Erecting prism diagonal for astro?

[Ratlet]

Just wondering if anyone has used any of the erecting prism diagonals on FLO for astronomy?  Lot of the stuff online seems older and possibly doesn't reflect (hah) the current state of things.

I'm going to slap one on the back of my Tair 3 300mm f4.5 and see how it fairs and would like to not have the diagonal be a weak link.

As it's a telephoto there's only about 45mm, maybe 50mm when I remove the m42 mount to play with.

I realise this isn't the greatest idea ever but the Tair has great image quality and low chromatic aberration so from dropping a few quid on a diagonal I could get another scope to play with.

 

[markse68]

Do you mean the Amici prisms? There was a quite detailed post about them a while back from Zeta Reticulan-

Mark

[NGC 1502]

I tried an erecting prism once for a refractor. My idea was to get a “correct” view, same as you get with binoculars or birding scope.  This gives more intuitive star hopping and navigating the moon’s features.

It works as above.  Decent views at low power, but at medium to high power the sharpness was compromised.

Could be ok for low power terrestrial views.

Possibly the TeleVue 45 degree erecting prism may be better. You could always phone them to ask, they don’t take emails. You will get an honest answer, not sales pitch.

Edit for correction- just checked, the TV erecting prism is 60 degrees.

Ed.

Edited October 30, 2022 by NGC 1502
Correction

[Jules Tohpipi]

This is where I've always found prism terminology confusing - when in the context of diagonals. I get that erecting means the image will be the right way up but does it necessarily mean left and right will be corrected also? I'm guessing not as some prism diagonals - such as a Celestron and the Tak - don't seem to be left/right corrected.

Generally speaking, it seems most people/websites/manufacturers don't stick to any standardised terminologies to define the nature of prism diagonals; therefore, you have to read any accompanying text to know whether they are L/R corrected (FLO is very good at clarifying this). Most get labelled 'erecting' at best. But 'Amici' often gets dropped (if it applies). Whereas, for example, finder scopes are more likely to be labelled RACI which makes the situation more clear from the outset.   

I found this thread comparing diagonals an interesting read.

 

Edited October 31, 2022 by Jules Tohpipi

[NGC 1502]

It’s been a long time I read up about erecting prisms. But my understanding is that the idea is to use an erecting prism with an astronomical refractor.  Such a refractor used without any diagonal (often called straight through viewing) gives an upside down view, so birders for instance won’t want that.  They may be ok with using a regular 90 degree (prism or mirror) diagonal, this does give a “right way up” view, but left and right reversed, and this may be acceptable.  If a “correct view” is required with an astronomical refractor (as seen by our eyes, binoculars, spotting scope) then an erecting diagonal is the answer.

The TeleVue 60 degree diagonal or a 45 degree erecting diagonal gives a “correct view” and at the same time a more comfortable access to the eyepiece for daytime terrestrial viewing.

[Ratlet]

I picked up a cheap circle T prism diagonal to try out.

I wasn't so fussed about whether it was a correct image or not and more that I minimised the optical path

[Louis D]

On 30/10/2022 at 11:22, Ratlet said:

As it's a telephoto there's only about 45mm, maybe 50mm when I remove the m42 mount to play with.

Regular 1.25" right angle prisms have a 63mm to 67mm optical path length.  I would think an Amici prism might add a bit to that.  It certainly won't be any shorter.  You might be able to make a straight through finder, but I have my doubts about a RACI finder.

[Ratlet]

11 hours ago, Louis D said:

Regular 1.25" right angle prisms have a 63mm to 67mm optical path length.  I would think an Amici prism might add a bit to that.  It certainly won't be any shorter.  You might be able to make a straight through finder, but I have my doubts about a RACI finder.

Cheers for that.  Hmmm.  Glad I didn't spend too much on the prism.  Hopefully get the focal point thrown enough with a Barlow to get it to work.