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Catalin Romania
member
Reged: 08/20/07
Posts: 57
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Hi,
I have just received one 300mm f/20 classical cassegrain mirror set and I want to put it in a scope :-).
Since I've built only newts until now, I want to ask you kindly to post some pictures here from your ATM cassegrain if you have. I'm interested (as the title says) in the mirror mountings ...I can handle the rest 
The central baffling tube of the primary...is one enigma for me....how to fix it the best way :-)
Thank you all!
Catalin
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Catalin Fus
Alexandria, Romania
www.catalinfus.ro
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mconnelley
super member
Reged: 03/14/06
Posts: 196
Loc: Fremont, CA
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Hello:
I made a 315mm f/15 classical Cassegrain a few years ago. I'll see if I can find a picture of it to post later. The primary mirror cell can be the same as for any equatorially mounted Newtonian. My mirror is pretty thick, so it sits on three pads on the bottom of the mirror. The mirror cell I have is like a pie pan, so the mirror can't slide around when the telescope is point round the sky. To hold the secondary mirror, you can just get a tip-tilt plate with collimation screws, then use silicone adhesive to glue the mirror to the plate. My secondary mirror holder is a bit different in that it has a retaining ring.
My primary mirror baffle is a bit odd. In my case, the optimal diameter of the baffle was around 3", and was larger than the hole in the middle of the mirror so I could not mount the baffle through the central hole. I first made the baffle tube from empty aluminum soda cans, which are the perfect size, with the ends cut off (be careful, the cut aluminum is thin and very sharp). I think I put about 5 cans end-to-end to make my baffle tube. The baffle tube is suspended in the middle of the optical tube with fine fishing wire and never touches the primary mirror. The end of each wire wraps around a screw so I can adjust the alignment of the baffle tube by adjusting the length of the wires. The extra wires in the light path produce a little diffraction, but I usually never notice it.
One thing to look out for is getting the position of the secondary mirror correct. This is very important so that the telescope is well corrected for spherical aberration and so that the focus will be within the range of your focuser, since a small error in the secondary mirror position will lead to a very large error in the location of the focal plane. When I made my scope, I did all of the measurements, then drilled the holes for the spider a few mm farther from the primary mirror than where my measurements said was the right spot. I then put washers between the spider and the secondary mirror holder to carefully adjust the secondary mirror position by about 1 mm at a time.
Cheers
Mike Connelley
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Mike I. Jones
Pooh-Bah
Reged: 07/02/06
Posts: 1102
Loc: Fort Worth TX
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Here's a zipped file of Version 1 of CassDesign. The inputs are straightforward, provided you know the values to key in. It calculates the optimal baffling that minimizes central obstruction. Let me know how it works for you.
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 a new design you'll like.
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Catalin Romania
member
Reged: 08/20/07
Posts: 57
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@ mconnelley - thank you for the information!!! I was thinking to use a secondary mount like yours, with retaining ring, but I haven't found some pictures in any ATM site to show how it's the best way to do this. I want to make this spider wisely and by not using any type of gluing for the secondary  ....so I'll gladly wait for your pictures!!!
@Mike Jones - your program was put to work some time ago when I was looking for anything that helped me with the construction :-). Thanks for sharing it with us!!
I have to apologies for my english, since I wasn't clearer in the post...with the baffling tube. I try to reformulate that question...:-)
how do I mount properly the baffle tube?
I ask you that because of this test
http://translate.google.com/translate?hl=en&u=http%3A%2F%2Fastro-foren.de%2Fshowthread.php%3Ft%3D9476
in which the baffle tube sits on the primary , holding it against a plate....and causes some 'minor' problems here. I want to avoid that problem but every ATM site that I've seen with cass construction uses this tube for fixing the primary on it's cell....
Can it be done in other way , avoiding this primary mirror fixing with the baffle tube?
The friend who made me this set , gave me the proper spacing between the primary and the secondary and it goes like that...
1064mm spacing => 5700mm FL
1058mm spacing => 6000mm FL
Thank you very much!
