mak5
member
Reged: 03/09/09
Posts: 42
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All Cassegrain telescopes have primary and secondary baffles but some have also some tube baffling (annular baffles) something like a refractor. Are they really worth the extra work or flocking the tube is enough? How is their position and diameter calculated?
Please help a puzzled man depuzzle himself  .
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Posts: 1667
Loc: Fort Worth TX
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I worked hard to put together a fairly detailed article on designing and optimizing Cassegrain baffling in the November 2008 issue of Astronomy Technology Today. If you are a subscriber, you can read it online in the back issues.
Baffling is about stray light control. In an unbaffled Cassegrain, stray light can usually slide right by the Cass secondary and reach focus, where it fogs the image with unfocused light. Two baffles are needed, one extending forward from the primary mirror hole and one backward from the secondary mirror. Their diameters and lengths have to be optimized to minimize the central obscuration of the telescope pupil.
See if you can access that article, that will give you a lot of information. It also has references for earlier articles on baffling published in Applied Optics.
Mike
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56 mirrors, lenses, 16" f/6 Newt, 6" f/10 refractor, TOA-130S, Tinsley 5" f/15 Mak, 6" f/4 RFT, Coronado PST. Still to build: 24" f/10 Modified Dall-Kirkham, 10" f/26 Mak, 8" f/12 apo, spectrohelioscope, Herrig, Schupmann, and others.
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mak5
member
Reged: 03/09/09
Posts: 42
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Thanks for the help, Mike.
I've just read your ATT article on Cassegrain baffles and I will also try CassDesign2.
I was mainly asking about the baffles installed on the tube of some Cassegrain scopes (like the AT 8" RC). I also saw these type of baffles inside the Intes Micro maks. Do they really help or they are not worth the extra work?
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Nils Olof Carlin
professor emeritus
Reged: 07/26/04
Posts: 690
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If you really want to be thorough, try this article by Mike Peck and companion spreadsheet to download from here
You might find the maths somewhat baffling 
Nils Olof
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davidpitre
Carpal Tunnel
Reged: 05/10/05
Posts: 1988
Loc: Central Texas
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Quote:
Thanks for the help, Mike.
I've just read your ATT article on Cassegrain baffles and I will also try CassDesign2.
I was mainly asking about the baffles installed on the tube of some Cassegrain scopes (like the AT 8" RC). I also saw these type of baffles inside the Intes Micro maks. Do they really help or they are not worth the extra work?
You are talking about baffles outside the primary mirror, correct ?
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David
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mak5
member
Reged: 03/09/09
Posts: 42
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Thanks for the links.
Yes, I am talking about the annular baffles (knife edge) that are on the inside of the telescope tube.
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Nils Olof Carlin
professor emeritus
Reged: 07/26/04
Posts: 690
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If you can't see the reflection of the tube wall by looking through the (empty)focuser, then no reflections from the tube wall can reach the EP (or other detector). If you can see part of the tube, then that part may benefit from a ring baffle. Otherwise, there is little point in baffling the tube.
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mak5
member
Reged: 03/09/09
Posts: 42
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Thanks, Nils. It makes perfect sense. However, I am trying to understand why the guys at Intes Micro and GSO put tube baffles in their maks and RCs, respectively.
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davidpitre
Carpal Tunnel
Reged: 05/10/05
Posts: 1988
Loc: Central Texas
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Quote:
Thanks, Nils. It makes perfect sense. However, I am trying to understand why the guys at Intes Micro and GSO put tube baffles in their maks and RCs, respectively.
...and Astrophysics and TEC.
I agree with Nils, It seems to be a waste unless perhaps the mirror is especially dusty.
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David
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TxStars
professor emeritus
Reged: 10/01/05
Posts: 561
Loc: Lost In Space
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These baffles help keep stray light from making it to the mirrors.
If you have the moon centered then move it to the edge or slightly out of view these baffles make a difference.
They also help provide support so your tube does not flex as much with that guidescope on top.
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http://txstars.org/
Tak FC-50 / Tak FCT-65 / Tak FC-100
Celestron 8" Schmidt camera,Sand Cast C-8/C-5
"SCUZZO" 300/f 1.1 , 16"f/7.5
Cameras: Nikon F2/Mamiya 645, press 6x7 ,6x9/ Hasselblad 6x6 / Lenses Yes Many - Yay Film
Mounts - GoTo by hand and eye
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tim53
Carpal Tunnel
Reged: 12/17/04
Posts: 2283
Loc: Highland Park, CA
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Baffling the tube will cause more problems than it solves, in my experience. Tube currents will tend to hug the wall of the tube until they encounter an obstruction that they'll flow around. This is okay on the top side of the tube if they're warm currents and the baffles are big enough ID to keep them out of the light path as they flow over the baffle, but warm currents from the bottom side of the tube will come off the baffles and go right into the light path.
I saw this with the first tube I made for my Cass several years ago. The next tube had no baffles, I just made it bigger inside (about 15.5 inches for a 12.5"). But my first primary baffle had a ring baffle near the sky end and was closed at the backplate (mounted to it, not the primary), and so I had tube currents in the primary baffle that weren't eliminated by turning on the fan. When I replaced that primary baffle with one without a ring at the end and with holes in front of the backplate to allow airflow through the baffle, the tube current plumes went away when the fan was on.
I have no baffle on the secondary, but my secondary spider is a full foot down inside the end of the OTA.
Tube currents are more obvious and obnoxious with high resolution planetary viewing/imaging. Probably much less so for DSO work.
-Tim.
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"In the old days, before the discovery of eruptions, the lava had to be carried by hand down the mountain and thrown on the sleeping villagers. This took a lot of time." - New Yorker Cartoon, ~ 1980
Hatch Observatory details:
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mak5
member
Reged: 03/09/09
Posts: 42
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Is there a way to calculate their positions and inside diameters? I have found nothing on this untill now.
Though tube currents might be a problem with these type of baffles, I'm tempted to give them a go.
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Nils Olof Carlin
professor emeritus
Reged: 07/26/04
Posts: 690
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Quote:
Is there a way to calculate their positions and inside diameters?
If these baffles don't actually hide the tube wall from being seen from within the FOV, or do anything else that is optically meaningful (unlike the baffles of a refractor that have a job to do), there is no obvious principle of design to follow.
But you can al least make sure the baffles stay out of the lightpath: imagine a truncated cone, top angle equal to your widest true field angle, and truncated by the primary mirror. Or if easier, draw a scale drawing with lines starting at the edge of the primary and tilting "outwards" by half the true feld angle each side. Keep the baffles outside this line, and they stay out of the light path - the tube currents, if any, won't.
Quote:
I'm tempted to give them a go.
well - but why?
Nils Olof
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mak5
member
Reged: 03/09/09
Posts: 42
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Thanks to everybody for making it clearer for me.
I'm thinking of using them for a better control of the stray light. 90% of my observing time is in my backyard, so I want to make sure that I have the best image given the conditions.
I agree that if the tube wall is not seen from the focuser, these baffles don't have an optical job to do. I'll have to make some drawings of the telescope to decide if they can help or not.
Flocking the tube may be a better solution.
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Nils Olof Carlin
professor emeritus
Reged: 07/26/04
Posts: 690
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Another possible temptation, in case you observe close to light sources, is to make a (lightweight) tube extension - slightly conical to stay out of the light path. It could limit the light that could scatter, for instance by dust on the primary... (such a tube extension might be even more useful with a Newtonian).
Nils Olof
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mak5
member
Reged: 03/09/09
Posts: 42
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Thanks.
I could use the dew shield from my C8.
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