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Equipment Discussions >> ATM, Optics and DIY Forum

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Ajohn
sage
*****

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
Postmaster
*****

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 (30 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
*****

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 (45 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
*****

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
*****

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 (37 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
*****

Reged: 08/17/07

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 (33 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
*****

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
*****

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 (31 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? [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
Postmaster
*****

Reged: 12/17/04

Loc: Highland Park, CA
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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careysub
Carpal Tunnel
*****

Reged: 02/18/11

Loc: Rancho Cucamonga, CA
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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cspell
journeyman


Reged: 05/14/11

Loc: NJ
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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Arjan
super member


Reged: 01/21/09

Loc: Netherlands
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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Ajohn
sage
*****

Reged: 12/03/07

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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Arjan
super member


Reged: 01/21/09

Loc: Netherlands
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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Ajohn
sage
*****

Reged: 12/03/07

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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Mark Harry
Vendor
*****

Reged: 09/05/05

Loc: Northeast USA
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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Arjan
super member


Reged: 01/21/09

Loc: Netherlands
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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