Catalin
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Catalin Fus
Alexandria, Romania
www.catalinfus.ro
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Mike I. Jones
Pooh-Bah
Reged: 07/02/06
Posts: 1102
Loc: Fort Worth TX
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On the three Cassegrains I've help build, we mounted the primary baffle tube directly on the backplate, not touching the mirror anywhere within the perforation. The alignment of the baffle tube was independent of mirror collimation. You don't want the tilting of the primary mirror in collimation to also tilt the primary baffle tube.
Likewise, the rear-pointing secondary baffle was mounted on the plate attached directly to the spiders that the secondary cell was mounted to. Again, you don't want the secondary baffle to change in alignment as you turn the collimation screws on the secondary cell.
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 a new design you'll like.
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 1952
Loc: Boechout, Belgium
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Quote:
..... you don't want the secondary baffle to change in alignment as you turn the collimation screws on the secondary cell.
Mike
Ok Mike, I can live with that, but why not just mount the primary baffel on the primary mirror itself. Collimating the primary will 'set' the baffel automatically in the correct position..
Or maybe I'm missing something here..
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Chris
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Catalin Romania
member
Reged: 08/20/07
Posts: 57
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Mike, I fully understand your point of view and it makes sense :-).
If the hole in the mirror is 68mm, how much clearance would you consider to leave between them?
Should I mount the focuser and the baffle tube via same screws on the backplate, so they will be centered by default?
Catalin
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Catalin Fus
Alexandria, Romania
www.catalinfus.ro
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Mike I. Jones
Pooh-Bah
Reged: 07/02/06
Posts: 1102
Loc: Fort Worth TX
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Misaligned baffle tubes produce non-uniform illumination and flat-fielding problems at focus over the FOV, and possibly allow stray light paths to focus to exist. My personal concerns with hard-mounting baffles to mirrors are:
1. The hole through the primary may not be cored precisely perpendicular to the optical axis of the front surface.
2. The hole may not be precisely centered with respect to the optical axis, which may also not be precisely centered to the edge of the mirror.
3. The mirror may not be (and often is not) precisely centered in the tube assembly and to the secondary mirror.
If the baffles are hard mounted to the mirrors, you could conceivably achieve good collimation with crooked baffle tubes, or good baffle tube alignment but not be in collimation. This could be overcome by having a tilt-adjust joint in the primary baffle tube. But strictly as a personal preference, I just found it more mechanically straightforward to decouple the two.
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 a new design you'll like.
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Catalin Romania
member
Reged: 08/20/07
Posts: 57
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Mike,
you don't want to answer my questions...? 
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Catalin Fus
Alexandria, Romania
www.catalinfus.ro
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tim53
sage
Reged: 12/17/04
Posts: 323
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Catalin:
When I built my 12.5" f/23 Cassegrain, I mounted the baffle tube to the back plate in such a way as to be able to align it separately from the focuser. Everything is adjustable individually that way, so I don't have to worry about inaccuracies in the machining of the various parts.
I also drilled holes in the baffle tube just ahead of the back plate and behind the mirror, so that the cooling fan blows air through the baffle tube as well as the main tube. This has helped tremendously for planetary definition.
-Tim.
I
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"We`re just waiting looking skyward as the days come down.
Someone promised there`d be answers, if we stayed around."
-Orchestral Maneuvers in the Dark, "The Romance of the Telescope"
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Catalin Romania
member
Reged: 08/20/07
Posts: 57
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If there is anyway that you can send me 2-3 pictures of your mirror mounts I would appreciate it a lot!!!
If not....I understand..that's life :-)
catalin.fus (at) gmail.com
Thank you!
Catalin
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Catalin Fus
Alexandria, Romania
www.catalinfus.ro
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Dick Parker
super member
Reged: 08/17/07
Posts: 143
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Tim -
"...holes in the baffle tube just ahead of the backing plate and behind the mirror...."
Thanks for sharing that idea.
Dick Parker
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Mike I. Jones
Pooh-Bah
Reged: 07/02/06
Posts: 1102
Loc: Fort Worth TX
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Sorry Catalin - I have been off taking care of home chores before a storm hit.
Here's a diagram I did for one design showing the baffle tube mounted separately from the primary. Notice how the baffle tube can be enlarged forward of the primary with support rings.
The primary cell mount would be something like 67.85mm outer diameter (OD) and 63.85mm inner diameter to give a 2mm wall thickness. The gap between the primary support tube and baffle tube support would be perhaps 1mm, thus the OD of the baffle tube support sleeve would be 61.85mm. If it has a wall thickness of 1mm, the ID would be 59.85mm. If CassDesign says it wants a larger baffle than that, then install larger OD rings on the baffle tube support sleeve.
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 a new design you'll like.
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MSO
member
Reged: 12/23/04
Posts: 32
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Why not just 'settle' the issues of Cassegrainian and instead go for a Nasmuth/Coude .It sure has it's Plus+..(no hole to perforate and better view positioning and keeping the bodily temps.`away` form the mirror Prime and so on).
If tho a perforation is a must have, try a water-jet(almost every city has one)And cut exact ctr. holes/no heat and no pressure-and cheap)..
Anyway, my question is/was for Mike Jones innitially~
Mike my question is:do you do,or know anyone who does a 16" SCT Corrector Lens Plate?
Thank you..
btw:I rarely am connected here at this site.
But,you may e-mail me(easier for me) (Or Call--806-796.3787 
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 1952
Loc: Boechout, Belgium
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Mike,
That's an issue we're talking about his very moment as we're building 7 DK scopes at the same time this very moment. )I think we have a DK-mania here at the observatory this moment...  )
I would consider mounting the baffel directly to the primary. If the baffel is correctly mounted to the primary itself there is no problem imo. Mike, give me one good reason not to and we'll consider changing our option.
Needles to say we would use an extra collar between the baffel (with flange) and the mirror of course..
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Chris
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Mike I. Jones
Pooh-Bah
Reged: 07/02/06
Posts: 1102
Loc: Fort Worth TX
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I listed my concerns in post 2353375 above. Read through them again and see if you still have questions.
Chris, with your level of experience and expertise and quality of optical and mechanical tooling available, I've no doubt you could mount your baffles right on your primaries, and everything would come together just fine, with collimated optics and perfectly aligned, concentric baffles.
My concern is that if Catalin's hole through his primary isn't centered and perpendicular, his primary baffle could be crooked and would need some method of independent adjustment. Neither of us know how well this hole was made; we don't have his mirror in our shops to measure it. Mounting his baffle directly to a poorly cored hole would introduce a problem with aligning the baffle tube concentrically to the secondary while at the same time achieving good collimation.
Mike
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 1952
Loc: Boechout, Belgium
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Mike,
I did not think of the central hole possibly not beeing at the exact centre of the mirror... In our case I will do the trepanning myself so I hope to have the central holes spot-on...
Still, I just need to find an even more lighter tube for that baffel as the one we have now. Not only because that baffel will be 'hanging' on that mirror, but also it'll be super-lightweight-telescopes. (no OT) So every gram counts...!
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Chris
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Mike I. Jones
Pooh-Bah
Reged: 07/02/06
Posts: 1102
Loc: Fort Worth TX
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Phil, check your PM's - I answered you there. I don't want to stray off Catalin's topic here. If you want to, start a new thread on that mirror - I'll be glad to post what I've done so far on that one.
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 a new design you'll like.
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Catalin Romania
member
Reged: 08/20/07
Posts: 57
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Thanks for the advices Mike!!
I have measured the center hole and it seems centered to millimeter (my scale was in mm so , anything below that I can't measure).
I have to figure out in the next days how to make the mirror cell..
1...like in the newts that I've built, 9 point mirror cell + lateral 3x120 degrees supports
2...adapt the design that Mike suggested for the conical one here, using the center hole to fix the mirror on the cell.
For the secondary mirror , I think I will go classic with just one small modification...
If you still have suggestions....I'll gladly accept them on this topic.
You have a picture attached with the mirror set....so you will now it's real :-)
Catalin
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Catalin Fus
Alexandria, Romania
www.catalinfus.ro
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 1952
Loc: Boechout, Belgium
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Catalin...
Get the mirrors out of that foil, some brand of foil tend to damage the coating after beeing to long in contact with it...!!
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Chris
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