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Equipment Discussions >> ATM, Optics and DIY Forum

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Brian Albin
Seeker


Reged: 08/22/06

Loc: Western Oregon
What has Become of the Cassegrainís Popularity?
      #5655298 - 01/31/13 11:20 PM

Have you noticed the lack of Cassegrain telescopes in the reviews section of Cloudy Nights?
They are filed under Compound Telescopes Ė Ritchey-Chretiens, Dall-Kirkhams, and Other Designs.
Except there are none.
http://www.cloudynights.com/category.php?category_id=22&pr=2x6

Have you an idea of why this design has so fallen from favor?


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cheapersleeper
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Reged: 01/22/10

Loc: Sachse TX
Re: What has Become of the Cassegrainís Popularity? new [Re: Brian Albin]
      #5655327 - 01/31/13 11:38 PM

I don't recall them haveing been in favor in the last few decades.

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BillC
on a new path
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Reged: 06/04/04

Loc: Lake Stevens, WA, USA
Re: What has Become of the Cassegrainís Popularity? new [Re: Brian Albin]
      #5655333 - 01/31/13 11:40 PM

Ritchey-Cretiens, Dall-Kirkhams and the like are all Cassegrains.

Just a thought.

BillC


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Crayfordjon
Vendor - Zerochromat
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Reged: 06/17/09

Loc: UK
Re: What has Become of the Cassegrainís Popularity? new [Re: BillC]
      #5655486 - 02/01/13 01:58 AM

Fashion dear chap, everybody wants designer label stuff, if you have not got an RC or a triplet you are a nobody. That is what ATM is all about these days, and ,Oh yes, it must be computerized to the hilt!.

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Mark Harry
Vendor
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Reged: 09/05/05

Loc: Northeast USA
Re: What has Become of the Cassegrainís Popularity? new [Re: Crayfordjon]
      #5655699 - 02/01/13 07:38 AM

I came up with a Gregorian Cass, where the primary is a hyperboloid, and the concave secondary a nice little sphere. I hate to aspherize little dinky mirrors.
*******
Total response- -ZERO.-
Problem: the very outer part of the FOV plots are bloated; but that ocurrs only at low powers. (big deal for some)
Otherwise, I think the idea has a lot of plusses.
M.


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polaraligned
professor emeritus


Reged: 12/26/08

Re: What has Become of the Cassegrainís Popularity? new [Re: Mark Harry]
      #5655716 - 02/01/13 08:00 AM

The primary hyperboloid is enough to stop most amateurs dead in their tracks.

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Mirzam
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Reged: 04/01/08

Loc: Lovettsville, VA
Re: What has Become of the Cassegrainís Popularity? new [Re: Mark Harry]
      #5655735 - 02/01/13 08:12 AM

I think there are a bunch of reasons that classical cassegrains are not commonly encountered these days.

They are relatively expensive to fabricate due to all aspherical optics, limited by inherent aberrations to long f-ratios and narrow fovs, and are not well corrected for wide field photographic applications.

My 12.5" classical cassegrain is used for observing and photographing planetary nebulae and obscure globular clusters. It really comes into its own for lunar and planetary binoviewing and lunar and planetary photography. Unfortunately there are not too many nights of good seeing quality to support this use. The cass requires a massive mount and is therefore not a telescope that I attempt to transport. If one wants to observe galaxies or other extended deep sky objects there are much better options.

JimC


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Ajohn
sage
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Reged: 12/03/07

Re: What has Become of the Cassegrainís Popularity? new [Re: Mark Harry]
      #5655757 - 02/01/13 08:20 AM

I would be interested in having the prescription for that Mark if it's available. I'm very interested in the gregorian.

Cassegrains used to be mentioned a lot in astro and telescope books written I would guess around the 40's or earlier. One phrase I remember - generally focused by moving a mirror usually the 2ndry. For amateur construction the difficulty of making the 2ndry is often mention still hence the DK. I would guess the manufacturers have the same problem. The tak scope for instance is a DK. Very expensive but not that expensive to make at home.

The other point is intended use. Texereau goes through that fairly well. He explains why a rather huge focal length is needed to record say shot's of the moon or planets on film that can actually make use of the scopes theoretical resolution. If I remember correctly that comes out at about f25 for his own personal 10in scope that he used through a hole in the roof.

I'm trying something a little more flexible F15 220mm with a field just big enough for the moon. A magnification of 5 gives a relatively small obstruction and the mirrors are harder to make.

The DK isn't much use for widish fields really as coma is 5 times as large as the Cass. Field curvature seems to be worse for the same F number on a Cass against a newtonian but coma is the same. End results on a flat field seem to be better than the DK but for visual use the curvature isn't a problem anyway.

John
-


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Mark Harry
Vendor
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Reged: 09/05/05

Loc: Northeast USA
Re: What has Become of the Cassegrainís Popularity? new [Re: Ajohn]
      #5655951 - 02/01/13 09:47 AM

A spherical secondary with my version alleviates collimation requirements. Correcting the hype isn't too bad, since the overcorrection is rather mild; -1.1 to -1.3~ in most cases.
It'll give a correct oriented image to be used terrestrially or astronomically if desired with no diagonal. Baffling can be pretty simple with a correctly placed aperture stop at the primary focus. Lots to like.
***
I'll get back later with a basic design shortly after errands...
M.


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Sean Cunneen
Let Me Think
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Reged: 08/01/07

Loc: Blue Island Illinois
Re: What has Become of the Cassegrainís Popularity? new [Re: Mark Harry]
      #5656062 - 02/01/13 10:33 AM Attachment (63 downloads)

I have a classic Cass which I have played with for the past year. Taking care of all the gremlins is seemingly a full-time job. Making the baffles especially is well, baffling. These scopes like tapers and lots of wide baffles which are very difficult to make economically.

The mirror spacing is a bit of a chore as well. The tolerances are very tight and testing/adjusting must be done outside and is a tedious process. If I wasn't such a masochist I would've have given up a long time ago!


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Ajohn
sage
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Reged: 12/03/07

Re: What has Become of the Cassegrainís Popularity? new [Re: Sean Cunneen]
      #5656206 - 02/01/13 11:49 AM

I suspect all of these scope even the DK have to be focused by moving the 2ndry mirror. I believe that used to be done via a rod along the scope and a belt drive of some sort to the focuser. These days an electric focuser would probably be easier. A linear bearing and some pre ground rod for the same might help some one make something suitable for either method. Electrics through the spider vanes or the belt/string or what ever over them.

My feeling is if this is done properly and the tube is stable enough collimation should be a rather complicated one time job providing the main mirror is adjustable for tilt and centring and can be kept in place. Done that way slight errors in the alignment of the focusing travel alignment can be adjusted at the mirror rather than trying to centre and square up etc the focuser. The 2ndry then just needs the usual tilt adjustment.

I haven't really thought through how to focus but another thought that would get rid of batteries is a lever running above or below a vane controlled via a push pull rod at the eyepiece end driven just like that or via a screw.

The part of this that does concern me is the spider. I don't think there is much chance of using tensioned thin vanes as something hefty enough to resist the strain might prove rather difficult to produce sensibly. I had a dob once that used part of a sort of deep oval that went out just past the 2ndry holder. That sort of thing might be a better way to go. On the dob the oval was used to shield the 2ndry mirror from stray light as there was no tube. I also made a dob with a single 1/4in dia steel bar running across the tube but feel that no spider diffraction would be a better solution.

Linear bearings - probably 2 spaced many bar diameters apart. The more over all length of the bearings the better the angular tolerance it should hold. The adjustment could work between them.

John
-


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Mirzam
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Reged: 04/01/08

Loc: Lovettsville, VA
Re: What has Become of the Cassegrainís Popularity? new [Re: Ajohn]
      #5656459 - 02/01/13 02:18 PM

My classical cassegrain does not have moving mirrors. The spacing between the primary and secondary is a critical design measure. Small changes in the spacing change the back focus by a large amount and can adversely impact the spherical correction. Mike Jones has written articles about this.

So you would not want to focus by moving mirrors in a classical cass. And you need to design in the amount of backfocus needed for your application from the beginning.

JimC


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Mark Harry
Vendor
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Reged: 09/05/05

Loc: Northeast USA
Re: What has Become of the Cassegrainís Popularity? new [Re: Mirzam]
      #5656558 - 02/01/13 03:08 PM

I have a model here done in WINSPOT.
(can't seem to do an OSLO at the moment) It's roughed in; probably pretty close to optimum; but that's my -opinion-.
**********
6" F/34.18 system.
6" F/4 primary, @ -1.185 conic.
1.15" dia spherical secondary; ROC = 9.063" concave.
separation, 28"
Focuses @ 34.09843" from the secondary.
This would make a reasonbly compact scope; but still a long focus instrument typical of the "Greg".

There's a comatic flare at the edge of the .3 degree field approx 2.5 x 1.5 times as large as the airy disc.
Certainly seems to be about what a DK Cass is. (planet shooter?) Might be fixable with a coma corrector.
Maybe one of the smart guys can take a look at it with ZEMAX or OSLO...?
As I said before, personally I find it somewhat difficult to make tiny little asphericals; but a sphere is a slam-dunk. I think this has some merit.
M.


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Mark Harry
Vendor
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Reged: 09/05/05

Loc: Northeast USA
Re: What has Become of the Cassegrainís Popularity? new [Re: Mark Harry]
      #5656569 - 02/01/13 03:18 PM

Does anyone have a link to Mike's
Cassdesign?


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Ajohn
sage
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Reged: 12/03/07

Re: What has Become of the Cassegrainís Popularity? new [Re: Mirzam]
      #5656628 - 02/01/13 03:54 PM

The fact that the spacing you mention is critical is why I feel it's best to focus one by moving the 2ndry mirror. That way the back focal length can always be bought in line with the design. I'm 100% sure that they used to be built that way - usually - in the UK probably every where.

Also the large telescopes. Tried to find some good drawings showing where the focusing motors are fitted but no luck but did find this "slightly" more complex scope that mentions it.

http://www.eso.org/sci/facilities/paranal/telescopes/ut/m2unit.html

Basically if you want a cassegrain to remain in alignment a lot more thought is needed in how the mirrors are located and how they are adjusted. It doesn't make much sense to try and centre the 2ndry on the primary it would be easier to do it the other way round and centre the main to the 2ndry. Similarly it doesn't make much sense to try and tilt the 2ndry into alignment with main. Then if you want to be sure that the 2ndry to main distance correct another adjustment is needed so may as well focus using the 2ndry as well. Strictly speaking there is no need for anything other than spacing adjustments on the 2ndry.

I don't see any problem at all making a main mirror holder that can be tilted and centred that will remain in place. I can see all sorts of problems trying to do that with a 2ndry. Some one who can't make an accurate carrier for the 2ndry easily might also want 3 levelling screws to set it up so that it was square to the axis of the distance or focusing adjustment if that is used. Saves a lot of money or time too. Much simpler than the usual focuser.

John
-


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Pinbout
Postmaster
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Reged: 02/22/10

Loc: nj
Re: What has Become of the Cassegrainís Popularity? new [Re: Mark Harry]
      #5656655 - 02/01/13 04:07 PM

Cassdesign

Hey Mark,

Whatzup!!!


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careysub
Carpal Tunnel
*****

Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: What has Become of the Cassegrainís Popularity? new [Re: Brian Albin]
      #5657045 - 02/01/13 07:55 PM

Were they ever popular among amateur astronomers?

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Tom and Beth
Post Laureate


Reged: 01/08/07

Loc: Tucson, AZ
Re: What has Become of the Cassegrainís Popularity? new [Re: Ajohn]
      #5657260 - 02/01/13 10:21 PM

Quote:

I suspect all of these scope even the DK have to be focused by moving the 2ndry mirror. I believe that used to be done via a rod along the scope and a belt drive of some sort to the focuser. These days an electric focuser would probably be easier. A linear bearing and some pre ground rod for the same might help some one make something suitable for either method. Electrics through the spider vanes or the belt/string or what ever over them.

My feeling is if this is done properly and the tube is stable enough collimation should be a rather complicated one time job providing the main mirror is adjustable for tilt and centring and can be kept in place. Done that way slight errors in the alignment of the focusing travel alignment can be adjusted at the mirror rather than trying to centre and square up etc the focuser. The 2ndry then just needs the usual tilt adjustment.

I haven't really thought through how to focus but another thought that would get rid of batteries is a lever running above or below a vane controlled via a push pull rod at the eyepiece end driven just like that or via a screw.

The part of this that does concern me is the spider. I don't think there is much chance of using tensioned thin vanes as something hefty enough to resist the strain might prove rather difficult to produce sensibly. I had a dob once that used part of a sort of deep oval that went out just past the 2ndry holder. That sort of thing might be a better way to go. On the dob the oval was used to shield the 2ndry mirror from stray light as there was no tube. I also made a dob with a single 1/4in dia steel bar running across the tube but feel that no spider diffraction would be a better solution.

Linear bearings - probably 2 spaced many bar diameters apart. The more over all length of the bearings the better the angular tolerance it should hold. The adjustment could work between them.

John
-




I'm just a hack of an ATMer, but once you have the spacing and alignment of the two mirrors correct, FOCUSING of the telescope is no more difficult than using a typical focuser such as found on a refractor or Newt.

And I surely hope a wire spider will work, as I plan on using one...if I ever finish this 300MM F20 DK.


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Tom and Beth
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Reged: 01/08/07

Loc: Tucson, AZ
Re: What has Become of the Cassegrainís Popularity? new [Re: Pinbout]
      #5657267 - 02/01/13 10:25 PM

Thanks for Cassdesign link. I had been looking for that recently.

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Ajohn
sage
*****

Reged: 12/03/07

Re: What has Become of the Cassegrainís Popularity? new [Re: Tom and Beth]
      #5657616 - 02/02/13 06:18 AM

Quote:

Quote:

I suspect all of these scope even the DK have to be focused by moving the 2ndry mirror. I believe that used to be done via a rod along the scope and a belt drive of some sort to the focuser. These days an electric focuser would probably be easier. A linear bearing and some pre ground rod for the same might help some one make something suitable for either method. Electrics through the spider vanes or the belt/string or what ever over them.

My feeling is if this is done properly and the tube is stable enough collimation should be a rather complicated one time job providing the main mirror is adjustable for tilt and centring and can be kept in place. Done that way slight errors in the alignment of the focusing travel alignment can be adjusted at the mirror rather than trying to centre and square up etc the focuser. The 2ndry then just needs the usual tilt adjustment.

I haven't really thought through how to focus but another thought that would get rid of batteries is a lever running above or below a vane controlled via a push pull rod at the eyepiece end driven just like that or via a screw.

The part of this that does concern me is the spider. I don't think there is much chance of using tensioned thin vanes as something hefty enough to resist the strain might prove rather difficult to produce sensibly. I had a dob once that used part of a sort of deep oval that went out just past the 2ndry holder. That sort of thing might be a better way to go. On the dob the oval was used to shield the 2ndry mirror from stray light as there was no tube. I also made a dob with a single 1/4in dia steel bar running across the tube but feel that no spider diffraction would be a better solution.

Linear bearings - probably 2 spaced many bar diameters apart. The more over all length of the bearings the better the angular tolerance it should hold. The adjustment could work between them.

John
-




I'm just a hack of an ATMer, but once you have the spacing and alignment of the two mirrors correct, FOCUSING of the telescope is no more difficult than using a typical focuser such as found on a refractor or Newt.

And I surely hope a wire spider will work, as I plan on using one...if I ever finish this 300MM F20 DK.




I'm more worried about a pure Cassegrain Tom and people say no way as they always need tweaking each time they are used. DK's are easier to tweak. If things need to stay put then tension should be ok if it's even in all of the vanes and what ever they connect to on the tube is reasonably rigid. They should settle down eventually anyway.

As to the rest there isn't much room round the 2ndry mirror for adjustments - very little on the one I am looking at with a 50mm dia mirror so to me it makes more sense to tilt and centre the main mirror. The tilt is usually there anyway.

The big telescopes probably focus using the 2ndry because of variations in spacing due to temperature changes but people have pointed out that the back focal length can change rather a lot due to variations in this so it has to be set accurately. I don't see any real problem doing that in a way that also focuses. A simple screw adjustment at the eyepiece end would then allow any other errors in construction / optics to be tuned out and it would only be used once. All much easier to do than making a good crayford and a lot cheaper than buying one.

Somehow I suspect this is area is part of the reason SCT's move a mirror for focusing other than the usual one quoted in mag's and books - closer focusing than could be obtained with a normal focusing arrangement.

John
-


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Ajohn
sage
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Reged: 12/03/07

Re: What has Become of the Cassegrainís Popularity? new [Re: Ajohn]
      #5657620 - 02/02/13 06:26 AM

Forgot to mention one worry is just how par focal eyepieces are -do some makes stay in focus as eyepieces are changed. My impression is that some makes do but a moving 2ndry mirror could still shift the focal plane anyway.

John
-


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tim53
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Reged: 12/17/04

Loc: Highland Park, CA
Re: What has Become of the Cassegrainís Popularity? new [Re: Ajohn]
      #5658775 - 02/02/13 07:38 PM

Love my Cassical Class!:


Next to my 8" f/7 Cave lightweight deluxe Newt.

Here it is after I replaced the 1 1/4" Jaegers refractor focuser with a 1 1/4" Crawmach:



It's an f/23 with optics by Ed Beck. 2.5" secondary for 20% obstruction. Does pretty well on planets. Mars a coulple years back, near opposition, when it never got much over 14 arc seconds. And this is the 'boring' side!:



I use a lasermax holographic collimator on mine to get everything close. Then I fine tune the primary while gee-whizzing through the eyepiece. Everything is independently adjustable, including the baffle tube and the focuser.

-Tim.


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Brian Albin
Seeker


Reged: 08/22/06

Loc: Western Oregon
Re: What has Become of the Cassegrainís Popularity? new [Re: tim53]
      #5659358 - 02/03/13 04:40 AM

I would like a slow Cassegrain (original formulation) for planets. The fast & portable Dobson idea is not for me, as I have a backyard. I understand the need for portability for all the city residing observers.

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Dick Parker
scholastic sledgehammer
*****

Reged: 08/17/07

Loc: Tolland, CT and Chiefland, FL
Re: What has Become of the Cassegrainís Popularity? new [Re: tim53]
      #5659957 - 02/03/13 01:05 PM Attachment (32 downloads)

I have to second tim. My 12.5 inch f/20 Classical Cass is my most used scope. It is a completely ATM project. Perhaps we can encourage more acceptance of this configuration. Yes, that includes DK, RC etc.

Seems to me that the popularity of shorter focal length telescopes has gained favor because of imaging and the Cassegrain configuration usually ends up with relatively long focal ratio and smaller fields

For visual observing, though, they can't be beat. I initially made mine for planetary observing, but became very impressed with performance on deep sky objects. Naturally, the Andromeda galaxy and Orion Nebula can not fit in the FOV, but star clusters are impressive. Eyepieces really like longer focal ratios.

In my workshop I have stimulated 3 other Cassegrains to be made. One, a 10 inch f/15 model is being made now. I have the Hindle sphere test apparatus to test the hyperbolic secondary, so the ones made here are Classical Cassegrains.

I'll put up some planetary images I have taken.

Dick Parker


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Dick Parker
scholastic sledgehammer
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Reged: 08/17/07

Loc: Tolland, CT and Chiefland, FL
Re: What has Become of the Cassegrainís Popularity? new [Re: Dick Parker]
      #5659970 - 02/03/13 01:11 PM Attachment (47 downloads)

An image of Mars taken through the 12.5 inch Cass during the Nov 2005 opposition. Image taken the night of opposition. Image taken with a pretty inexpensive webcam.

Dick Parker


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Mark Harry
Vendor
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Reged: 09/05/05

Loc: Northeast USA
Re: What has Become of the Cassegrainís Popularity? new [Re: Dick Parker]
      #5659980 - 02/03/13 01:17 PM

Doggone!
Nice work, both of you!
M.


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Dick Parker
scholastic sledgehammer
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Reged: 08/17/07

Loc: Tolland, CT and Chiefland, FL
Re: What has Become of the Cassegrainís Popularity? new [Re: Dick Parker]
      #5659983 - 02/03/13 01:17 PM Attachment (39 downloads)

And my 16 inch f/17 Cassegrain. Also a completely ATM project. So, yes, they can be made as ATM projects, although this is probably an "advanced" project.

Being a Classical Cassegrain, it is a superb performer on star clusters, galaxies, small nebulas, and double stars as well as planets. Until I made my Cassegrains I never really knew how well they worked. The focal ratio, for visual telescopes, provides nice image scale, use of clean simple eyepieces and generous eye relief.

Let's hear it for Cassegrains.

Dick Parker


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Dick Parker
scholastic sledgehammer
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Loc: Tolland, CT and Chiefland, FL
Re: What has Become of the Cassegrainís Popularity? new [Re: Dick Parker]
      #5659988 - 02/03/13 01:20 PM Attachment (35 downloads)

An image of Jupiter through the 16 inch Cass. Same inexpensive webcam (i.e. not many pixles resolution) and I wish the seeing were better.

Dick Parker


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John Jarosz
Astro Gearhead
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Reged: 04/25/04

Loc: Fairfax, Iowa
Re: What has Become of the Cassegrainís Popularity? new [Re: Mark Harry]
      #5659993 - 02/03/13 01:22 PM

Quote:

Seems to me that the popularity of shorter focal length telescopes has gained favor because of imaging




I think is also has to do with the advent of the Paracorr and modern short FL eyepieces with wide fields and long eye relief. Collimation is probably equally difficult with a short focus Newtonian and a Cass.

John


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davidmcgo
Pooh-Bah
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Reged: 10/09/04

Loc: San Diego, CA
Re: What has Become of the Cassegrainís Popularity? new [Re: John Jarosz]
      #5660437 - 02/03/13 05:42 PM Attachment (33 downloads)

I've had experience with 2 Cassegrain scopes. One was a Star Instruments 10" f15 classical Cass. It was fitted in a fiberglass tube, solid back plate, typical Cave or Parks Newtonian style components. It was a bear to get collimated and didn't cool well. Principal problem with collimating Classical Cass and RCs is that the centering of the secondary optical axis to the primary mirror's has to be within a few thousands of an inch. Typical secondary holders can tilt the mirror in a way that shifts the axis.

So that scope went away. A cople of years ago I got an old set of 3B Optical 10.25" Dall Kirkham optics based on an f4 primary. I made a wood open frame tube with adjustable trusses to work the spacing and then recently remounted in a lined aluminum tube with fans, etc. This scope is working at about f17 with the focuser and diagonal I'm using and is very insensitive to misalignment of the secondary. Since the secondary is spherical there is no optical axis per se, and I never have on axis coma or astigmatism that can't be collimated out by adjusting the primary.

Also, the secondary is on an AstroSystems holder and I can set the focus range wherever I need to move it by moving the secondary fore/aft via the threaded rod and nuts on either side of the sider hub and then when set, I use a Feathertouch 2015BCR I have on the back end. Movement of the secondary is only needed to change to accommodate a camera or binoviewer and is pretty delicate to get set right. But the focusing in use is with typical Newt style focuser on the rear cell.

Lastly, I'll second Dick Parker and Tim53's observations on how good these long f ratio scopes are for visual deep sky. They aren't limited to planets. And visually I never notice coma, it would show in imaging but visually even with a 50mm Axiom or 40mm Pentax XW, the field is visibly sharp to the edge. I've had fantastic views of even larger objects like M8, M42, and M31. I just need to pan the scope around to see all of them, but the views are stunningly sharp and contrasty. Center of M42 around the trapezium looks like cotton candy its so filamented, and the E and F stars just snap even at 90x or thereabouts.

I'm very partial to DKs based on experience with this scope, as the tuning of the collimation isn't so daunting as the classical cass or RC and it works superbly well visually.

Dave

Edited by davidmcgo (02/03/13 05:44 PM)


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Kaelin
sage


Reged: 11/16/09

Loc: Midwest Great Lakes
Re: What has Become of the Cassegrainís Popularity? new [Re: Brian Albin]
      #5660529 - 02/03/13 06:26 PM

Built my own 6-inch Cass about 15 years ago. Took awhile to get the secondary correct, as I recall. Compact to stow and great for galaxies. I'll want to get it to more star parties when warmer weather arrives. A classic ATM-built Cass usually draws some attention because it isn't often seen in the field.

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tim53
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Re: What has Become of the Cassegrainís Popularity? new [Re: Kaelin]
      #5661575 - 02/04/13 11:28 AM

As to the original question. Marketing!

An sct is shorter than true cassegrains, and they've been modified over the years to correct for their inherent weaknesses. Sadly, before most amateurs think about understanding the optical trade offs in selecting an optical configuration that will work well for what they want to do (and maybe learn how to make it themselves if its not readily available) they'll plunk down a few thousand dollars for the current "Barbie Doll of telescopes". And the. A few more $K for the accessories.

Tim


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Re: What has Become of the Cassegrainís Popularity? new [Re: davidmcgo]
      #5661824 - 02/04/13 01:49 PM

Thanks for the perspective on the DK.

Even several books on telescope design do not provide this sort of understanding of the design's merits.


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Re: What has Become of the Cassegrainís Popularity? new [Re: careysub]
      #5662079 - 02/04/13 04:29 PM

I really enjoyed reading about these Cassegrains! Great pictures too (of the planets and scopes) . I have a 10 inch f/4- f/15 newt-classical cassegrain.

I used Ken Novaks' Cassegrain notes for design, it is posted here:
http://bobmay.astronomy.net/CassNotes/
links there to calculate distances, baffle length etc.
Some good Cassegrain info on Stellafane's links page too.

Used Novaks's Cass mirror cell, etc., and it has always worked well- not sure who is selling such stuff now.
Now working towards doing variable star photometry with it- Casses being well suited for this.
Cassegrains are a path less followed, but a good one...
Cheers to all


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Re: What has Become of the Cassegrainís Popularity? new [Re: Mark Harry]
      #5667309 - 02/07/13 03:36 PM Attachment (28 downloads)

A regular Gregorian also is not that hard. I made a 6" F/24 to try out the concept. Bath interferometry requires quite precise measurement of RoC however, so now my primary turned out under corrected with less than sharp images as a result. The magnification then starts working against you...

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Re: What has Become of the Cassegrainís Popularity? new [Re: Arjan]
      #5669484 - 02/08/13 05:38 PM

I'm thinking about a gregorian as well. Conics seem to be lower on the 2ndry and far more simple testing that a cassegrain 2ndry. Nice thing with the gregorian is the source can go at one point and the knife edge at the other. I have a feeling a web cam with suitable lens may make it easier to look for the null.

John
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Re: What has Become of the Cassegrainís Popularity? new [Re: Ajohn]
      #5670077 - 02/09/13 02:33 AM

I would recommend building a Bath interferometer. About as difficult as a foucault tester, but much more objective results. My secondary was 40mm and F/2.8 (k=-0.62), making KE measurements quite hard to do. See here for a report.

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Re: What has Become of the Cassegrainís Popularity? new [Re: Arjan]
      #5670177 - 02/09/13 05:43 AM

I have hung around on the bath group. I mentioned an F3 main mirror somewhere and was pointed at the group by Dale. I have seen comments that it isn't reliable at that focal ratio and also that it is so am not sure yet. Also had some unanswered questions. Have bought most of the bits though. I found a microscope stage to build it on - cheap xy movements, z to if whole scope is used. I have too many microscopes. Microscopes for polarised light work also have rotating stages but have a tendency to be rather expensive.

Thanks for your link.

John
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Re: What has Become of the Cassegrainís Popularity? new [Re: Ajohn]
      #5670221 - 02/09/13 07:21 AM

Precisely why I suggest using a hyperboloidal primary, and spherical secondary.
I worked up a design using an F/5 primary, and the plots are cut in half over the earlier F/4. The hyperboloid is mild, and rather easy to do.
********
If you -can- make the small prolate secondary, performance can be astounding. But this would be easier when making a larger aperture than a 6" scope with small secondary. FWIW.
M.
Mark


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Re: What has Become of the Cassegrainís Popularity? new [Re: Ajohn]
      #5670298 - 02/09/13 08:38 AM

I bet that the Bath will be a lot less unreliable than foucault is. I've tried both... For this focal ratio you need a wide laser beam though, the 1 or 2 mm output by the diode assembly won't give you full illumination. I used an objective from an old film projector as a replacement collimation lens, to obtain a 5mm beam.

Mark is right, the 40mm secondary was difficult because of its small size. However, scaling up also leads to a bigger system. My OTA is about 700mm now.


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Re: What has Become of the Cassegrainís Popularity? new [Re: Mark Harry]
      #5670373 - 02/09/13 09:37 AM

The one I am looking at is 220mm F3 to F15 but I do have an old meade 10in F4.5 cardboard tube newtonian kicking about. I was going to sort it out and sell it but with the cost of bits for making scopes it might be better to use it for something else. There is something seriously wrong with the 2ndry holder. Not possible to catch anything like all of the light from the mirror even with offsets etc.

John
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Re: What has Become of the Cassegrainís Popularity? new [Re: Ajohn]
      #5670862 - 02/09/13 02:39 PM

You need a secondary of 83mm, given a focal plane diameter of 25mm at 270mm behind the primary. It's a lot of obstruction...
RoC=384mm, k=-0.45, f1=230mm, f2=1160mm. That's an F/2.4 mirror.


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Re: What has Become of the Cassegrainís Popularity? new [Re: Arjan]
      #5672071 - 02/10/13 10:00 AM

It's an F3 mirror! The obstruction will work out larger than a cassegrain but it can be advantageous to use the 2ndry as a stop as some do in newtonians. Loose spec, not checked but wont be far out is

Primary 220dia F3 K 1
2ndry less than 52mm dia but more than 48mm, 359mm rad K 0.444
Separation 875mm, Focus 200mm behind mirror.

The aim being an semiangle of view of 0.3 degrees.

No comment on Bath reliability. I read the forum every now and again.

John
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Re: What has Become of the Cassegrainís Popularity? new [Re: Ajohn]
      #5674137 - 02/11/13 02:10 PM Attachment (20 downloads)

Not sure your calculations are OK. Did you take field into account for sec size?
This is what I get based on your input:


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Re: What has Become of the Cassegrainís Popularity? new [Re: Arjan]
      #5675738 - 02/12/13 12:24 PM Attachment (21 downloads)

I used a web java applet and you are right it's entirely wrong. For a fully illuminated 0.3 degree field the 2ndry needs to be nearly 83mm dia and 74mm axial. It should get more attractive at higher magnification factors. Is the program you used available on the web? Free?

Sounds like I will be switching to a cassegrain. The 2ndry can be smaller than 55mm for that. I'm after a multipurpose scope with interchangeable ends. The baker reflector corrector being one alternative and greg was another possibility. I also hoped to find an F8 to F10 solution with a small obstruction. I believe Dall did that a long time ago but probably not from an F3 mirror.

The big problem with the classic compound scopes is field curvature. They give the same amount of coma as an equivalent newtonian but the field curvature seems to be worse. This shows the effect of a 157mm curved field against a flat one. It's pretty obvious which is which. The petzval rad is about 266mm which is still rather severe. Not a problem visually really as eye accommodation should look after it.

John
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Re: What has Become of the Cassegrainís Popularity? new [Re: Ajohn]
      #5676457 - 02/12/13 07:20 PM

My cass, too:
http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/4124808/Main...


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Re: What has Become of the Cassegrainís Popularity? new [Re: Ajohn]
      #5677077 - 02/13/13 06:38 AM

John,

Its a spreadsheet of my own device... but you can have it of course. I'll put it up on my website and post the link.
When magnification is high, field curvature is less a problem than you may think. Angular field is rather small, and the depth of focus is large.

Arjan


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Re: What has Become of the Cassegrainís Popularity? new [Re: Arjan]
      #5677130 - 02/13/13 07:32 AM

John, here's the link: atm.udjat.nl
Click on "Downloads" and then select under Data files, excel the file "Scope design tools". It's a 1M excel with all kinds of calculators I use for designing.


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Re: What has Become of the Cassegrainís Popularity? new [Re: Arjan]
      #5679060 - 02/14/13 09:53 AM

Thanks Arjan. I used the modas java web page. The full program seems to be rather odd on Gregorians as well. I was going to knock up a spread sheet for designing these scopes but it probably would never be as tidy as yours. I seemed to have got myself tied up on what else I might be able to do with a 220mm F3 mirror at the moment as well as some form of compound.

Thanks for your links as well. I run linux but seem to be having a lot more luck running windows apps on an emulator these days. LibreOffice though has no problems reading your spreadsheet.

John
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Re: What has Become of the Cassegrainís Popularity? new [Re: Ajohn]
      #5680695 - 02/15/13 05:35 AM

I can highly recommend OSOL-LT for optical analysis. I recently found out that you can go up to 10 surfaces, which is enough to cover most amateur systems. I can send you some lens files, also for a Gregorian.

Wouldn't you consider a Kutter if you're thinking about planet scopes? See my website under articles, down left.

-Arjan


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Re: What has Become of the Cassegrainís Popularity? new [Re: Arjan]
      #5680740 - 02/15/13 06:43 AM

I downloaded oslo edu rather than LT as last time I used oslo a lot probably about 10 years ago the unregistered LT was severely crippled so I managed to find an early version of edu on the web. Not sure how things are now as I went for edu straight away. I've also added the edu version of the opic optimisation routine. It will now optimise with GENII,opic, aberrations and ray. Not used ray yet so not sure what it does.

My biggest problem so far is schmidt type correctors. There seems to be no way of forcing the neutral zone to be in a particular place. Seems from others that if it did REAL or something like that it would. Of course they never explained what REAL was. Useful conversation. Other parts of the thread have been though - in places.

On the compound I'm aiming for something more general than a planetary scope which is why I aimed at F15. Basic idea is a moon sized field in a 2in eyepiece or across a 35mm frame. For a planetary scope I would look at the Stevick Paul or a much longer focal length compound. I've played with the S P numerous times of the years with oslo. Latest idea was to work back from that to Paul's original constants to see if there is any more flexibility in the design. I've done that now but it's on the back burner. I am not at all sure I would ever make one of those but the concept is interesting. The baker reflector correct is too and given what it should be able to do I may well have a go at making one of those. It's a tough design to change though and I haven't got the time to go out and digest the entire content of the optics section of a reference library. I doubt if my brain has sufficient capacity left either. It's full of many other technical areas You may gather that the answers to some of the questions I ask don't exactly please me.

John
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Re: What has Become of the Cassegrainís Popularity? new [Re: Ajohn]
      #5681844 - 02/15/13 03:56 PM

Ha, ha, know that feeling. There's plenty of room in your brain though, just need to time-share. I' also doing a lot of electronics, embedded SW and also drew-up and built a model ASK13 glider. Oh, and of course there's also work...

I find the Stevick-Paul too complicated, although its optical performance is outstanding. Just need to tinker a bit more with the Kutter, until I find the right compromise. I currently arrive at a sort of non-tilted flat field of 0.4deg with a field-wide 95% Strehl, for a 200mm F20 system. Tube is less than 1.6m long and it is all-spherical hence easy to make.


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Re: What has Become of the Cassegrainís Popularity? new [Re: Arjan]
      #5681849 - 02/15/13 03:59 PM Attachment (15 downloads)

Here's the OSLO output:

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Re: What has Become of the Cassegrainís Popularity? new [Re: Arjan]
      #5683112 - 02/16/13 10:06 AM

My next "look at" is what can be done with an F3 mirror based cassergrain. I'm wondering about types of sub aperture maksutov style correctors. The idea of being able to put a baker reflector corrector on the end of that is very tempting and his basic idea of using a full aperture corrector to cure problems is also interesting. I have a feeling that a simple sub aperture corrector can sort out a cassegrains problems. Probably wrong though.

First problem really though is can I make a good accurate F3 mirror. It was hogged out by a machine and the person who did that said make sure you maintain the chamfer round the edge when you rough grind it to a slower sphere. That's red rag to a bull as far as I'm concerned. One idea is to use a method that was used to make some of the early fast mirrors for rc's. Ring laps. The other is lap shapes etc according to the wear rate that is needed.

Trouble is paper telescopes in Olso are so interesting.

John
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Re: What has Become of the Cassegrainís Popularity? new [Re: Ajohn]
      #5683199 - 02/16/13 10:50 AM

An F3 is not an easy mirror, you really need to have some experience. Because you magnify its image to get a usable system F/ratio, it is also quite unforgiving.

If you make it a bit longer, you could also consider a Lurie-Houghton corrector. It's a lot of glass to grind though...


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Re: What has Become of the Cassegrainís Popularity? new [Re: Arjan]
      #5685705 - 02/17/13 06:31 PM

I'm determined to use it at F3 - until I find I can't do it.

I've just spent several hours on the web looking for olsolt 5.4. It doesn't seem to be around any more. I think I had it on a cd but have lost it. Annoying as I understand it's more capable than the more recent versions.

John
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Re: What has Become of the Cassegrainís Popularity? new [Re: Ajohn]
      #5685878 - 02/17/13 08:14 PM

Quote:

I've just spent several hours on the web looking for olsolt 5.4. It doesn't seem to be around any more.



You may ask Sinclair Optics for a copy. If I remember correctly, many of the functions that are now restricted in the .edu freebie versions were unrestricted in 5.4LT version. It also allowed systems up to 12 surfaces vs 10 in the .edu version.


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Re: What has Become of the Cassegrainís Popularity? new [Re: MKV]
      #5686417 - 02/18/13 04:29 AM

I'm sort of hoping some one offers to mail me an installation file.

John
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Re: What has Become of the Cassegrainís Popularity? new [Re: Ajohn]
      #5687021 - 02/18/13 02:25 PM

Oldest I have is 6.3.3 from Sep 2005

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Re: What has Become of the Cassegrainís Popularity? new [Re: Arjan]
      #5687507 - 02/18/13 06:31 PM

I still have 6.1 and it's not really any difference to the current edu. When I downloaded last which seems to be more recent than I thought it was more severely limited than 6.1 and I found 6.1 on the web on a .edu page.

