Diagonals

[alan potts]

A short time before Christmas I noticed that FLO were advertising Williams Optic diagonals with 1/12PV mirrors, I ordered a new one through my local dealer as I never seem to have enough of them, this will be 4. Whether the WO that I have yet to pick up from Sofia is the new spec remains to be seen.

Now I see Teleskop Services are doing the same degree of PV on two lines, are Tele-Vue going to join the optical flatness race, mine are only 1/10 ?

Alan 

[John]

I thought Zeiss prisms were the way to go these days for the ultimate in planetary views ?

[Tinker1947]

i think as with a lot of other stuff that gets hyped up (PC's a good one) the diffrence between a 1/10 and a 1/12 is all in the mind for viewing through would there be any noticable diffrence, i know with PC's the latest Video card may sound nice costs more money but doesn't look any different to one half the price...

[alan potts]

I am seriously thinking of a Zeiss Prism, after all they are only the same price as the TV diagonal 329 euros. I would like to now arrive a situation where I have a diagonal for every scope that uses one. I tend to do astronomy on a scope rotational basis and I always pick the one that has no diagonal in it, so when the WO arrives that is the job sorted until another scope comes along.

If I get one it will be my first and probably last Zeiss buy.

It just goes to show I don't have a clue what I am doing I thought I posted this in Eyepieces, just spent 10mins looking for it.

Alan

[Tinker1947]

Do you have a Baader i used one with my SCT, it showed a nice clear view, the Click-Lock really was a good addition....

http://www.firstlightoptics.com/diagonals/baader-clicklock-2-dielectric-star-diagonal.html

[Macavity]

I suppose, at some stage in my unillustrious career, I was supposed to understand "errors" 

I would have to think *very carefully* about all these quoted values re. optical quality... 

As above, computers depend on the "weakest link". Recently I became interested posts on 

ultimates of visual observing. I Googled "Sirius Pup". Not convinced it was pertinent. lol 

Aside: Am about to by a "click lock". No point in collimating Fast Newts without such things?

[YKSE]

Alan,

If you're getting another diagonal, I'd assume that a prism is better than another dielectrical one, simply because it might should you some differences.

[Tinker1947]

I suppose, at some stage in my unillustrious career, I was supposed to understand "errors" 

I would have to think *very carefully* about all these quoted values re. optical quality... 

As above, computers depend on the "weakest link". Recently I became interested posts on 

ultimates of visual observing. I Googled "Sirius Pup". Not convinced it was pertinent. lol 

Aside: Am about to by a "click lock". No point in collimating Fast Newts without such things?

I have 2" version on both my Newt and Dob wouldn't do without them......

[jetstream]

I thought Zeiss prisms were the way to go these days for the ultimate in planetary views ?

And star color IMHO. They also add very little scatter to the path and allow somewhat higher mags to be used because of it.

[John]

And star color IMHO. They also add very little scatter to the path and allow somewhat higher mags to be used because of it.

I'm getting sold on one of these now. I assume that you get the T2 Baader Zeiss body / prism and then add a click lock adapter on one side for the eyepiece and a T2-2" barrel adapter on the other ?

[jetstream]

John, there are two versions that have the Zeiss prism, mine is the 2" version, with their standard 2" EP holder with brass compression ring. Some say the t 2 version has even less scatter, but mine shows very little. Baader uses the SCT threads in my body (wedge too) and a clicklock can be fitted easily.

When buying, getting the part #'s right is needed as Baader also sells a non Zeiss version, but this should be good too. I think BigMak had both ( Zeiss's), not sure which one(s) he has now.

[jetstream]

Mine is a "PRISM-2" # 2406010

The T 2 is T2-01B, # 2456095

Both have Zeiss prisms, the T2-01B is just the body, needs the EP holder and nosepiece. Hope this helps.

[John]

Thanks Gerry - very useful information for me and anyone else contemplating these items

[Mark at Beaufort]

Having a TeleVue Everbrite diagonal I thought I had just about the best diagonal until I read this thread. I suppose we are talking about this diagonal - http://www.telescopes.com/telescope-accessories/diagonals-and-prisms/baaderplanetarium125inchprismdiagonal.cfm

I going to the States in a few months to visit my son and his family - I might order one if it is that good.

[jetstream]

Mark, the one in the link might be the Baader 245 6005K that Baader sells for 115 Euros. These are not the same. I'm not saying their bad, just not the same. http://www.baader-planetarium.de/sektion/s28/s28.htm

[alan potts]

Mark,

Me too I have 3 of them, they are well made though and can't be that bad surely. I have a feeling Zeiss will be selling 2 more diagonals soon though.

[John]

The Baader product range can be quite confusing !

My current diagonals are a TV Everbright and an Istar. Both are mirror diagonals and seem extremely well made. They perform very well too, to my eyes, so I'm going to be cautious here to avoid the possibility of chasing a slight performance edge that I'm not going to benefit from 99% of the time or even worse a theoretical performance edge that I don't notice 100% of the time.

Thats not saying I'm not considering a quality prism diagonal but I'm not about to throw £'s at it until I'm happy that I know the pros and cons of the different options.

[AndyH]

There's the top end CARL Zeiss @ €329 / $439 . Does incl nosepiece & 2" eyecup.  http://www.teleskop-express.de/shop/product_info.php/info/p612_Carl-Zeiss-2--Diagonal-Prism-in-CNC-housing-from-Baader.html or http://agenaastro.com/baader-2-90-star-diagonal-zeiss-prism-2406010.html

Then the Zeiss Jena @ €185 / $259 . Does not incl nosepiece & eyecup. http://www.teleskop-express.de/shop/product_info.php/info/p609_Baader---Zeiss-90--diagonal-prism---T2-thread-on-both-sides.html or http://agenaastro.com/baader-t-2-prism-star-diagonal-zeiss-prism-t2-01b-2456095.html

Finally the Baader prism(no mention of Zeiss) @ €95 / $129 . Does not incl nosepiece & eyecup.  http://www.teleskop-express.de/shop/product_info.php/info/p605_Baader-T2-diagonal-prism-90----dual-T2-connection---short-back-f.html or http://agenaastro.com/baader-t-2-prism-star-diagonal-t2-01c-2456005.html

Not forgetting the Takahashi 1.25"/31.75mm prism @ roughly £70 from UK Tak dealer trutek-uk.com . Very nice prism*. Whilst not as wide a clear aperture as the above offerings, perfectly fine if just using 1.25" eyepieces. From memory, the c/a is at least 27mm.

Be aware all the above are Porro Prisms. Not to be confused with the amici type.

Andy.

* I've been using the wee Tak for many years, along side a bigger Tak with 34mm c/a and I believe to a similar quality to the Zeiss type. Originally bought to cure back focus problems, I soon realised they were the equal, at least, to my mirror diagonals. Tres pleased with them on my slow scopes(I have no other type;))

[Ruud]

Roland Christen of Astro-Physics made a few interesting diagonal observations

First, he says that 1.25" dielectric diagonals aren't that good because of curved edges. Dielectric coatings cause edge curving because they are so many layers thick. It's like snow lying on a roof: the edge is not sharp, but round. Having a curved edge only 0.625" from the centre is just too close in according to Christen. 

For 2" diagonals a curved edge will not deteriorate the view appreciably because being at the edge of a 2" beam, the curved edge only influences the widest field you can see in your 2" diagonal. This coincides with low magnifications, and at low magnifications wave front errors don't affect image sharpness very much.

Second. For higher magnifications, the centre part of the mirror is used. There the mirror is at its best, and at higher magnifications the edge of the diagonal is not involved in the image at all, being blocked by the field stop of the eyepiece.

Third, In a telescope a 1/10th wave diagonal usually performs at 1/40th wave accuracy. This is because the light beam of any individual star coming from the objective is quite narrow by the time it reflects off the diagonal. Each star, in a sense, reflects off its own little piece of diagonal and small bits of the surface cause less of a wave front error than the diagonal as a whole.

That's what Christen says, but I forget where he said it. I hope these are useful considerations for those who are thinking about a new diagonal.

The best reason for getting dielectric is for the durability of the surface and the easy cleaning that comes with it. That dielectric diagonals reflect 99% rather than 95% is a nice bonus, but not really visible. (The difference is 0.045 magnitudes.)

[Mark in Macc]

I have the T2 version and it is a quality piece of kit. Beware however that the T2 version does not have a 2" aperture. It is only 35mm. I use mine with binos, so it doesn't matter for me. I bought a 2" Click lock and a 2" nosepiece. You will need adaptors and a spacer to fit these properly. I can post details if anyone is interested.

[Mark in Macc]

Should this post be in the eyepiece section?

[GavStar]

Mark,

I have a T2 Zeiss arriving this week and also have got the 2 inch click lock and nose piece. I have on order a t2 to sc adapter for the clicklock - do I need more bits?

Thanks

Gavin

[Mark in Macc]

Hi Gavin, the 2 components will fit but what I found was that the clicklock is quite wide and interferes when you push the nosepiece into the visual back. The nosepiece comes with a stop ring to seat against the visual back, but the clicklock gets in the way. When you get your bits, try it out and you will understand what I mean. I bought a spacer to move the stop ring position away from the clicklock.

[Dave In Vermont]

I have 3 of the William Optics. All-metal ones. I get enough propaganda on the wonders of carbon-fibre in my bicycle work as stands. I couldn't care less. Sure it weighs a lot less, but when it breaks - it's like glass. Shattered. Lots of fun on your new 5000-quid bicycle - when you have to sweep it off the roadway with a dust-pan. I don't want to know about the "dropped-diagonal" tragedy.

Anywho - I LOVE my WO-diagonals! I've never tried the TeleVue offerings, or the other ones mentioned. But to use an old "Yankee Adage"- "If it ain't broke - don't fix it."

My Two-P,

Dave

[AndyH]

In case anyone is wondering what is being talked about on this thread, here are a couple of links to threads that started the prism discussion......

All started here, by BillP(Bill Paolini) http://www.cloudynights.com/topic/443262-eyepiece-performance-using-a-prism-diagonal/

Then continued to this 18 page thread http://www.cloudynights.com/topic/448635-mirror-dielectric-prism-performance-comparison/

Rather long discussions. If you want to skim, it may be an idea to first flip through the pages to Bill's posts, especially the ones where he jots down his notes after a session, trying out various combinations of prisms/eyepieces and scopes. As always with BillP's posts, a fascinating read.

Andy.