AE classical Cassegrain 12" F:5/20 telescope: cnanges in focal position using Barlow/Powermate or eyepiece projection methods.

[astrognosis]

First posting and not sure if this is the best section.

I have an AE classical Cassegrain 12" F:5/20 telescope for which I'm going to mount into a new truss tube once I do some initial testing with the existing original skeleton tube. I'm going to have the focal length of both primary and secondary measured and importantly the centre of radius of the primary   mirror measured so that I am able to calculate the  optimum spacing  between the primary and secondary.  The telescope is planned to be used for high resolution  lunar and planetary imaging.

My question:

Thinking ahead I will have to choose a suitable focuser, its positioning and allow for  changes in focusing positions when using with Barlow/Powermate   eyepiece projection.  So I need to know ;

1.  with these optical additions is focus movement inwards or outwards,

2.  what lengths would these movements be  in general.

 

I'd also be interested to get in touch with  anybody who has  one of the AE classical  Cassegrains.

Many thanks,

Eric

 

 

[CraigT82]

Some useful info here on barlow focal lengths and how to calculate...

https://britastro.org/node/15666

 

[JeremyS]

10 hours ago, astrognosis said:

First posting and not sure if this is the best section.

I have an AE classical Cassegrain 12" F:5/20 telescope for which I'm going to mount into a new truss tube once I do some initial testing with the existing original skeleton tube. I'm going to have the focal length of both primary and secondary measured and importantly the centre of radius of the primary   mirror measured so that I am able to calculate the  optimum spacing  between the primary and secondary.  The telescope is planned to be used for high resolution  lunar and planetary imaging.

My question:

Thinking ahead I will have to choose a suitable focuser, its positioning and allow for  changes in focusing positions when using with Barlow/Powermate   eyepiece projection.  So I need to know ;

1.  with these optical additions is focus movement inwards or outwards,

2.  what lengths would these movements be  in general.

 

I'd also be interested to get in touch with  anybody who has  one of the AE classical  Cassegrains.

Many thanks,

Eric

 

 

Sorry, can’t answer your Qs, but this sounds a fascinating project. AE cassegrains have a superb reputation. It would be great if you could keep us updated on progress.

Any pics?

Edited February 21, 2021 by JeremyS

[andrew s]

8 minutes ago, JeremyS said:

Sorry, can’t answer your Qs, but this sounds a fascinating project. AE cases grains have a superb reputation. It would be great if you could keep us updated on progress.

Any pics?

Don't you just live ATI correct!

Yes pics please. @Peter Drew should be able to advise you.

Regards Andrew 

 

Edited February 21, 2021 by andrew s

[astrognosis]

Hi Jeremy Andrew and Peter,

The telescope came with its original  AE B type mount.

Looking through the Classical Cassegrain mirror formula I have come to realise that I should really try and find the radius of curvature for the secondary and then I will be theoretically working to  the optimum.  I can send the secondary off to get this measured.

I have also this last night been working on how changes in temperature  alter the focal position.  I wish to see if it's worth changing the OTA to a truss design using carbon fiber tubes as opposed to the existing original aluminium.

Once I get the CR value I will try out using a couple of pieces of software to get a suitable configuration.  Hence also the need to know how far the focal point moves from prime focus with various projection methods. 

Part of the input will be the size of the FOV which I require as not vignetted.  The telescope is unlikely to be used visually so I could get away with a smaller on to suite the size of imaging chips.   I was thinking of  perhaps 15mm  or  10 arc mins.  This together with back focus requirements  may help in easing the configuration requirements.

 

Eric

[dweller25]

Great project 👍

[Peter Drew]

I can't specifically directly answer the optical characteristics as these will vary being that the optical sets will be unique to each telescope.  I can confirm that amplifying additional optics will give increasing back focus dimensions and focal reducing elements, the reverse.  As a rule of thumb, the increase of back focus due to an inter mirror change is the amount of change x the amplification factor squared and the amplification factor in your case will be x4.  All Cassegrain types are susceptible to  inter mirror variation due to varying temperature, one of the reasons one seems to always be fiddling with the focuser on this design so any mechanical modifications that can be included to mitigate this are worthwhile.  When making Schmidt cameras, the most sensitive of Cassegrains, I used to separate the optics on Invar rods!         🙂 

[astrognosis]

Hi Peter,

thanks for your comments.  I have spent the night working out the change in mirror separation due to thermal expansion/contraction.  At F:20 I would also have the advantage of critical focus zone which is 1mm and it would take a temperature change of about 2.4 degree C  at prime focus to  span this zone.   This  will be ok to test the telescopes optics.  Construction of a new OTA using carbon truss once I'm happy with the  optics and configuration will improve these figures  significantly.  I'm hoping inquiries with Vacuum Coating who test mirrors to get me a radius of curvature of the secondary will go along way to  answering my  mirror separation,

you may recall in the mid-80's you finished off a 16"  F:6 Newtonian rotating tube for me, by visiting my home , a job passed on by Rob Miller before he left for the States.  It never really got used.  The mirror has now been placed into a carbon truss OTA and mounted onto a Paramount ME  and the mirrors  collimated. I'm waiting for it to clear up long enough to give it a first light try out.  The mirrors were by Jon Owen.  It will be used mostly for an automated super nova hunting with CCD.

 

Regards, Eric

[Peter Drew]

Hello Eric, yes I remember it well.  Good to know that you are still "up and doing".   Cheers.  Peter.       🙂