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Prism or mirror for FS-102?

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#26 peleuba

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Posted 06 March 2020 - 09:34 AM

If you use a extension tube to get the focus it is wise to use the diagonal first and then the tube. This way you use a much larger area of the mirror, which reduces additional aberrations.

 

I am late to see this as the post is 2 months old...  

 

I don't understand this point of view.  On a mirror, the smaller the area in use, the lower the total aberrations will be.  How can you say a larger surface area contributes less error?  

 

What am I missing.

 

Thanks.


Edited by peleuba, 06 March 2020 - 04:07 PM.

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#27 j.gardavsky

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Posted 06 March 2020 - 10:48 AM

Next to the 2" BBHS, and 2" Wild Heerbrugg prism, the little Tak has arrived,

 

Tak prism has arrived.jpg

 

Best,

JG


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#28 Nippon

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Posted 06 March 2020 - 08:54 PM

I am late to see this as the post is 2 months old...  

 

I don't understand this point of view.  On a mirror, the smaller the area in use, the lower the total aberrations will be.  How can you say a larger surface area contributes less error?  

 

What am I missing.

 

Thanks.

I have the same view. I've read that if a diagonal has any issues they are likely to be on the edges of the mirror or prism. So the less used by the light cone the better in theory I guess.


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#29 stevew

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Posted 06 March 2020 - 09:42 PM

I have both the Baader Zeiss spec T2 prism with a 2” nosepiece and 1.25” micro focuser eyepiece holder and a Baader 2” BBHS mirror diagonal. 
 

When I had first light with my FS-102 a couple of nights ago I found that when using the 24mm Panoptic and 18.2mm DeLite (parfocal) and other 1.25” eyepieces  with the prism I had to rack the focuser all the way out and then use the micro focuser to reach focus. With the mirror the focuser was racked out about an inch as shown below. This may or not matter to you, but I thought it worth mentioning. 

 

 I ended up sticking with the mirror as I was looking at the moon and really couldn’t tell much of a difference between the two. 
 

gallery_7167_7839_73564.jpeg

Very nice rig Doug!


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#30 Moondust

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Posted 07 March 2020 - 12:49 PM

I had my 92mm Stowaway out last night looking at the moon switching between 3 different diagonals using the same Tak Toe 3.3 eyepiece. Among the the diagonals were a really cheap (around $25 ) older GSO .965 to 1.25 prism diagonal I bought about 12 years ago to be able to use my 1.25 inch eyepieces on a classic Tasco refractor I have. I used a 1.25  adapter on the .965 end to be able to use it in the Stowaway. The other 2 diagonals were a Tele-vue 1.25 Ever-Brite mirror diagonal and my new as of yesterday Takahashi 1.25 prism diagonal. I read that for refractors f/7 is roughly the cut off point for using a prism so when I received my f/6.5 Stowaway last year I was looking for a quality mirror. An Astro-Physics Max Brite was on my short list but I wanted to keep my setup relatively light so selected the highly rated 1.25 Tele-vue Everbrite. I also read that for the moon a prism vs mirror doesn't make much difference, but tonite found this to be totally false. The seeing was excellent with the moon placed high in the sky and the results between the 3 diagonals turned out to be a bit shocking, so much so that I spent a good 2 hours switching between diagonals just to check my vision and sanity. To put it bluntly the 2 prism diagonals both trashed the TV 1.25 Everbrite! Detail was sharper and tighter, contrast higher and to my dismay and going against everything I understood about prisms the color correction was better!!! With the mirror diagonal there was a subtle but definite orange tint on all the highlighted crater rims. I had noticed this before but always attributed it to atmospheric dispersion and at times it may have actually been the case. When switching to either prism this orange tint to the crater highlights was gone. I was also very surprised the views in the cheap GSO diagonal easily bettered the Tele-vue and equaled or at times seem to even slightly better the Takahashi. With the mirror diagonal it was though I was looking through a pane of window glass or a cheaper eyepiece vs a high end one. I also brought out my Stellarvue f/7 80mm Access an excellent fpl 53 doublet with very good color correction (not as good as the triplet) and it also benifited from the prism. The orange highlights on crater rims were again there with the mirror and absent with either one of the two prisms. Can someone please try to explain what is going on here, I would expect the exact opposite to be true after reading prisms can add color on faster refractors .

 

I really like the Tak 1.25 prism diagonal so far but it's not without its quirks. I really have to rack my focuser out to reach focus, even much more so than with the smaller GSO prism. The Feather Touch on the Stowaway is quit beefy and the one on the Stellarvue is quit good as well so I don't think I should be too concerned about focuser tube sag, but this diagonal turns my short scopes into much longer beasts than I would like. I haven't had any big issues with the silver clamping collar on the Tak diagonal with any of my eyepieces including some Tele-vues, in fact I kinda like it. I found out though if you make it too tight you can actually start to unscrew the bottom of the eyepiece barrel, so I tighten just till I feel it snug up. Seems they should have made the collar tighten by turning to the left instead of to the right to avoid the possibility of an eyepiece barrel gradually unscrewing which could be disaster spilling glass elements on the ground with some eyepiece designs. By the way the lunar views through the Stowaway and Tak prism/ Toe 3.3 eyepiece combo was impressive. I was pulling in detail on crater Schickard not even shown in my Antoni'n Rukl (map 62) Atlas to the moon. If some knowledgable person can explain what's going on with the orange highlights on my TV Everbrite vs the prisms I'd appreciate it. Thanks  


Edited by Moondust, 07 March 2020 - 02:12 PM.

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#31 BillP

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Posted 07 March 2020 - 03:18 PM

Adding prism path to a main objective can affect it in various ways.  It can add SA and CA, and it can also be corrective somewhat for CA of the objective.  There is no set rule of thumb though to determine what the effect will be.  As Tom Back noted on this topic, the only way you will be able to tell is if you model the specific objective with the added prism path to see if anything positive or negative may happen, and if it does then if it will sneak into the visual threshold or not.

 

As far as the contrast differences you noted that is a no brainer as all the dielectrics I have run through my scopes always showed marked more scatter than prisms.  The only scatter you are going to get off the prism is from the two air-glass surfaces and whatever negligible amount from the light path through the prism.  40 layers of dielectric coatings however are a whole other story and each layer adds its scatter and any imperfection or micro-dust between the layers will wreak havoc.  Only the most very expensive premium ones come close to how little scatter a quality prism shows, like the AP and Baader Maxbrights.  But even these still show more than a quality prism.

 

As far as those orange highlights, I suppose, depending on how the orange spectrum was corrected in your main objective that a prism path might shift it in the opposite direction some, but only way to tell if that might be happening is to know the prescription of the main objective and model that with a prism's prescription.  While I have, with my scopes, experienced very minimal addition of CA when prism path is added, and SA, it never reached the visual threshold though unless the target was glaringly bright and the magnification was up near 40-50x/in at least.  When I add a binoviewer to a large prism diagonal (2") then SA reaches the visual threshold at planetary magnifications so when I do that I will either use the short path Baader Zeiss 1.25" prism or use a mirror diagonal so the SA does not reach an impactful visual threshold with my scopes.

 

What I would be interested in hearing though is if that orange CA you are seeing is there if you view straight through without any diagonal also.  At least doing this would let you know if it is something about the particular diagonal or not.  Btw, I assume that when you see this color it happens when the crater in directly on-axis in the eyepiece so that lateral color from the eyepiece is not the issue.


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#32 Moondust

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Posted 07 March 2020 - 04:32 PM

Thanks Bill,  I tried a few times to view direct without a diagonal but my neck was about to give out in the awkward position so I didn't come to any definite conclusion. I'll have to try it with some white highlights in terrestrial viewing where the tube is horizontal. I am almost certain though the orange is not there without a diagonal, after all this is a Stowaway we are talking about, supposed to have unprecidented color correction according to Roland. The same orange highlights on crater rims were also showing in my Stellarvue. This is not lateral color I'm positive as it was on-axis and the Tak Toe 3.3 eyepiece is no slouch. I wonder if another Tele-vue 1.25 Everbrite would behave in the same manner or if mine is somehow defective. Tele-Vue must have some kind of warranty on their diagonals, I may try to get a hold of another to compare with mine and then act accordingly. Thanks again.      


Edited by Moondust, 07 March 2020 - 04:34 PM.


#33 mikeDnight

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Posted 07 March 2020 - 04:41 PM

I had my 92mm Stowaway out last night looking at the moon switching between 3 different diagonals using the same Tak Toe 3.3 eyepiece. Among the the diagonals were a really cheap (around $25 ) older GSO .965 to 1.25 prism diagonal I bought about 12 years ago to be able to use my 1.25 inch eyepieces on a classic Tasco refractor I have. I used a 1.25  adapter on the .965 end to be able to use it in the Stowaway. The other 2 diagonals were a Tele-vue 1.25 Ever-Brite mirror diagonal and my new as of yesterday Takahashi 1.25 prism diagonal. I read that for refractors f/7 is roughly the cut off point for using a prism so when I received my f/6.5 Stowaway last year I was looking for a quality mirror. An Astro-Physics Max Brite was on my short list but I wanted to keep my setup relatively light so selected the highly rated 1.25 Tele-vue Everbrite. I also read that for the moon a prism vs mirror doesn't make much difference, but tonite found this to be totally false. The seeing was excellent with the moon placed high in the sky and the results between the 3 diagonals turned out to be a bit shocking, so much so that I spent a good 2 hours switching between diagonals just to check my vision and sanity. To put it bluntly the 2 prism diagonals both trashed the TV 1.25 Everbrite! Detail was sharper and tighter, contrast higher and to my dismay and going against everything I understood about prisms the color correction was better!!! With the mirror diagonal there was a subtle but definite orange tint on all the highlighted crater rims. I had noticed this before but always attributed it to atmospheric dispersion and at times it may have actually been the case. When switching to either prism this orange tint to the crater highlights was gone. I was also very surprised the views in the cheap GSO diagonal easily bettered the Tele-vue and equaled or at times seem to even slightly better the Takahashi. With the mirror diagonal it was though I was looking through a pane of window glass or a cheaper eyepiece vs a high end one. I also brought out my Stellarvue f/7 80mm Access an excellent fpl 53 doublet with very good color correction (not as good as the triplet) and it also benifited from the prism. The orange highlights on crater rims were again there with the mirror and absent with either one of the two prisms. Can someone please try to explain what is going on here, I would expect the exact opposite to be true after reading prisms can add color on faster refractors .

 

I really like the Tak 1.25 prism diagonal so far but it's not without its quirks. I really have to rack my focuser out to reach focus, even much more so than with the smaller GSO prism. The Feather Touch on the Stowaway is quit beefy and the one on the Stellarvue is quit good as well so I don't think I should be too concerned about focuser tube sag, but this diagonal turns my short scopes into much longer beasts than I would like. I haven't had any big issues with the silver clamping collar on the Tak diagonal with any of my eyepieces including some Tele-vues, in fact I kinda like it. I found out though if you make it too tight you can actually start to unscrew the bottom of the eyepiece barrel, so I tighten just till I feel it snug up. Seems they should have made the collar tighten by turning to the left instead of to the right to avoid the possibility of an eyepiece barrel gradually unscrewing which could be disaster spilling glass elements on the ground with some eyepiece designs. By the way the lunar views through the Stowaway and Tak prism/ Toe 3.3 eyepiece combo was impressive. I was pulling in detail on crater Schickard not even shown in my Antoni'n Rukl (map 62) Atlas to the moon. If some knowledgable person can explain what's going on with the orange highlights on my TV Everbrite vs the prisms I'd appreciate it. Thanks  

I'm just thinking out loud here, so it's in no way a criticism. Orange to me seems an unusual tint to observe, and I'm wondering if you may be seeing the Orange hue that can at times be seen around lunar craters, and which are caused by tiny glass beads created during the impact? Ive noticed this many times around certain craters.



#34 peleuba

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Posted 07 March 2020 - 08:43 PM

I don't understand this point of view.  On a mirror, the smaller the area in use, the lower the total aberrations will be.  How can you say a larger surface area contributes less error?  

 

 

 

Let me answer my own post.  To be clear, the smaller the area in use on a (mirror) diagonal the lower the aberration will be.  What was posted above is wrong.  If you place the diagonal forward to intercept the light cone so that it (light cone) takes up a larger surface area on the diagonal, you run the risk of additional aberration being added to the image.

 

Smaller the area = less aberration; larger the area = greater the aberration.

 

Regarding prisms - they will add something to the image  - CA and SA.  Whether this is beneficial to the the image depends entirely on the sign of the the existing error in the objective lens.  If the prism contributes opposite sign correction, then the image may improve.  If not, it won't. 

 

Prisms don't scatter - that is one benefit over a mirror diagonal.  Mirror diagonals do not add SA or CA - they can only add astigmatism and scatter.


Edited by peleuba, 07 March 2020 - 08:44 PM.

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#35 jeffmac

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Posted 07 March 2020 - 09:32 PM

Bill, have you directly compared an enhanced TV to the Everbrite or the Maxbright? I don't remember if you made notation of this in your excellent diagonal review.



#36 BillP

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Posted 07 March 2020 - 10:25 PM

Bill, have you directly compared an enhanced TV to the Everbrite or the Maxbright? I don't remember if you made notation of this in your excellent diagonal review.

If you mean the TV Enhanced Aluminum mirror diagonal, no.  I would guess though that this being a conventional aluminized mirror with enhanced overcoats, it should perform as well as premium secondaries on Dobs as it is the same tried and true technology.  I would expect it to perform on-par with the Takahashi 2" Aluminum Mirror diagonal from my review.  This means visibly more scatter than a prism and a bit more scatter than the AP dielectric which was the lowest of the dielectrics.  ENhanced coatings on aluminized mirrors are said to add scatter over conventional coatings.  I read this in many posts and the premium mirror manufactures say this as well.  Below is what it says on the Webster Telescopes site:

 

Enhanced coatings offer slightly higher reflectivity over semi-enhanced coatings, but at the expense of a slightly rougher surface profile, and a slight change in the mirror's figure.  This results in some additional light scatter and a lowering of contrast.  Semi-enhanced  coatings are built up of two layers (aluminum and protective overcoat).   Enhanced coatings are made up of many layers of coating.  http://www.websterte...mirror coatings


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#37 peleuba

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Posted 08 March 2020 - 07:03 PM

If you mean the TV Enhanced Aluminum mirror diagonal, no.  I would guess though that this being a conventional aluminized mirror with enhanced overcoats, it should perform as well as premium secondaries on Dobs as it is the same tried and true technology. 

 

I think you are absolutely correct. Not only equivalent performance, but perhaps even the same part.    I have seen photos of the TeleVue diagonals take apart and on the back of the mirror is stamped Antares.  The very same Antares Optics that makes the excellent Newtonian diagonals.  


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#38 Moondust

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Posted 09 March 2020 - 01:02 PM

Thanks Bill,  I tried a few times to view direct without a diagonal but my neck was about to give out in the awkward position so I didn't come to any definite conclusion. I'll have to try it with some white highlights in terrestrial viewing where the tube is horizontal. I am almost certain though the orange is not there without a diagonal, after all this is a Stowaway we are talking about, supposed to have unprecidented color correction according to Roland. The same orange highlights on crater rims were also showing in my Stellarvue. This is not lateral color I'm positive as it was on-axis and the Tak Toe 3.3 eyepiece is no slouch. I wonder if another Tele-vue 1.25 Everbrite would behave in the same manner or if mine is somehow defective. Tele-Vue must have some kind of warranty on their diagonals, I may try to get a hold of another to compare with mine and then act accordingly. Thanks again.      

Well I got to borrow another 1.25 Everbrite and it was about the same as mine so must be the nature of the beast. I'll give it credit for a few things though. It is better built than the Tak, came with covers (Tak didn't) and as the name implies is brite. I  much prefer the 1.25" Tak prism view though.  


Edited by Moondust, 09 March 2020 - 01:02 PM.

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#39 Dave1066

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Posted 09 March 2020 - 05:18 PM

Well...I think the answer here would be more a matter of principle than anything else!  I mean you are in a bit of a refractor pursit land having a fluorite Apo, and a nice classic doublet makes it purer still, and of course a Takahashi branding is the icing on the cake.  Now with all that purity in hand, why spoil it with mirrors in the optical chain shameonyou.gif.   I think you need to keep that little OTA an all-glass affair and go with a nice prism  hamsterdance.gif

Does that mean then Bill, that you now rate the Baader Zeiss prism diagonals over the Baader BBHS mirror diagonals on planets? With small refractors of an F ratio of F9 or more?


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#40 jeffmac

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Posted 09 March 2020 - 08:46 PM

If you mean the TV Enhanced Aluminum mirror diagonal, no.  I would guess though that this being a conventional aluminized mirror with enhanced overcoats, it should perform as well as premium secondaries on Dobs as it is the same tried and true technology.  I would expect it to perform on-par with the Takahashi 2" Aluminum Mirror diagonal from my review.  This means visibly more scatter than a prism and a bit more scatter than the AP dielectric which was the lowest of the dielectrics.  ENhanced coatings on aluminized mirrors are said to add scatter over conventional coatings.  I read this in many posts and the premium mirror manufactures say this as well.  Below is what it says on the Webster Telescopes site:


Enhanced coatings offer slightly higher reflectivity over semi-enhanced coatings, but at the expense of a slightly rougher surface profile, and a slight change in the mirror's figure.  This results in some additional light scatter and a lowering of contrast.  Semi-enhanced  coatings are built up of two layers (aluminum and protective overcoat).   Enhanced coatings are made up of many layers of coating.  http://www.websterte...mirror coatings


Thanks, Bill.
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#41 StarAlert

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Posted 18 March 2020 - 11:02 PM

I have a question about something I noticed in my Baader T2 Amici Prism diagonal the last two nights. When I point it at a very bright object like Venus or Sirius or Regal, there is a horizontal line of light that runs through the entire FOV. The width of the line is the same width as the object. I swapped eyepieces... no change. I then swapped to my Baader Maxbright diagonal and it was gone. 

 

Does anyone know what’s going on? 



#42 YAOG

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Posted 18 March 2020 - 11:33 PM

Subject title says it all. Whats better?

For your signature refractor telescopes, a 2" Baader BBHS Sitall mirror. You need f/9 or slower to be sure of getting the best out of a top shelf prism like the Baader BBHS coated Zeiss spec prism. 

 

I have both the Baader Zeiss spec T2 prism with a 2” nosepiece and 1.25” micro focuser eyepiece holder and a Baader 2” BBHS mirror diagonal. 
 

When I had first light with my FS-102 a couple of nights ago I found that when using the 24mm Panoptic and 18.2mm DeLite (parfocal) and other 1.25” eyepieces  with the prism I had to rack the focuser all the way out and then use the micro focuser to reach focus. With the mirror the focuser was racked out about an inch as shown below. This may or not matter to you, but I thought it worth mentioning. 

 

 I ended up sticking with the mirror as I was looking at the moon and really couldn’t tell much of a difference between the two. 
 

gallery_7167_7839_73564.jpeg

The Baader T2 prisms have only half the optical path length of the 2" Baader BBHS Sitall mirror diagonals. Add a 35mm or so long extension to the  nose of the Baader T2 prism and you will be fine. with a similar focuser position. 



#43 YAOG

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Posted 18 March 2020 - 11:36 PM

I have a question about something I noticed in my Baader T2 Amici Prism diagonal the last two nights. When I point it at a very bright object like Venus or Sirius or Regal, there is a horizontal line of light that runs through the entire FOV. The width of the line is the same width as the object. I swapped eyepieces... no change. I then swapped to my Baader Maxbright diagonal and it was gone. 

 

Does anyone know what’s going on? 

Any Amici prism, even a Baader will have this line visible on bright objects due to the double prism design of an Amici diagonal. These are really meant for terrestrial use not astronomical use though for the most part the line is not visible. 


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#44 YAOG

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Posted 18 March 2020 - 11:40 PM

I've always bought the big name diagonals for my Takahashi Flourites and Triplets, TMB and APM refractors to give my scopes every performance advantage I can coupled with high quality eyepieces from Televue, Explorer Scientific but in all honestly, I've never seen any visual difference between a AP Maxibrite, a TV Everbrite or a $100 Celestron  or Meade 10th wave Diagonal. 

 

This doesn't stop me from owning the big name diagonals however. I think this is more in our heads then anything else. There may be a detectable visual advantage if your eyes are 20 years old but after 50, I believe the differences mostly show up on a data sheet and not in real world visual observing.

 

The quality of your eyepiece has a much more significant advantage then the quality of anyone's 10th or 20th wave diagonal, but even the best 2" eyepieces from Televue still cant compete with a very inexpensive 1.25" binoviewer with a simple set of plossls in them. 

 

Just being realistic.

 

...Ralph 

Ralph,

 

I'm 65 this year and the differences are at the eyepiece. 



#45 YAOG

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Posted 18 March 2020 - 11:43 PM

I would be interested in reading more comparison between mirror and prism at high power (e.g. planetary and double stars observing). 

 

When it is said that a mirror introduces more light scattering than a prism, how much detail is lost? Could you describe features that were noticeably enhanced with a prism and lost or washed with a mirror? 

 

Let's assume a decent mirror diagonal, like the Baader BBHS Silver mirror, and a decent prism like Baader T2 zeiss prism or 1.25" Takahashi prism. 

You should read Bill P's articles on mirror diagonals vs prism and the update on the Baader BBHS coated mirror diagonals. Just google it.  



#46 YAOG

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Posted 18 March 2020 - 11:49 PM

Does that mean then Bill, that you now rate the Baader Zeiss prism diagonals over the Baader BBHS mirror diagonals on planets? With small refractors of an F ratio of F9 or more?

I certainly do but I'm not Bill P. But the differences show up more easily at high magnifications such as I am prone to use on planetary use and when trying to cut my eyeballs on Luna's craters.   


Edited by YAOG, 18 March 2020 - 11:52 PM.


#47 StarAlert

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Posted 19 March 2020 - 03:04 AM

Any Amici prism, even a Baader will have this line visible on bright objects due to the double prism design of an Amici diagonal. These are really meant for terrestrial use not astronomical use though for the most part the line is not visible. 

It’s not present on a regular prism diagonal? 


Edited by StarAlert, 19 March 2020 - 03:05 AM.


#48 YAOG

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Posted 19 March 2020 - 04:46 AM

It’s not present on a regular prism diagonal? 

No.



#49 Terra Nova

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Posted 20 March 2020 - 04:09 PM

I use prism diagonals in my F8 and above refractors- 0.965” and 1.25” Takahashi and a Baader/Zeiss 1.25” prisms. For my faster refractors I like mirror diagonals- 1.25” and 2” TeleVue Everbrites and a 2” Altair quartz dielectric. It’s not an issue with my Questar or my Coronado Maxscope.


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#50 daquad

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Posted 09 April 2020 - 07:00 PM

It’s not present on a regular prism diagonal? 

No, because an Amici prism is called a "roof" prism, because of its shape.  The ridge line of the roof is what you see with bright objects.  With an ordinary prism there is no line at the reflection surface.  With a roof prism there are two reflections, insteaed of one.  The even number of reflections give the correctly oriented image. 

 

The original image is "upside down",  i.e. up is down and left is right.  I you stand on your head while looking at the image it would appear right side up.  This acrobatic feat is not recommended.  So enter the Amici roof prism.  The first reflection reverses "up/down" and the second reverses "left/right", thereby providing a correctly oriented image.

 

Dom Q.




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