My biggest problem with it is schmidt correctors. There doesn't seem to be any way of controlling where the neutral zone. Not that this has much to do with this thread.

John
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Re: What has Become of the Cassegrainís Popularity? new [Re: Ajohn]
      #5688281 - 02/19/13 03:42 AM

John,

You probably have it, but on ATMSite.org, when you scroll down to the Oslo keyword there's a lot to be found on this topic. Also example lens files including a Schmidt design.

-Arjan


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Re: What has Become of the Cassegrainís Popularity? new [Re: Ajohn]
      #5688593 - 02/19/13 10:03 AM Attachment (18 downloads)

If you want to go with a Schief, an all spherical equal radius design were the secondary is slightly warped into a toroidal gives excellent on and off axis performance. The spot diagram is for 1/2 field of view.
From a fabrication stand point, it is simple to make since both the primary and secondary have the same radii, just opposite in sign so the tool used to grind the primary can be used as the secondary. Also once the primary is polished spherical it becomes the test plate for the secondary. The secondary is polished spherical and tested by interference then it is very slightly mechanically warped in one axis to form the needed toroid. The warping is much less then a Yolo. I'm working a 6" version. The primary is done and I'm in the process of figuring the secondary. I hope to have to finished for this years Stellafane.
- Dave


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Re: What has Become of the Cassegrainís Popularity? new [Re: DAVIDG]
      #5689234 - 02/19/13 04:34 PM

Dave,

Interesting. Why do you use warping instead of a corrector, is it easier in your opinion? I'd like to know how you mechanically solve this.
In your design, what is the tilt of the image plane?

-Arjan


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Re: What has Become of the Cassegrainís Popularity? new [Re: Arjan]
      #5689278 - 02/19/13 05:00 PM Attachment (11 downloads)

I sometimes have a problem with Oslo. I came across a tilted that uses 2 identical spherical mirror and a flat. The 2nd mirror is used twice but oslo ignores it. The 2nd time the mirror is used it's entered with a corrected tilt. Tilt and bend set on all surfaces. When I focus it the rays just shoot past the mirror after the flat. I have had this happen with cas 2ndry mirrors but that just went away. This is the layout - not focused as it shrinks dramatically due to the rays going past the mirror that should reflect them back between the main and the flat. What an I doing wrong?

John
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Re: What has Become of the Cassegrainís Popularity? new [Re: Ajohn]
      #5689284 - 02/19/13 05:02 PM Attachment (20 downloads)

This is the file. Very high F number but the spot size is tiny - in winlens.

John
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Re: What has Become of the Cassegrainís Popularity? new [Re: Arjan]
      #5689453 - 02/19/13 06:37 PM

Quote:

Dave,

Interesting. Why do you use warping instead of a corrector, is it easier in your opinion? I'd like to know how you mechanically solve this.
In your design, what is the tilt of the image plane?

-Arjan



In my opinion it is easier to warp. In my design both mirrors are spherical and of the same radius, so as I said the tool becomes the secondary and the primary the test plate for the secondary. Only a little more additional optical work is needed over making a newtonian and as you can see the spot diagram is excellent and there is no chromatic aberration from a corrector lens. The telescope is 1/2 the size of typical 6" f/15 refractor with much better theorical optical performance. The on axis polychromatic Strehl at best focus is 0.992. It is also easier to align the optics. Making a corrector requires a lens pair which adds fours additional surfaces. In some designs you can use off the self lenses but the quality can not be assumed and you need to test them as well and the addition problems of mounting and aligning them which includes having the ablity to off set them from the optical axis. Other designs with a corrector lens may require a different radius on the secondary so more additional optical work.
The warping is very slight, much less then a Yolo so a simple cell will work and it only needs to be warped in one axis since the other radii is the same as the primary. The amount of pressure required is small. I'm using a piece of Quartz for the secondary that is about 10mm thick. When testing the secondary by interference I can easily warp it into a toroid by just using finger pressure at two points This not a new idea and it has been done successfully before.
Once the optics are finsihed I plan to build the OTA then test the system by double pass autocollimation so I'll be able to see how warping is needed to the secondary and how effective my mechanical cell design works in achieving and holding the needed toroidal figure.
The image plane tilt is about 5 degree.

- Dave


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Re: What has Become of the Cassegrainís Popularity? new [Re: Ajohn]
      #5690664 - 02/20/13 11:24 AM

Try negative 202 twice for the second mirror

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Re: What has Become of the Cassegrainís Popularity? new [Re: DAVIDG]
      #5690680 - 02/20/13 11:33 AM

Quote:

This not a new idea and it has been done successfully before.



Yes, I've seen it before where the toroid was actually polished in. However, I was just curious how you plan to mechanically solve the cell.

I see in all models of especially large aperture Kutters that image plane tilt becomes a real spoiler. There seems to be no way to solve this, other than making a tilted drawtube.


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Re: What has Become of the Cassegrainís Popularity? new [Re: Arjan]
      #5690857 - 02/20/13 01:06 PM Attachment (11 downloads)

That was a bit silly of me. Forgot winlens's sign conventions.

Oslo doesn't give such good spots. The fan goes out of the airy disc, Weak but well out giving p/v's from 0.2 to nearly 0.5 across a 0.3 degree field. Any idea which one is correct. This is what winspot shows. p/v's would be very very low.

John
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Re: What has Become of the Cassegrainís Popularity? new [Re: Ajohn]
      #5690915 - 02/20/13 01:35 PM

Not so many mental "gymnastics" with winspot. From what I've seen so far, winspot is fairly consistent with OSLO and ZEMAX. Easy to use; though different nomenclature.
M.


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Re: What has Become of the Cassegrainís Popularity? new [Re: Ajohn]
      #5691160 - 02/20/13 03:33 PM

Quote:

That was a bit silly of me. Forgot winlens's sign conventions.

Oslo doesn't give such good spots. The fan goes out of the airy disc, Weak but well out giving p/v's from 0.2 to nearly 0.5 across a 0.3 degree field. Any idea which one is correct. This is what winspot shows. p/v's would be very very low.

John
-




John,
Check the units that you are using in Winspot, they might
be in millmeters instead of inches.

- Dave


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Re: What has Become of the Cassegrainís Popularity? new [Re: DAVIDG]
      #5692228 - 02/21/13 06:14 AM

That was the problem Dave. If only it was like that and in inches. F58 though. Not sure what that would be like.

John
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Re: What has Become of the Cassegrainís Popularity? new [Re: Ajohn]
      #5692320 - 02/21/13 08:05 AM

Quote:

I see in all models of especially large aperture Kutters that image plane tilt becomes a real spoiler. There seems to be no way to solve this, other than making a tilted drawtube.






Actually your design with 2 correcting lenses would be a Dittie but yes they have a considerable tilt. One fix is to use a much flatter curve on the secondary and to shorten up the secondary to image distance. You can get it back to zero tilt.


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Re: What has Become of the Cassegrainís Popularity? new [Re: Ed Jones]
      #5692803 - 02/21/13 01:48 PM

Quote:

Actually your design with 2 correcting lenses would be a Dittie but yes they have a considerable tilt. One fix is to use a much flatter curve on the secondary and to shorten up the secondary to image distance. You can get it back to zero tilt.



And then it becomes a Chief!
Conserving all other parameters, will it then also get rid of coma/astig?


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Re: What has Become of the Cassegrainís Popularity? new [Re: Arjan]
      #5692858 - 02/21/13 02:17 PM

Quote:

Quote:

This not a new idea and it has been done successfully before.



Yes, I've seen it before where the toroid was actually polished in. However, I was just curious how you plan to mechanically solve the cell.

I see in all models of especially large aperture Kutters that image plane tilt becomes a real spoiler. There seems to be no way to solve this, other than making a tilted drawtube.




