Return to the Cloudy Nights Telescope Reviews home pageAstronomics discounts for Cloudy Nights members
Get a Cloudy Nights T-Shirt Submit a Review / Article

Click here if you are having trouble logging into the forums

Privacy Policy | Please read our Terms of Service | Signup and Troubleshooting FAQ | Problems? PM a Red or a Green GuÖ uh, User

Equipment Discussions >> ATM, Optics and DIY Forum

Pages: 1 | 2 | 3 | 4 | 5 | (show all)
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?


Post Extras: Print Post   Remind Me!   Notify Moderator  
cheapersleeper
Carpal Tunnel
*****

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.

Post Extras: Print Post   Remind Me!   Notify Moderator  
BillC
on a new path
*****

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


Post Extras: Print Post   Remind Me!   Notify Moderator  
Crayfordjon
Vendor - Zerochromat
*****

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!.

Post Extras: Print Post   Remind Me!   Notify Moderator  
Mark Harry
Vendor
*****

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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
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.

Post Extras: Print Post   Remind Me!   Notify Moderator  
Mirzam
Post Laureate
*****

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


Post Extras: Print Post   Remind Me!   Notify Moderator  
Ajohn
sage
*****

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
-


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mark Harry
Vendor
*****

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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Sean Cunneen
Let Me Think
*****

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 (62 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!


Post Extras: Print Post   Remind Me!   Notify Moderator  
Ajohn
sage
*****

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
-


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mirzam
Post Laureate
*****

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


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mark Harry
Vendor
*****

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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mark Harry
Vendor
*****

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?


Post Extras: Print Post   Remind Me!   Notify Moderator  
Ajohn
sage
*****

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
-


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pinbout
Postmaster
*****

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!!!


Post Extras: Print Post   Remind Me!   Notify Moderator  
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?

Post Extras: Print Post   Remind Me!   Notify Moderator  
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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Tom and Beth
Post Laureate


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.

Post Extras: Print Post   Remind Me!   Notify Moderator  
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
-


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pages: 1 | 2 | 3 | 4 | 5 | (show all)


Extra information
14 registered and 25 anonymous users are browsing this forum.

Moderator:  ausastronomer, richard7, Starman81 

Print Thread

Forum Permissions
      You cannot start new topics
      You cannot reply to topics
      HTML is disabled
      UBBCode is enabled


Thread views: 5079

Jump to

CN Forums Home


Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics