Prism Diagonal and the Skywatcher 66ED

[Darren B]

After reading a lengthy thread and report on Star diagonal prisms on the cloudy Nights forum, I had to have a go at this and bought a cheap prism diagonal (metal nosepiece, plastic prism housing, metal eyepiece holder and prism looks uncoated).I tried it this morning with my Skywatcher 66ED with the prism and a dielectric diagonal. The eyepices I used were 30mm Ultima, 12.5 WO SPL & 5mm BO TMB. The moon and M42 were the main objects. First target was the moon. It was a bit hard to tell about false colour, sometimes I thought the prism had edge it but on second viewing with the mirror I wasn't sure. One thing I did notice the prism was sharper and objects seem to snap into focus more. By the end of the session it was starting to get light and I turned the scope on M42, using 5mm BO TMB giving 80x mag. Both digonals showed 4 stars of the trapezium and the view with the prism was sharper or better focused perhaps and the nebula stood out more. I'm hoping to go out this evening for further tests. One thing I have found with the prism is that you need to rack out the focuser further than a mirror. With my 66ED, virtually to the end, so an extension may be needed.

[Brinders]

Just taken delivery of a 1 1/2" prism from Smart Astronomy with the intention of using it on my little WO Megrez 72 in place of the WO 2" dielectric diagonal. Haven't had a chance to try it yet though, but I can see that it is multicoated. Just a little concerend about what you say about having to rack out the focuser on yourSkywatcher 66ED - I already have to when using my Celestron 1 1/2" mirror diagonal. I was hoping the prism might not have been as bad. No problems with the WO 2", is that because of the larger size I wonder?

[Darren B]

The length of my focus tube is 6cm and my eyepieces came into focus at around 5.5cm +/-2mm. I thought this had to be mentioned. I used an extension, basically an old barlow with the lens removed. Since I'm planning to do lunar observing with this scope, so far I think the difference could be worth it. Give it a go.

[Darren B]

Had a quick go last night. Objects viewed were M57, M31/M32, Jupiter and the Moon. The ring nebula looked a bit sharper with the prism, with the galaxies it was hard to tell. With Jupiter I saw the biggest difference. With it being so low in the sky I wasn't expecting to much. With the mirror, I could see the two belts though not clearly due to the atmosphere. I also some CA around the planet, mainly red. This again is due to Jupiter's low position. With the prism the belts stood out more and there was a lot less red surrounding the planet.

[Neon]

I brought a basic 45 degree angle prism from Scopes 'n Skies over the weekend, with the intention to use it for terrestrial observation. Maybe it's just me, but I think the prism loses contrast at a greater rate than my 90 degree mirrors.

No real difference around the 15x or 20x magnification, but as I took my scope up to 37x and then 60x the difference became more pronounced. Nothing really debilitating, but just an obvious difference. That's not a major concern for me, since I doubt I'll be using it much above 37x and even then the loss of contrast isn't an issue, but I don't think I'll use it for astronomy purposes.

That was with my Semi Apo 80mm refractor, so obviously it may differ with other telescopes.

Mark

[John]

I brought a basic 45 degree angle prism from Scopes 'n Skies over the weekend, with the intention to use it for terrestrial observation. Maybe it's just me, but I think the prism loses contrast at a greater rate than my 90 degree mirrors.

No real difference around the 15x or 20x magnification, but as I took my scope up to 37x and then 60x the difference became more pronounced. Nothing really debilitating, but just an obvious difference. That's not a major concern for me, since I doubt I'll be using it much above 37x and even then the loss of contrast isn't an issue, but I don't think I'll use it for astronomy purposes.

That was with my Semi Apo 80mm refractor, so obviously it may differ with other telescopes.

Mark

Thanks for your notes on this Mark.

I think the 45 degree prisms have more internal surfaces that the light has to bounce off than the 90 degree ones which probably accounts for the loss of contrast compared to your 90 deg mirror diagonal. My 45 deg prism certainly has a longer light path through it than a 90 deg one.

TBH the 45 deg prisms have never been thought much good for astro purposes - very useful for birdwatching though

John

[Lady Isabella]

Hello

I use a prism diagonal with my Takahashi refractors, as the scopes are colour free it has no effect on colour. However, I do tend to find that the prism does seem to pick -up more subtle planetary detail when compared to a mirror version.

[A McEwan]

Interesting to hear the observations of the prisms showing more. I', curious - why should this be?

[Lady Isabella]

Hi

In my case, several people who have used my set-up, seem to think that the prism

is acting like an extremely mild filter. The difference between the prism and mirror is subtle indeed, but the other night on Jupiter I could pick out the broken North Temperate belt more easily after switching from the mirror to the prism.

It has also been mentioned to me that the prism acts as a seal, stopping changes

in tube currents when changing eyepieces. I remain a bit skeptical about this. Putting a filter on the nose piece of the mirror diagonal would have the same effect if this method is true.

Also, refractors tend to be slightly under-corrected. While a prism is said to be over-corrected for spherical aberration, and it is said that the two can null the spherical correction. Again, I not sure how much an effect this will have, as I also thought that an under-corrected lens will improve as the scope reaches an ambient operating temperature.

[Darren B]

I have heard about the under corrected doublets and over corrected prisms, also another point is that while achro's show purple, apo's doublets tend to show red which our eyes have difficulty seeing in the dark. Therefore the lack of seen CA on bright objects. The prism diagonal corrects this and bring the red end of the spectrum into focus. You could say that the prism is the third glass element. I have heard someone trying a prism with a triplet and got red false colour on bright objects where as with a mirror, he had none. I hope this make some sense.

[Lady Isabella]

Some of the best Zeiss APO scopes were designed to be used with a prism diagonal,

and for years Takahashi have been making a prism for use with their scopes.

Last night I used my £40 prism on the Tak triplet scopes. Even on Vega there was zero false colour in magnifications ranging from 25x up to 300x.

[dweller25]

Does anyone know where you can buy a 90 degree non erecting prism ?

[Lady Isabella]

Hi

I've been using one of these with my APO scopes and it gives very nice images.

http://www.telescopehouse.com/acatalog/info%5f07202%2ehtml

I prefer it over my WO diagonal that cost twice as much.

If you want better quality then you can splash out on a Takahashi one, but I doubt that it will give better views.