Erecting prism woes

[Ukastronomer]

I bought one of these

https://www.amazon.co.uk/Erecting-Telescope-Adaptor-Os-336276/dp/B00AZKNIWW

mainly because I wanted to also use my Ed 72mm as a birding scope, but as I have found out it will not work with either scope (in the images)

Is it just me, can not focus over 50', appears not enough inward travel on focusser and I am not going to start cutting my tube down :(


Thanks

[astro744]

The light path of the Amici prism is longer than a standard diagonal. Your only solution may be to get a 1.25" version or just use a standard diagonal and view with left/right reversed images.

Note before you go changing find out the light path length of the 2" version, 1.25" version Amici and any standard diagonal you have (especially one that works) so you know how much travel is left on the draw tube when focussing at infinity.

What eyepiece are you using with the 2" Amici? Try a 1.25" eyepiece with an in-travel adapter such as http://www.televue.com/engine/TV3b_page.asp?id=23&Tab=_intrav

[Ukastronomer]

So what is the point of the diagonal if they don't work,

thanks for the link

[astro744]

Some refractors have more focuser inward and outward travel than others.

I'm not sure about the path length of the 2" Amici diagonal you have but the Baader one is 121mm plus an additional 10mm for the 1.25" adaptor.

See https://www.baader-planetarium.com/en/accessories/optical-accessories/prism-and-mirror-star-diagonals/baader-2%22-amici-erecting-prism-45°-with-2%22-sc-thread-and-2%221%C2%BC%22-reducer.html

This could only be a few mm too long but enough for you to not reach focus. However 121mm is long compared to many standard 2" diagonals which have around 110mm path lengths. (Brands can vary quite a bit depending on construction).

I use the Tele Vue 60 deg. diagonal for terrestrial and astronomical viewing on my Tele Vue 60, (left/right reversed). It also has a slightly longer path length compared to the 90 deg. version but works well on my 'scope.

[Ukastronomer]

Thanks

A 1.25 works fine, it IS inward travel that is the problem, I have found this out from the help here

[erick]

This "Ultra low eyepiece adapter" from Astrosystems should work. I expect it is cheaper than the Televue and it would seem to provide more in-travel.

https://www.astrosystems.biz/eyepieceadapter.htm

I've just ordered two for the same reason - just put a 45 deg 2" erecting image diagonal on a refractor and I have the same problem. These plus 1.25" eyepieces should sort the problem.

I already have one somewhere in my stuff but cannot find it :sadeyes:

[Ukastronomer]

:thumbsup::thumbsup::thumbsup: