 02/10/08 05:23 PM |
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Re: Tell me about Focal Reducers / Field Flattener
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On my CPC925, more in-focus is required. I cannot use the FR with my binoviewers, for example. I also find that there is some CA at high magnifications. Of course it's primary visual purpose is not for high power viewing.
Best,
Ed L.
| serambin |
| (sage) |
| 02/10/08 05:29 PM |
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Re: Tell me about Focal Reducers / Field Flattener
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Great help, this was what I needed. CN comes through again!
Stan
| Joe Ogiba |
| (Post Laureate) |
| 02/11/08 09:41 AM |
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Re: Tell me about Focal Reducers / Field Flattener
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Quote:
On my CPC925, more in-focus is required. I cannot use the FR with my binoviewers, for example. I also find that there is some CA at high magnifications. Of course it's primary visual purpose is not for high power viewing.
Best,
Ed L.
Using my Denk II binoviewer with dual arm Power X Switch in focal reducer StarSweeper mode is no problem in my C9.25.
| hudson_yak |
| (Pooh-Bah) |
| 02/11/08 09:53 AM |
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Re: Tell me about Focal Reducers / Field Flattener
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Presumably you're talking about the Denk that has the powerswitch slider downstream of the diagonal mirror. That's what allows that to work, since it keeps the spacing between reducer and eyepieces under control.
Mike
| Craig Smith |
| (sage) |
| 02/11/08 12:03 PM |
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Re: Tell me about Focal Reducers / Field Flattener
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Not to hijack this topic, but I've always wondered, how does a focal reducer flatten the field? I would think that faster scopes would have more field curvature, not less.
| Eddgie |
| (Carpal Tunnel) |
| 02/11/08 01:18 PM |
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Re: Tell me about Focal Reducers / Field Flattener
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The speed of the system doesn't really have any affect on the field curvature. This is mostly influenced by the design of the optic.
SCTs have probably the most field curvature of any commercial telescope made. Field curvature simply means that when the center of the field is in focus on the film plane, the outside of the field is out of focus. So the field is "Curved". If you used an inward curving film (like a shallow bowl), this would allow you to have stars sharp at the center and at the edge.
The Focal reducer/field flattner simply adds more strength to the optics as you move further from the center to the edge, so that stars near the outside are focused further back than those at the center.
Most other telescopes suffer from other types of abberations (Coma for mirrors, and you can get coma correctors to compensate) and Astigmatism (fast refractors, and usually the manufacturer will sell a special telecompressor lens designed for a specific telescope design) that closely matches the amount of astigmatism introduced off axis, but places at 90 degrees, so that that they cancel out.
So, fast Newtonian telescope don't have enough field curvature to be noticed over the primary abberations, which are off-axis Coma (Dominant abberation in that kind of system) and Astigmatism.
For refractors, often the field is almost perfectly flat. A telecompressor that is not designed for the system (SCT reducer used in a refractor) will actually CURVE the field. But usually, because these telescopes are smaller and faster, the blur circle remains so small as to be not visible at the detector. Some people report using SCT focal reducers on their small refractors with some success.
Regards.
| hudson_yak |
| (Pooh-Bah) |
| 02/11/08 01:18 PM |
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Re: Tell me about Focal Reducers / Field Flattener
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A simple focal reducer wouldn't, however, the Celestron/Meade R/C is not a simple reducer, it has additional lens elements designed to flatten the field of SCTs.
Mike
| Craig Smith |
| (sage) |
| 02/11/08 04:19 PM |
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Re: Tell me about Focal Reducers / Field Flattener
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Thanks!
| hudson_yak |
| (Pooh-Bah) |
| 02/11/08 04:27 PM |
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Re: Tell me about Focal Reducers / Field Flattener
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I should add that, even though the Denk reducer in their PowerSwitch and SCT StarSweeper products is a simple reducer, that shouldn't scare you from using those instead of the Celestron/Meade R/C piece, if for purely visual use. Their convenience swapping in and out likely outweighs any R/C flattening advantages. Does for me anyway.
Mike
| UrthWorm |
| (member) |
| 02/13/08 06:53 AM |
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Re: Tell me about Focal Reducers / Field Flattener
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Quote:
A simple focal reducer wouldn't, however, the Celestron/Meade R/C is not a simple reducer, it has additional lens elements designed to flatten the field of SCTs.
Mike
I have tried using the Meade Variable focal reducer with my Intes 703 (a Mak-Cass) with unacceptable results for imaging.
With the reducer installed, I get vignetting, which in and of itself is correctable with Flats, I suppose.
What is intolerable, however, is elongated stars all around the edges, with the degree of elongation becoming greater the further from the center of the image. The direction of star elongation radiates from the center of the image, following the same orientation as would imaginary rays of light coming from a central point in the image.'
I also noticed that outside of focus, stars at the edges of the image are not completely donut-shaped, but rather crescent-shaped, the missing half of the donut being towards the outside of the image, away from center.
Is this coma? Is it due to the fact that the Meade focal reducer is also a field flattener, intended to flatten the field of a parabolic mirror, and in so doing, distorts the field of my spherical-optics Mak?
| hudson_yak |
| (Pooh-Bah) |
| 02/13/08 10:14 AM |
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Re: Tell me about Focal Reducers / Field Flattener
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The crescents are normal, happens when used in an SCT too. That's what the vignetting looks like when out of focus, it just shows clearly that less than the entire aperture's rays are getting all the way through.
SCTs have spherical mirrors too, but still, seems like any device designed to compensate for their characteristics isn't likely to work well in a different type of scope.
Mike
| UrthWorm |
| (member) |
| 02/13/08 10:54 AM |
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Re: Tell me about Focal Reducers / Field Flattener
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Mike,
Thanks for the correction about mirrors . . . I suppose I was referring to the curvature properties resulting from the employment of Schmidt corrector vs. meniscus.
Thanks also for the clarification on the crescents. So that is what vignetting looks like in a defocused state . . .
Even at best focus, I'm still getting elongation of the stars, so apparently, the Meade R/C is not a good match for my system . . . darn. I guess I'll just have to keep looking. I wish it wasn't such a guessing game, but with a long focuser, filter wheel and uncommon telescope, I guess that's what you have to contend with.
Thanks for your help.