TS/Gso 14" Truss Rc backfocus with CCD47 reducer

[krisch2]

Anyone have this setup that can help me to figure the correct backfocus on the 14" Rc with the 0.67 Reducer whrn the ccd chip is 85mm from the Reducer? 

I'm planning to setup the Rc for a dual camera setup with a flip mirror for ccd deepsky and CMOS planetary imaging. 

But I'm not sure if there is enough backfocus to introduce a flip mirror between a low profile focuser and the reducer? 

 

Any feedback would be appreciated. 

 

[kens]

The formula to work out the reduction for a given spacing on the CCDT67/CCD47 is:
reduction = 0.65 - 0.3 x (spacing - 90) / 90

And the resulting reduction in backfocus from no reducer is:
delta = spacing x (1 / reduction - 1)

so at 85mm:

reduction = 0.67x

delta = 42.5mm

[krisch2]

Hi Ken

This information is VERY helpful.

Can I ask where you have this information from?

Anyway, this is much more information than what the supplier can give me!

Thanks 

Kristian

 

 

[Northernlight]

On the ccd67 the optimal spacing from ccd is 85mm. I use 85mm spacing for mine and get pefect stars to corners on my 8" RC

[krisch2]

Hi  Northernlight

Thanks for your information!

Do you get a focus delta -42,5mm from the 8" RC original backfocus when you have the 85mm spacing?

 

Kristian

[kens]

47 minutes ago, krisch2 said:

Hi Ken

This information is VERY helpful.

Can I ask where you have this information from?

Anyway, this is much more information than what the supplier can give me!

Thanks 

Kristian

 

 

I think someone posted it over at Cloudy Nights and from there I plugged it nto my own spreadsheet of astro calcs for future reference. I'll see if I can find the original reference

[krisch2]

Hi Ken

Ok, is this a general calculation for reducers or only for the CCD67/CCD47 reducers?

I also made my own spread sheet to easy calculate the backfocus for my hopefully new setup!

No urgent need for the original reference, Ken.

 

THANKS!

 

[kens]

Actually a quick google search came up with this:

http://www.astro-physics.com/products/accessories/photo_acc/Telecompresssor-techdata.pdf

This says for the CCDT67:

reduction = (305-spacing)/305 (305 refers tothe focal length of the reducer in mm)

So not sure where I got mine from - I'm not clever enough to have worked it out myself

 And I've always understood the CCD47 to be the same as the CCDT67 - now I'm not so sure

[kens]

Ok - I think I got it by back calculation from this graph.

http://www.astro-physics.com/products/accessories/photo_acc/CCDT67 & 27TVPH.pdf

The formula from the AP paper is based on the spacing from the centre of the optics whereas the one I gave is for the spacing from the back flange. Sorry for any confusion.

[krisch2]

Ken

CCD47 and CCDT67 looks similar, but data from Astro Physics about the CCDT67 really differ from the data of the CCD47.

On Astro Physics site, they reccomend the CCDT67 for f/9 up to f/18. For f/8-f/9 and lower they reccomend 27TVPH. TS says CCDT67 designed for f/9 and slower so they give the same info.

According to AP the working distance of CCDT67 is 101mm for 0,67x reduction, but TS says 85mm, so here they are not giving the same info.

Have you check your true f/ratio?

Can the working distance be different for a refractor system and a reflector system?

But I'm now very curious where you got the formula that gives 0,67x @85mm spacing  

 

 

 

[krisch2]

Didn't see your last post before I posted mine.

Your explanation makes sense!

Thanks!