Jump to content


Photo

Suggestions for 2' Erecting Diagonals/Prisms

  • Please log in to reply
28 replies to this topic

#1 Bart

Bart

    Mercury-Atlas

  • *****
  • Posts: 2730
  • Joined: 28 May 2006
  • Loc: Somewhere near Charlottesville, VA

Posted 10 January 2013 - 08:48 PM

Suggestions for 2' Erecting Diagonals/Prisms would be appreciated.

Should this question be here in the Eyepiece forum or in the Equipment forum? :question:

Thanks
Bart

#2 ibase

ibase

    Aurora

  • *****
  • Posts: 4502
  • Joined: 20 Mar 2008
  • Loc: Manila, Philippines 121*E 14*N

Posted 10 January 2013 - 10:18 PM

Have had good experience with the William Optics 1-1/4" version:

Posted Image
Left - WO 45-deg erecting prism diagonal; right - generic 45-deg diagonal

It's as if a veil was lifted from the view compared to the generic diagonal at right above. Click here for WO 2" version.

Best,

#3 Bart

Bart

    Mercury-Atlas

  • *****
  • Posts: 2730
  • Joined: 28 May 2006
  • Loc: Somewhere near Charlottesville, VA

Posted 10 January 2013 - 11:22 PM

Thanks. The 2' looks great. Yet, I'm hoping to not have to spend that much. But, if that's the only one of quality available, I might have to bite the bullet and get it.

#4 Richard Low

Richard Low

    Viking 1

  • *****
  • Posts: 698
  • Joined: 27 Nov 2005
  • Loc: 1 deg N, GMT+8hrs

Posted 11 January 2013 - 07:55 AM

WO used to have a 2" 90 degree Erecting Prism Diagonal; you may be able to find one on the used market.

#5 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 43918
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 11 January 2013 - 08:29 AM

Suggestions for 2' Erecting Diagonals/Prisms would be appreciated.

Should this question be here in the Eyepiece forum or in the Equipment forum? :question:

Thanks
Bart


Bart:

Correct image prisms (all diagonals erect the image) typically have two issues when compared to a standard prism or mirror diagonal. First, a restricted clear aperture, this is means for low power, wide field viewing, there will probably be some vignetting.

The second issue is the loss of image sharpness at higher magnifications due to the fact that the Amici roof prism splits the light into two paths and then recombines it.

It is my understanding that the Baader T2 is the best in terms of image sharpness but it has a limited clear aperture. It is also quite expensive.

I do a fair amount of birding with my smaller refractors, I just use a standard mirror diagonal, the left-right reversal has never bothered me, in fact a correct image diagonal takes some getting used to.

Jon

#6 Hermie

Hermie

    Ranger 4

  • *****
  • Posts: 338
  • Joined: 20 Apr 2005
  • Loc: Cloudy HKG

Posted 11 January 2013 - 08:31 AM

Post deleted by Hermie

#7 neotesla

neotesla

    Apollo

  • *****
  • Posts: 1121
  • Joined: 18 Nov 2010
  • Loc: Canada

Posted 11 January 2013 - 09:46 AM

University Optics also has one as well...

http://www.universit...inch.html#PRISM

#8 Bart

Bart

    Mercury-Atlas

  • *****
  • Posts: 2730
  • Joined: 28 May 2006
  • Loc: Somewhere near Charlottesville, VA

Posted 11 January 2013 - 10:00 AM

Suggestions for 2' Erecting Diagonals/Prisms would be appreciated.

Should this question be here in the Eyepiece forum or in the Equipment forum? :question:

Thanks
Bart


Bart:

I do a fair amount of birding with my smaller refractors, I just use a standard mirror diagonal, the left-right reversal has never bothered me, in fact a correct image diagonal takes some getting used to.

Jon


But, doesn't a "standard mirror diagonal" cause the image to be upside down?

Thanks

#9 Eigen

Eigen

    Explorer 1

  • -----
  • Posts: 81
  • Joined: 26 Jul 2012

Posted 11 January 2013 - 10:54 AM

No, image is right side up, but reversed left-right.

