Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Collimating spider for convertible newtonian-cassegrain questions

  • Please log in to reply
3 replies to this topic

#1 myleskoppelman

myleskoppelman

    Sputnik

  • -----
  • topic starter
  • Posts: 37
  • Joined: 03 Aug 2024
  • Loc: Minneapolis, MN

Posted 05 October 2024 - 03:50 PM

I have an old Takahashi CN-212 convertible telescope from 1996. Heres the link to the manual: https://www.astropic...al_english.pdf 

 

It says on page 17 that if you switch between cass and newt mode you will have to recollimate. That makes sense but what i'm wondering is do you have to center the spider in newtonian mode? On page 20 it talks about adjusting the spider in cass mode, but theres no mention of adjusting the spider for newtonian mode. Does this mean that once ive centered the spider in cassegrain mode, It should be fine for newt mode, and I just have to collimate the mirror?

 

More generally, say for a regular newtonian, does the spider have to be perfect centered? When I center the spider in cass mode, looking at the secondary mirror in newt mode it looks like the image i've attached where you can see one of the sides of the mirror since the spider isn't perfect, but does that matter?

 

I just want to know essentially if I have to center the spider every time I switch modes or if centering it in cassegrain mode is enough. Thanks!



#2 TOMDEY

TOMDEY

    James Webb Space Telescope

  • *****
  • Posts: 16,997
  • Joined: 10 Feb 2014
  • Loc: Springwater, NY

Posted 05 October 2024 - 04:24 PM

One would hope and expect that a Premium Convertible telescope would be extremely close to optimal alignment when switching configuration. This can (and should) be achieved by providing extra "co-alignment" degrees of freedom in the mirror mounts... so that there will be no subsequent/operational need to fiddle with the spider or anything else. Some of the professional observatory telescopes comprise this rapid changeover feature. I was involved in a lot of that kind of stuff where I worked.   Tom



#3 myleskoppelman

myleskoppelman

    Sputnik

  • -----
  • topic starter
  • Posts: 37
  • Joined: 03 Aug 2024
  • Loc: Minneapolis, MN

Posted 05 October 2024 - 06:05 PM

One would hope and expect that a Premium Convertible telescope would be extremely close to optimal alignment when switching configuration. This can (and should) be achieved by providing extra "co-alignment" degrees of freedom in the mirror mounts... so that there will be no subsequent/operational need to fiddle with the spider or anything else. Some of the professional observatory telescopes comprise this rapid changeover feature. I was involved in a lot of that kind of stuff where I worked.   Tom

what does co-allignment entail exactly?



#4 TOMDEY

TOMDEY

    James Webb Space Telescope

  • *****
  • Posts: 16,997
  • Joined: 10 Feb 2014
  • Loc: Springwater, NY

Posted 05 October 2024 - 08:44 PM

what does co-allignment entail exactly?

The term co-alignment, in this context, is similar to our more familiar use of ~parfocal eyepieces~. Remove one, stick in the other --- and it's ready to use. For the convex aspheric classic cass secondary, the 5 "hard" degrees of freedom are roll, pitch, x, y, and z. For the Newt folding flat, the 3 "hard" degrees of freedom are tip, tilt and z of its surface normal vector, and the 2 "soft" degrees of freedom are in-plane x and y. For parfocal eyepieces, only yaw doesn't matter.    Tom




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics