PiotrM
Carpal Tunnel
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Loc: Poland
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Reducing secondary mirror in an RC?
#5323779 - 07/18/12 03:32 PM
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GSO RC has around 50% of central obstruction. The secondary is big and the scopes gives a very big imaging circle... but what if I want small circle in favor to low obstruction. Is it doable to use smaller secondary in such RC?
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fetoma
Carpal Tunnel
Reged: 09/26/06
Loc: Southern New Jersey
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Re: Reducing secondary mirror in an RC?
[Re: PiotrM]
#5323830 - 07/18/12 04:24 PM
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I'm no expert, but I doubt it. The secondary is designed to match the mirror, so to speak. A reflector could get away with it much easier.
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idealistic
scholastic sledgehammer
Reged: 12/31/10
Loc: massachusetts
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Re: Reducing secondary mirror in an RC?
[Re: fetoma]
#5323847 - 07/18/12 04:38 PM
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A small hyperbolic mirror would be hard to find. Could you shave down the original? Then wouldnt you have to lengthen the tube to take advantage of the full aperture? Where would that leave the focal point? I might be totally wrong. Sounds like a difficult but interesting project.
Edited by idealistic (07/18/12 04:40 PM)
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laconicsax
super member
Reged: 10/05/10
Loc: Portland, OR
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Re: Reducing secondary mirror in an RC?
[Re: fetoma]
#5323848 - 07/18/12 04:39 PM
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RCs are reflectors...
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idealistic
scholastic sledgehammer
Reged: 12/31/10
Loc: massachusetts
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Re: Reducing secondary mirror in an RC?
[Re: laconicsax]
#5323850 - 07/18/12 04:40 PM
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I assume he meant Newtonian.
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PiotrM
Carpal Tunnel
Reged: 01/03/10
Loc: Poland
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Re: Reducing secondary mirror in an RC?
[Re: idealistic]
#5323860 - 07/18/12 04:50 PM
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I asked GSO but now they are rather on vacations.
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Eddgie
Postmaster
Reged: 02/01/06
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Re: Reducing secondary mirror in an RC?
[Re: PiotrM]
#5323874 - 07/18/12 04:58 PM
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The obstruction in an astrograph is usually made large simply to provide a much larger fully illuminated field.
Most cassegrain telescopes start with a field that is only fully illuminated at the very center. In many cases, the outside of the field is allowed to fall in illumination down to about 75% for visual use, because the eye really can't detect the loss of illumination easily.
The camera can though. This is why the corners of many images taken with cassegrain telescopes is darker than the center.
The larger secondary mirror in your scope is there to provide a more fully illumiated field.
The problem with reducing the size of the secondary though is that once again, if you make it smaller, you may start to see some illumination falloff.
If you only make it a little smaller (maybe 40%) it is possible that for visual use, you may not even see the difference.
But if you make it too small, most likely the illumination falloff will start to become severe. For example, if you tried to make it 33%, you might loose 50% to 60% of the edge of field illumination, and perhaps as much as 20% illumination at the center of the field.
There is always a tradeoff to making a modification to a telescope that was optimized for a particular usage.
So, while you may be able to make the obstruction smaller, the spacing of the secondary mirror from the primary will determine how much of the light cone is lost.
In other words, you might be able to go a bit smaller, but not enough to make it worth doing.
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rmollise
Postmaster
Reged: 07/06/07
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Re: Reducing secondary mirror in an RC?
[Re: PiotrM]
#5323875 - 07/18/12 04:59 PM
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If you need a smaller secondary, you need to go to a different design. Maybe an SCT or MCT. The size of the secondary goes with the rc territory. Reduce it and the field illumination and usefulness for imaging goes down.
It is only a consideration for visual use, anyway, and most folks don't choose an rc if visual observing is their primary application for the telescope...
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PiotrM
Carpal Tunnel
Reged: 01/03/10
Loc: Poland
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Re: Reducing secondary mirror in an RC?
[Re: rmollise]
#5323931 - 07/18/12 05:53 PM
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Planetary imaging only. 16" if possible  still quite likely to be cheaper than non-mass made 16" f/20 DK.
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JoeR
Pooh-Bah
Reged: 03/07/10
Loc: Columbus, OH
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Re: Reducing secondary mirror in an RC?
[Re: PiotrM]
#5324195 - 07/18/12 09:54 PM
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Reducing a secondary mirror would also require increasing the focal length, or else there would be light loss from the primary mirror.
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elwaine
Pooh-Bah
Reged: 06/18/06
Loc: S.W. Florida
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Re: Reducing secondary mirror in an RC?
[Re: JoeR]
#5324204 - 07/18/12 10:07 PM
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I'm not trying to be snarky, but what is the point of buying an RC - which is designed primarily to be an astrograph - and then altering its design to turn it into something it was never intended to be?  Am I missing something here? If so, I apologize in advance.
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rmollise
Postmaster
Reged: 07/06/07
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Re: Reducing secondary mirror in an RC?
[Re: PiotrM]
#5324238 - 07/18/12 10:32 PM
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Quote:
Planetary imaging only. 16" if possible still quite likely to be cheaper than non-mass made 16" f/20 DK.
Well...hmm...an rc would not be my choice if planetary imaging were the goal...
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Jared
Post Laureate
Reged: 10/11/05
Loc: Piedmont, California, U.S.
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Re: Reducing secondary mirror in an RC?
[Re: PiotrM]
#5324380 - 07/19/12 01:31 AM
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Quote:
GSO RC has around 50% of central obstruction. The secondary is big and the scopes gives a very big imaging circle... but what if I want small circle in favor to low obstruction. Is it doable to use smaller secondary in such RC?
While it is theoretically possible, I would say it's not practical. One thing to worry about besides field illumination is baffling. The primary baffle is designed with a particular sized secondary in mind. Thos matters a lot on a Cassegrain since the baffling is critical to the performance of the scope.
The biggest problem, of course, is simply the challenge of finding an appropriately sized and figured secondary.
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freestar8n
Carpal Tunnel
Reged: 10/12/07
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Re: Reducing secondary mirror in an RC?
[Re: PiotrM]
#5324432 - 07/19/12 03:02 AM
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If you look up in the sky you might see an RC with a 12% secondary obstruction - the Hubble Telescope. The size of the secondary depends on the first order properties of the system - how big an illuminated field you need and how long the tube is - and doesn't depend on the particular curves.
But RC is harder to make and has field curvature and astigmatism - so it only makes sense to go to the trouble if you need a wide field with no coma - but can still tolerate field curvature. Note that RC is not really an "astrograph" because it has field curvature. The original intent of the design for imaging was to work with curved film holders - and that was a big factor in the design not catching on (e.g. 200 inch Palomar is not RC).
But an RC will work fine for imaging if the field is small enough - which it is on the HST. For wider field professional work, they work well for photometry and astrometry - but not pretty picture imaging.
If you wanted a small obstruction, narrow field scope for planets, you could certainly make an RC and it wouldn't have coma - but for such a narrow field you might as well make a DK since coma isn't a big deal. It is only a big deal on the HST because the aperture is very large and there is no atmosphere - so coma would immediately limit the field size. The effect of coma would be much less dramatic in a smaller scope in the atmosphere, where diffraction and seeing would still dominate.
So such an RC can certainly be made, but it might as well be DK or classical cassegrain with such a narrow field.
Frank
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freestar8n
Carpal Tunnel
Reged: 10/12/07
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Re: Reducing secondary mirror in an RC?
[Re: freestar8n]
#5324436 - 07/19/12 03:11 AM
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... and for planetary work, the HST design is pretty much what you are looking for - with an f/2.3 primary and overall f/24. Just scale it down to the aperture you want. Baffling wouldn't be a special problem since the intended field is narrow anyway. But there would be minimal gain by reducing coma over a DK, and a lot more work figuring the curves.
The old school approach would be an f/4-f/15 classical cassegrain in a long tube that worked as both Newtonian and Cassegrain.
Frank
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PiotrM
Carpal Tunnel
Reged: 01/03/10
Loc: Poland
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Re: Reducing secondary mirror in an RC?
[Re: freestar8n]
#5325018 - 07/19/12 01:01 PM
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The problem is such planetary scopes aren't mass made. I can buy a state-of-the-art f/20 Dall Kirkham... For 315 mm I got 7590 EUR and 6500 EUR prices from two sources. For 16" I got 15 000 EUR price There is also Hubble Optics f/20 DK with a sandwich mirror bit cheaper but also their mirrors are bit of an hot topic and not widely used.
Used second hand C14 bought from US would fit in around 2300 EUR. New in Europe - 5700 EUR. TS 16" f/4,5 Carbon Newtonian 3000 EUR but a long OTA that has a focuser at the top... (folded Newtonians - interesting, but also not made widely).
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