I made a couple of Schiefs with field tilts and visually you can't see the problem. You don't want to tilt the focuser. It's mechanical axis and the optical axis need to be parallel as what is usually done. You need to tilt the eyepiece or camera to match the tilted focal plane. A simple solution is to take a piece of tubing that one end with fit inside the focuser and the other will accept an eyepiece. If the field tilt is 5 degrees you cut the tube in half but instead of making a 90 degree cut you make it 87.5 degrees (90 - 5/2). Then you rotate the two pieces 180 degree and glue or soldered them back to together Now one piece is tilted 5 degrees to the other and the eyepiece is now tilted at the correct angle. Also this insures that the optical axis of the eyepiece passes through the center of the tilted field of view.

- Dave


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Re: What has Become of the Cassegrainís Popularity? new [Re: Ajohn]
      #5693120 - 02/21/13 04:51 PM

John,

I've reproduced the design in OSLO, but can't seem to get much better results than you are reporting, whatever I try. Are you sure about the Winspot plot?

-Arjan


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Re: What has Become of the Cassegrainís Popularity? new [Re: DAVIDG]
      #5693136 - 02/21/13 05:01 PM

Dave,

I recall an (old) article which showed a straight thick-walled tube where the eyepiece accepting side was drilled in under an angle. Seems more difficult than your suggestion...

You say visually you don't notice the image tilt. What would you see with a camera?
The off-axis smear you get in typical spot diagrams is not too far beyond the airy boundary, probably twice the diameter. The problem I can see is that the size of the airy disk is quite large in such a slow system (tens of micron), and hence the smear will cover many pixels.

-Arjan


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Re: What has Become of the Cassegrainís Popularity? new [Re: Arjan]
      #5693283 - 02/21/13 06:18 PM

Arjan,
For me a Schief is a high powered instrument that excels at planetary, lunar and double star observing. If you use it for high power imaging of the planets and double stars, the objects will be centered and the effect of field tilt very small and most likely not even visible. It's only if you did lunar imaging would the problem of the field tilt be an issue if you didn't tilt the camera. As for wide imaging that is over a few seconds in exposure times which would be required for deepsky imaging, seeing and tracking errors smear out the star images to many times the Airy disk size so the effects of the field tilt if the camera is not tilted, many not be visible.

All the Best,
- Dave


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Re: What has Become of the Cassegrainís Popularity? new [Re: Arjan]
      #5693307 - 02/21/13 06:36 PM Attachment (14 downloads)

Quote:

John,

I've reproduced the design in OSLO, but can't seem to get much better results than you are reporting, whatever I try. Are you sure about the Winspot plot?

-Arjan




I missed that I had loaded it as mms and it should have been inches. One other thing I missed on the oslo file. There should be a conic of 0.34 on the 1st mirror. That cleans up the central spot somewhat. It's not too bad really considering how simple it is. I've attached the oslo file converted to mms. A negative conic shows slightly better mtf.

Sorted my oslo problems as well. Installed an older version that I have had for some time. This time though I gave it no chance of finding I had a newer version. It's unregistered oslolt 6.1. The old equiv of edu. I can use the schmidt tools now but it's really strange oslo wont use the normal schmidt formulae.

John
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Re: What has Become of the Cassegrainís Popularity? new [Re: Ajohn]
      #5693316 - 02/21/13 06:43 PM

As I play with tilted scopes from time to time is there a way of getting a full spot diagram out of oslo on both axis? I think that is what Dave has done but not sure. I have other problems with spot diagrams such as more spots over more closely spaced focusing distances but can't find any way of changing them. Other than right clicking etc which has to be done every time I look that them.

John
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Re: What has Become of the Cassegrainís Popularity? new [Re: Ajohn]
      #5694097 - 02/22/13 07:10 AM

I substitute a negative value for the field, which shows me the "other" side. Not high tech, but simple.
M.


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Re: What has Become of the Cassegrainís Popularity? new [Re: Mark Harry]
      #5694267 - 02/22/13 09:28 AM

Don't think that works Mark. If you look at that plan view I posted the spot diagrams normally come up and out of it vertically. Making the field negative just switches that through 180 degrees. What's really needed is spots running across the view. I probably have that the wrong way round. From memory oslo uses an x,y,z axis system z being along the axis of the optics - spacing effectively. For tilted systems spots and curves are needed for both the x and y plains really.
I think winlens another freebie with sliders can show the spots correctly. It would be interesting to look at certain tilted scopes such as the stevick paul with mirrors tilted in both plains.

John
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Re: What has Become of the Cassegrainís Popularity? new [Re: Ajohn]
      #5694307 - 02/22/13 09:45 AM Attachment (9 downloads)

Quote:

As I play with tilted scopes from time to time is there a way of getting a full spot diagram out of oslo on both axis? I think that is what Dave has done but not sure. I have other problems with spot diagrams such as more spots over more closely spaced focusing distances but can't find any way of changing them. Other than right clicking etc which has to be done every time I look that them.

John
-




John,
I'm using a custom CCL routine that afriend wrote for me to display a full field spot diagram. As Mark said you can switch between a positive field angle and a negative one to display each 1/2 of the diagram. To change the number of spots displayed and adjust the scale go to Evaluate, Spot Diagram and Report Graphics. You'll see a text box like I have attached and you can manually adjust the number of spots displayed and the X and Y scaling factors.

- Dave


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Re: What has Become of the Cassegrainís Popularity? new [Re: DAVIDG]
      #5694452 - 02/22/13 11:04 AM Attachment (8 downloads)

I found the box Dave. It's a pain having to right click the spot diagram every time it updates while optimising something.

The documentation suggests that the spot diagram is an ccl but so far I haven't found it. Actually I think the X axis is really the Z axis but would have to plough through the docs again. My problem is summed up with this shot. The rms etc focuses are useless sometimes. In this case I'm more interested in -0.0125mm focus shift MKV did warn me. I suppose a slider to shift the focus is one way round the problem but as focus also shifts as things are changed it doesn't always work out.

John
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Re: What has Become of the Cassegrainís Popularity? new [Re: Ajohn]
      #5694559 - 02/22/13 12:00 PM

Quote:

I suppose a slider to shift the focus is one way round the problem but as focus also shifts as things are changed it doesn't always work out.



That's what I usually do, have a slider for TH of final leg to IMS. If you select the multispot output in the slider definition you get three Y-axis spots, middle one on optical center and the other at the two field extremes. For schiefs this is quite useful.
I haven't found either how to show X-axis variation. Under Evaluate->Spot diagram->Single spot you can define object coordinates (X,Y,Z). Don't know what it brings though.


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Re: What has Become of the Cassegrainís Popularity? new [Re: Arjan]
      #5694719 - 02/22/13 01:17 PM

The trouble with the slider is that say it's set for 0.010 mm steps the focus might shift by mms when something is changed. What I hope to do is get the slider call back function to work so that it auto focuses on each and every slider change. There are also some interesting ccl calls in the schmidttools ccl's. Optasp.ccl in particular. It demo's how to "throw up" graphic windows that update as things change. "Puke" just about sums up my feeling about oslo C at the moment. I can't get the schmidt file to compile and from the error messages I had when optasp wouldn't work with the latest osloedu it looks like they can be meaningless.

John
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Re: What has Become of the Cassegrainís Popularity? new [Re: Ajohn]
      #5694752 - 02/22/13 01:35 PM Attachment (11 downloads)

Maybe I misunderstanding the problem but when you set up the sliders if you click on the box "SPOT DIAGRAM" it will display a spot diagram window that changes in real time as you move the slider. You can also look at other spot positions besides the on axis spot by changing the FY position.

- Dave


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Re: What has Become of the Cassegrainís Popularity? new [Re: DAVIDG]
      #5694883 - 02/22/13 02:43 PM

That's exactly what I meant, and checking 'all points' gives you three spots at FY=-1, 0 and 1 .
But yes, if focus shifts you need to refocus with another appropriately defined slider.

Edited by Arjan (02/22/13 02:45 PM)


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Re: What has Become of the Cassegrainís Popularity? new [Re: DAVIDG]
      #5694895 - 02/22/13 02:48 PM

Could be that I notice the problems more because I have spent a lot of time working on the lenses and corrector in the baker thingy in the other thread on it. It's possible to add a solve to that focus tracks or keep hitting auto focus with the mouse. Neither are optimum really. It can be a real pain working on the correct rad and asphere at the same time. SA isn't too bad as the focus can always be shifted back in with a slider. Spots often aren't so easy.

I've now lost my sliders - been there before. I may be missing a windows dll for C.

John
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Re: What has Become of the Cassegrainís Popularity? new [Re: Arjan]
      #5695002 - 02/22/13 03:38 PM Attachment (6 downloads)

Dave,

I've tried a toroidal secondary on a 200mm F20-ish Kutter. It gets an almost perfect on-axis spot at 99.8% Strehl. The secondary tilt has to be quite high, resulting in an image tilt of almost 8deg and a mechanical challenge to build.

Sec RoC are 5000mm in Y direction and 4875mm in X direction (can this be correct?). If my calculations are right, this means that the edge of the 90mm dia mirror has to be bent back about 5 micron.


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Re: What has Become of the Cassegrainís Popularity? new [Re: Arjan]
      #5695334 - 02/22/13 06:21 PM Attachment (9 downloads)

Arjan
I'm sure you know that when designing a Schief you can fully correct for coma or astigmatism but not both that the same time. I first design the system using the secondary tilt which fully corrects for coma. This is also the smaller of the two tilt angles which also results in the least amount of image plane tilt. Then I apply the toroidal figure to the secondary to correct for the astigmatism. You also might want to go to f/24-25 since this will reduce the difference in the radii on the toroid.
Here is a spot diagram for a field of view 1/2 degree for 200mm f/24 Schief with 5.7 degree field tilt. The on axis Strehl is 0.987 and is diffraction limited out to just about the very edge of the field of view.

- Dave


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Re: What has Become of the Cassegrainís Popularity? new [Re: DAVIDG]
      #5696249 - 02/23/13 09:45 AM

I was talking to Gary Fishorn [SP??] About his 10 inch Herschelian he made by putting weights on the primary under knife edge test then polish till spherical . Not sure how to get in contact with him. I think his way has great merit and he has the weights needed known . One mirror and a eyepiece That is as good as it gets . Gary used my17inch CHief at Winter star party . And had some good ideas for it . Hope some one can get me his email as I should have ask .Thanks

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Re: What has Become of the Cassegrainís Popularity? new [Re: DAVIDG]
      #5696303 - 02/23/13 10:31 AM

Hi Dave,

Anastigmat has the smallest secondary tilt, increasing this tilt leads to the coma-free solution at the cost of overcorrected astigmatism.
Following Kutter in his article (page 10), starting from the coma-free situation and making the secondary toroidal allows you to compensate for the difference in meridional and sagittal focus.

A 200mm F/24 is huge, I get Pri-Sec of 1500mm and Sec-IMS of 1800mm. But worst of all is the Pri to final optical axis separation, more than 650mm.
It would be nice to have a coma corrector and use the anastigmatic tilt...

-Arjan


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Re: What has Become of the Cassegrainís Popularity? new [Re: Arjan]
      #5696392 - 02/23/13 11:16 AM Attachment (7 downloads)

Arjan,
Your right about the tilt angles, I was at work when I replied and was thinking of some else. Here is my design for 200mm f/24 Schief with a Toroidal secondary. As with my other designs the primary secondary have the same but opposite radii and spherical in figure. Then the secondary is mechanically warped in only one direction to form the toroid. The field tilt is 5.7 degrees. As I said I'm building 6" version.
When you consider that a classic 8" f/15 refractor is 2x longer then the schief and has worse optical performance, I don't consider the length of OTA at about 6 feet a problem. The OTA can be made very light, so a much smaller mount can used. By the any we have a yahoo group to discuss Schiefs, Schiefbuilders.
- Dave


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Re: What has Become of the Cassegrainís Popularity? new [Re: DAVIDG]
      #5696573 - 02/23/13 01:07 PM

I think the tilt of the secondary orientation has to be realized as a variable a builder has direct control over.
Next, astig can be controlled by either polishing in a toroidal shape on the secondary. Coma is not so easy. I would think the easiest thing would be to select a tilt that cancels coma to a large degree, then tackle the astig by a compromise tilt; and if not enough, to warp/polish in the radius difference needed on either secondary or primary.
A choice is available here, and I think that working out the compromise that you are most able to effect will get the best results.
My 2 millicents and regards,
M.


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Re: What has Become of the Cassegrainís Popularity? new [Re: Mark Harry]
      #5696578 - 02/23/13 01:10 PM

Dave, I think the post you made at 6:21 pm yesterday, would knock your shorts off over a 1/2 degree field! Excellent.
M.


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Re: What has Become of the Cassegrainís Popularity? new [Re: DAVIDG]
      #5696730 - 02/23/13 02:35 PM

Quote:

By the any we have a yahoo group to discuss Schiefs, Schiefbuilders.




Thanks Dave, yes I'm a member of that list. This thread has deviated a long way from Cassegrains.
Your analysis looks very promising. Mine still had the -0.5 conic in the primary, but this is probably not needed. You loose a bit of definition in the central spot, but those at the field edges seem better with k=0.
Still tossing around the parameters, the thing needs to fit in my garage too...

-Arjan


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Re: What has Become of the Cassegrainís Popularity? new [Re: Arjan]
      #5696924 - 02/23/13 04:22 PM Attachment (11 downloads)

If you realize that the Kutter 2 mirror type Schief is nothing more then a tilted Dall Kirkham Cassegrain, the thread hasn't drift that far off topic. The Dall Kirkham uses a ellipical primary and spherical secondary and so do most Schiefs.

Mark, thanks for the comment. The 6" toroidal schief is making progress. I get to work on the secondary every few days, in between a full time job, finishing up painting the complete inside of the my house, restoration of my '79 Camaro and the restoration the Hale spectrohelioscope for Stellafane.

So to get back on target, this thread got me to dig out a 6" set of Dall Kirkham optics that I purchased a couple of years ago. They were in a homemade scope and the builder said he could never get a good image from them. I believe Coulter made them and they are an f/15 set. The primary has a smooth figure with a ROC 46.5" and what looks like the proper amount of correction but I need to measure it along with the radius on the secondary.

Something didn't look right when I looked at the primary and a little bit of measuring with a ruler and I discovered why the optics gave a poor image or at least one of the problems. The hole in the primary wasn't centered on the optical surface but it was on the back. The hole was drilled on an angle and it wasn't a small one. The mirror must have not been mounted flat when bored. If you tried to align the optics visually they looked sort of OK but in reality they were way off optically.

A couple of days ago I brought the primary into work and chucked it up in the lathe late in the afternoon. When the mirror was spinning it was real easy to see how far off center the hole was on the front surface as it wobbled around. Luckily the hole was under sized. I chucked a grinding stone up in the tool post and slowly ( about 90 minutes ) ground the hole concentric with the OD of the mirror. I then used a bottle placed in the hole and used some 220 grit to grind a nice chamfer on the edge of the hole. Here is a picture of optics.

- Dave


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Re: What has Become of the Cassegrainís Popularity? new [Re: Dick Parker]
      #5697577 - 02/23/13 11:36 PM

Dick Parker,
It was an oversight on my part to not mention when you posted your Mars picture, You got a nice capture of the crossed lines on Hellas.


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