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Equipment Discussions >> ATM, Optics and DIY Forum

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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]
      #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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Arjan
super member


Reged: 01/21/09

Loc: Netherlands
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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Ajohn
sage
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Reged: 12/03/07

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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Arjan
super member


Reged: 01/21/09

Loc: Netherlands
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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Ajohn
sage
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Reged: 12/03/07

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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valy
sage


Reged: 07/28/09

Loc: Bucharest, Romania
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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Arjan
super member


Reged: 01/21/09

Loc: Netherlands
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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Arjan
super member


Reged: 01/21/09

Loc: Netherlands
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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Ajohn
sage
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Reged: 12/03/07

Re: What has Become of the Cassegrainís Popularity? [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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Arjan
super member


Reged: 01/21/09

Loc: Netherlands
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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Ajohn
sage
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Reged: 12/03/07

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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Arjan
super member


Reged: 01/21/09

Loc: Netherlands
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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Arjan
super member


Reged: 01/21/09

Loc: Netherlands
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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Ajohn
sage
*****

Reged: 12/03/07

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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Arjan
super member


Reged: 01/21/09

Loc: Netherlands
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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Ajohn
sage
*****

Reged: 12/03/07

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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MKV
Carpal Tunnel
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Reged: 01/20/11

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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Ajohn
sage
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Reged: 12/03/07

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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Arjan
super member


Reged: 01/21/09

Loc: Netherlands
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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Ajohn
sage
*****

Reged: 12/03/07

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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