I will mirror the previous opinion. I use my smaller SCTs with standard prism or mirror diagonals for terrestrial observation regularly, the left-right flip is not a big bother.

#10 Doug D.

Doug D.

    Mercury-Atlas

  • *****
  • Posts: 2736
  • Joined: 23 Aug 2005
  • Loc: Virginia

Posted 11 January 2013 - 11:02 AM

Jon is correct, standard diagonal reverses left and right but image should be upright. I use a Baader Amici prism diagonal with 2" click lock eyepiece adapter and nose for terrestrial and dark sky and I am quite fond of it. Plus, the T2 system gives you a lot of adaptability.

They aren't always in stock (like now for instance) but AlpineAstro has a good description on their site .

There was also a discussion on CN about the Baader Amici a while back.

There are only 2 cons as I see it. First, it is relatively expensive (but you get what you pay for in this case) and second, it is a 90 degree diagonal. That is fine for me but a 45 degree is arguably more comfortable for terrestrial viewing. I don't know of any 2" versions, however.

#11 DRodrigues

DRodrigues

    Vostok 1

  • -----
  • Posts: 146
  • Joined: 08 Aug 2011

Posted 11 January 2013 - 02:09 PM

Bart,
Have a look at http://www.pt-ducks.... image erectors since there is info that might interest you

#12 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 43918
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 11 January 2013 - 02:18 PM

Suggestions for 2' Erecting Diagonals/Prisms would be appreciated.

Should this question be here in the Eyepiece forum or in the Equipment forum? :question:

Thanks
Bart


Bart:

I do a fair amount of birding with my smaller refractors, I just use a standard mirror diagonal, the left-right reversal has never bothered me, in fact a correct image diagonal takes some getting used to.

Jon


But, doesn't a "standard mirror diagonal" cause the image to be upside down?

Thanks


Bart:

If the diagonal is vertical, the normal orientation, then the image will be erect but it will be reversed left and right, everything looks normal, natural, it's just that a bird facing north will appear to be facing south, not a real problem.

If there is no diagonal, then everything will be upside down and backwards but it only takes a simple diagonal to erect the image.

Jon

#13 Bart

Bart

    Mercury-Atlas

  • *****
  • Posts: 2730
  • Joined: 28 May 2006
  • Loc: Somewhere near Charlottesville, VA

Posted 11 January 2013 - 03:50 PM

Thanks guys, ya learn something new everyday!

#14 Lane

Lane

    Soyuz

  • -----
  • Posts: 3591
  • Joined: 19 Nov 2007
  • Loc: Frisco, Texas

Posted 11 January 2013 - 07:09 PM

WO used to have a 2" 90 degree Erecting Prism Diagonal; you may be able to find one on the used market.


I have the Orion branded version of this one. I use it for moon viewing sometimes since it makes the image match my detailed moon charts. It is also great for terrestrial viewing. It is actually pretty good, but I noticed on planets that you can definitely tell there is a loss of sharpness, not huge but noticeable. Sometime you see the line too at low power, the line where they glued the two prisms side by side to make this thing.

#15 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 43918
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 11 January 2013 - 07:20 PM

Sometime you see the line too at low power, the line where they glued the two prisms side by side to make this thing.



I think the prisms are one piece of glass. I believe the line comes from splitting and recombining the optical paths. It is most noticeable when splitting closely separated double stars.

Jon

#16 BillB9430

BillB9430

    Vostok 1

  • *****
  • Posts: 198
  • Joined: 02 Dec 2006
  • Loc: Illinois

Posted 13 January 2013 - 11:09 AM

Jon is correct about the cause of the "line". This part of an Amici or Schmidt prism is called a "roof" because two planes of the prism meet at a right angle. There is a horizontal diffraction spike on bright astronomical objects caused from light hitting the "ridge" where these planes come together. This is why even a high quality Amici or Schmidt prism is less than optimal for all astronomical viewing. The diffraction spike is not noticeable for terrestrial daytime viewing.

There are several prism arrangements that can be used as 2" erectors that do not have a roof, hence do not produce the diffraction spike. Porro type 1 and 2 straight through erecting clusters can be cemented up using 4 large low cost right angle prisms, but are not available as a finished commercial product. Smaller Porro Type 1 erecting systems are used in Porro prism binoculars. These provide straight through viewing with an offset. (See my past posts for a 5" scope with a 2" Porro Type 2 built in.) If you are able to build a diagonal rather than buy one, another type of right angle "roofless" diagonal that gives a "correct" image is a single Porro-Abbe prism. This prism can be cemented up from just two big right angle prisms. I built one for a friend who moved to Montana and has a great view of the mountains with his scope. I'll attach a photo. The main drawback to the Porro-Abbe diagonal is that to view the "correct" image, the observer must not only view at right angles vertically, but must also stand 90 degrees to the side of the scope. This awkward viewing position is likely why this sort of diagonal has never been commercially available, as far as I know. Lots of alternatives here that are interesting to consider. Hope this helps. - Bill

Attached Files



#17 Sarkikos

Sarkikos

    Hubble

  • *****
  • Posts: 16741
  • Joined: 18 Dec 2007
  • Loc: Per sylvam ad astra

Posted 13 January 2013 - 12:02 PM

I have an erecting prism diagonal in my 70mm finder scope. I received it with a used ST80. I'm not sure of the original source of the diagonal.

The 70mm achromat serves as a 15x finder scope on my 10" Dob when I go to dark sites. I've never noticed any line from the diagonal going through the FOV, but then I've never looked for one either. In any case, there is no obvious line from the diagonal.

I never use this finder scope for high power views, much less try to split any doubles with it. But it does serve as a very nice rich field telescope for objects that are too big for the main scope.

IME & IMO, an erect image non-reversed view is the best for star hopping and finding DSO. Personally, I don't like to see a reversed image. I'd rather deal with one that's upside down. The only time I don't mind a reversed image is when viewing planets or the Moon.

Mike

#18 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 43918
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 13 January 2013 - 01:03 PM

I have an erecting prism diagonal in my 70mm finder scope. I received it with a used ST80. I'm not sure of the original source of the diagonal.

The 70mm achromat serves as a 15x finder scope on my 10" Dob when I go to dark sites. I've never noticed any line from the diagonal going through the FOV, but then I've never looked for one either. In any case, there is no obvious line from the diagonal.

I never use this finder scope for high power views, much less try to split any doubles with it. But it does serve as a very nice rich field telescope for objects that are too big for the main scope.

IME & IMO, an erect image non-reversed view is the best for star hopping and finding DSO. Personally, I don't like to see a reversed image. I'd rather deal with one that's upside down. The only time I don't mind a reversed image is when viewing planets or the Moon.

Mike


Mike:

The line is a visible spike through a bright star at high magnifications, it most affects the planets and double stars. Several years ago I picked up another 80mm F/11 Achromat, a Vixen I believe. Viewing the double-double, it was just not sharp, the split was marginal. Typically this is an easy mark for an 80mm F/11. Then I realized that the pair was reversed left to right from what I was used to. It had a 90 degree correct image diagonal. That was quickly replaced with a star diagonal and all was well.

At low powers that's not a problem but what is a problem is that most correct image diagonals including the Orion's I have owned have a limited clear aperture. My Orion 45 degree has a clear aperture of 20mm, an issue if one is looking for the widest field of view.

As far the need for erect/correct images with a finder, I think we all deal with this differently. I am fine with an inverted, rotated image, maybe if I had started with a correct image finder, I would feel the need for one but my mind seems to handle the different orientations without much difficulty.

Jon

#19 Sarkikos

Sarkikos

    Hubble

  • *****
  • Posts: 16741
  • Joined: 18 Dec 2007
  • Loc: Per sylvam ad astra

Posted 13 January 2013 - 02:13 PM

Jon,

Yes, I agree that an Amici erecting prism isn't best for high-power work. For that I have mirror diagonals. I always use mirror diagonals in my 90mm Mak and 150mm Mak.

But I find that a decent quality erecting prism is best for rich field telescopes and finder scopes. As I said, I've never seen the bright line artifact in my finder and I don't use it for high power - or even mid-power - only low power.

Mike

#20 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 43918
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 13 January 2013 - 02:25 PM

Jon,

Yes, I agree that an Amici erecting prism isn't best for high-power work. For that I have mirror diagonals. I always use mirror diagonals in my 90mm Mak and 150mm Mak.

But I find that a decent quality erecting prism is best for rich field telescopes and finder scopes. As I said, I've never seen the bright line artifact in my finder and I don't use it for high power - or even mid-power - only low power.

Mike


As I said, at the low powers, its the restricted clear aperture. Consider the NP-101 + 31mm Nagler, it's a wonderful richest field scope but a correct image prism would vignette the 4.5 degree TFoV. Used on an alt-az mount, the views are reversed left to right, it never bothers me.

Jon

#21 Sarkikos

Sarkikos

    Hubble

  • *****
  • Posts: 16741
  • Joined: 18 Dec 2007
  • Loc: Per sylvam ad astra

Posted 13 January 2013 - 03:03 PM

My 70mm finder scope with a 20mm illuminated eyepiece gives me 4.7 degrees TFOV. That's plenty for rich field open clusters and such, and great for star hopping. Never seen the bright line, never seen vignetting. It works very well.

Mike

#22 ensign

ensign

    Viking 1

  • -----
  • Posts: 791
  • Joined: 16 Dec 2008
  • Loc: Southwestern Ontario

Posted 13 January 2013 - 04:30 PM

I have a William Optics 2" erecting diagonal and have used it in a 110mm refractor and am currently using it with an Equinox 80. The copy on the WO website used to say that this diagonal was designed specifically for astronomical use. I have compared the views side by side with my WO 2" dialectric diagonal and noted that the views are essentially identical, except, of course that one is mirror-reversed. I noted no difference in brightness and no lines bisecting any of the views.

I have to admit that this diagonal was a tad on the expensive side at around $240. They don't appear to be available new any more.

#23 Sarkikos

Sarkikos

    Hubble

  • *****
  • Posts: 16741
  • Joined: 18 Dec 2007
  • Loc: Per sylvam ad astra

Posted 13 January 2013 - 05:10 PM

Yes, I've also wanted a good 2" true erecting prism, with no reversed image. I know there are some 2" erecting prisms which reverse the image, and are considered good for astronomy. Some think they are better than mirror diagonals, at least for some telescopes. There are threads about this on the Refractor Forum.

But a true 2" erecting prism with no reversed image is something else.

Mike

#24 DRodrigues

DRodrigues

    Vostok 1

  • -----
  • Posts: 146
  • Joined: 08 Aug 2011

Posted 13 January 2013 - 05:38 PM

Consider the NP-101 + 31mm Nagler, it's a wonderful richest field scope but a correct image prism would vignette the 4.5 degree TFoV.


At http://www.pt-ducks....tm#Nikon_FSA-L2 I tested the Nikon FSA-L2 and it doesn't vignette with a 30mm UWA that has a similar field stop of the Nagler 31mm... ;)
You loose a bit of brightness and contrast but you gain a 3.5x zoom!!! :jump:

#25 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 43918
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 14 January 2013 - 06:24 AM

My 70mm finder scope with a 20mm illuminated eyepiece gives me 4.7 degrees TFOV. That's plenty for rich field open clusters and such, and great for star hopping. Never seen the bright line, never seen vignetting. It works very well.

Mike


Did you look at the erecting diagonal to see what the clear aperture is? At least look at it. As I said, I measured my Orion at 20.0mm, I have seen them much smaller but generally on inexpensive scopes.

Jon






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics