32 replies to this topic

[TheBigEye]

I'm looking for a 2" diagonal for use in a 6" f/6 refractor. I want to be able to use my 20mm T5 Nagler for wide field viewing. Has anyone compared the two offerings from Baader (AMICI-2 and AMICI-DX2) and the APM (APM-2-diag-EI-PR-FL). The Baader AMICI-DX2 version ($719) has a 44mm clear aperture, but the AMICI-2 ($299) only has 38mm, which may cause vignetting with the Nagler. That would be a non-starter. The DX2 is more than twice the cost of the AMICI-2, but as I understand it, the DX2 has a very high-quality Zeiss prism. 

 

APM's offering has a 46mm clear aperture and sells for $295. It probably doesn't have a Zeiss prism, so I might have the dreaded "spike" to deal with in the FOV. 

 

Would love to get your thoughts and experience if you have used one of these diagonals in a relatively fast refractor.  Thanks!    

[SeattleScott]

The Nagler only has a 27mm field stop, same as a 1.25” 24 Panoptic, so any of those would be more than fine.

The dreaded “spike” is more for high power viewing of bright objects. Are you planning to only use it as a low power sweeper? Or will you also do planetary viewing with it?

I haven’t used a prism below F7.7 so can’t help with respect to how much abberation the prism would add, but probably not a big deal if it will only be used at 45x.

Edited by SeattleScott, 19 February 2025 - 06:54 PM.

[spnc]

APM currently has their 2" Amici diagonal reduced to 225 Euros. After deducting the 19%VAT and euro to dollar conversion,  that is $195 +/-. I got one last week for use on the 66mm f6 that I have piggy backed on the 6" f8 refractor. The Baader 2" Astro Amici is listed at 670 euros on their website, 3x the cost of the  APM. Is it 3x better? I don't know and unfortunately the weather hasn't allowed me a chance to use the APM yet. Just be aware that it is a big, heavy hunk of glass.

 

Their 2" prism diagonal is currently 179 euros. I have no idea how long these prices will be on "sale."

[Starman1]

See:

https://agenaastro.c...m-diagonal.html

You're apparently looking for a correct image diagonal.  The link is a good one made in Taiwan.

Disadvantages:

--chromatic aberration of the prism due to the fast f/ratio scope

--there is a line in the middle of erecting prisms that causes a spike on stars as they cross the center line

--you will need more in-travel at the focuser

 

The Nagler you mention has a 27.4mm field stop.  You will not see vignetting.

[Highburymark]

The Baader T2 amici prism is excellent. The tiny ‘spike’ can only just be perceived on very bright stars. But who would use such a diagonal for studying stars? On the Moon, through a binoviewer, which is how I use it, it’s superb. Though I have slower and smaller scopes than yours. Another thing to be aware of - these things are difficult to make well, so there is some variability among examples - at least that was what I was told.

[TheBigEye]

The Nagler only has a 27mm field stop, same as a 1.25” 24 Panoptic, so any of those would be more than fine.

The dreaded “spike” is more for high power viewing of bright objects. Are you planning to only use it as a low power sweeper? Or will you also do planetary viewing with it?

I haven’t used a prism below F7.7 so can’t help with respect to how much abberation the prism would add, but probably not a big deal if it will only be used at 45x.

Hey, thanks for the info! I have a 31mm Nagler with the field stop at 42mm. I never went and checked the 20mm, but you're right - it's only 27mm. Yes, I will use the scope primarily as a low-power wide starfield sweeper. And for that, I really want a RACI diagonal, as long as it "gets along" with this fast refractor.  But that probably won't stop me from checking out the moon and planets at higher powers - especially if I'm hauling the scope somewhere to do family/public outreach. If it doesn't do well at higher powers with the RACI, I figure I'd buy a 1.25" mirror diagonal. Thanks again for your comments! Very helpful! 

[TheBigEye]

APM currently has their 2" Amici diagonal reduced to 225 Euros. After deducting the 19%VAT and euro to dollar conversion,  that is $195 +/-. I got one last week for use on the 66mm f6 that I have piggy backed on the 6" f8 refractor. The Baader 2" Astro Amici is listed at 670 euros on their website, 3x the cost of the  APM. Is it 3x better? I don't know and unfortunately the weather hasn't allowed me a chance to use the APM yet. Just be aware that it is a big, heavy hunk of glass.

 

Their 2" prism diagonal is currently 179 euros. I have no idea how long these prices will be on "sale."

Hi - thanks for the pricing info.! (This makes the decision tougher, you know!)  Would really like to hear what you have to say about the diagonal after you've had a chance to test it. As far as the weight is concerned, that's a "plus" in my situation because the COG of the OTA is just behind the objective lens due to the weight of front cell and the short tube length. A heavy diagonal might save me some money on a tube counterweight! Come to think of it, if this all works out, maybe I'll buy a binoviewer and get TWO of those diagonals . . . I appreciate your time!   

[TheBigEye]

The Baader T2 amici prism is excellent. The tiny ‘spike’ can only just be perceived on very bright stars. But who would use such a diagonal for studying stars? On the Moon, through a binoviewer, which is how I use it, it’s superb. Though I have slower and smaller scopes than yours. Another thing to be aware of - these things are difficult to make well, so there is some variability among examples - at least that was what I was told

Hi - thanks for the info on the T2. I haven't looked at those, just figuring that having to buy "nose" and "tail" pieces would drive up the cost. I don't plan to do any astrophotography or use the diagonal in multiple scope where reconfiguring to nose or tail pieces would be necessary. But, maybe there are other advantages of the T2 that I should consider? 

 

I did read about the prism quality variability issue. Certainly something to consider when buying a less expensive unit. Thanks!    

[SeattleScott]

If the refractor is an achro, then the CA imparted by a prism might be the least of your concerns when viewing planets. If it is an Apo, then yeah swap in a 1.25” mirror when looking at planets. That will help with CA, as well as eliminate the spike when Jupiter is centered in the view.

Even at low power, you might see a spike as bright stars pass through the view. Depends on the diagonal. I hear the $700 Zeiss prism controls this pretty well, but a $200 one might show a spike. So it just depends on how distracting you find that, and whether you are willing to pay a substantial premium to deal with it, or deal with a reversed image instead.

[TheBigEye]

See:

https://agenaastro.c...m-diagonal.html

You're apparently looking for a correct image diagonal.  The link is a good one made in Taiwan.

Disadvantages:

--chromatic aberration of the prism due to the fast f/ratio scope

--there is a line in the middle of erecting prisms that causes a spike on stars as they cross the center line

--you will need more in-travel at the focuser

 

The Nagler you mention has a 27.4mm field stop.  You will not see vignetting.

Hi - thanks for weighing in on my question! Also for the Long Perng referral. I completely missed this one! I checked it out - apparently, views with this diagonal are right side up, but mirror reversed - which was not exactly what I was looking for. Thanks for the tip also on the true field stop size of the Nagler 20 T5. The scope I will be using this with is a Astro Telescopes 152mm f/5.9. It has a fair amount of in-travel, so hopefully it'll be enough. I may upgrade to an APO at some point. Thanks! 

[Starman1]

Hi - thanks for weighing in on my question! Also for the Long Perng referral. I completely missed this one! I checked it out - apparently, views with this diagonal are right side up, but mirror reversed - which was not exactly what I was looking for. Thanks for the tip also on the true field stop size of the Nagler 20 T5. The scope I will be using this with is a Astro Telescopes 152mm f/5.9. It has a fair amount of in-travel, so hopefully it'll be enough. I may upgrade to an APO at some point. Thanks!

All star diagonals on refractors have an upright image reversed left to right, but they are not described as "erect image" diagonals. I am sorry I misidentified this as a "correct image" diagonal. 2" prisms are uncommon, so I'll remember this one for later. It looks like maybe APM and Baader are your choices if you can't find one of the Barsta ones sold under the Olivon and other brand names, probably used only.

[SeattleScott]

All star diagonals on refractors have an upright image reversed left to right, but they are not described as "erect image" diagonals. I am sorry I misidentified this as a "correct image" diagonal. 2" prisms are uncommon, so I'll remember this one for later. It looks like maybe APM and Baader are your choices if you can't find one of the Barsta ones sold under the Olivon and other brand names, probably used only.

It seems like it is a correct image diagonal, at least according to the ad copy?

Product Details

Long Perng 2" 90° Erecting Amici Prism Diagonal w/ 1.25" Eyepiece Adapter # K2T-E

Specifications:

Barrel Size: 2"
Viewing Angle: 90 degrees
Materials: BK7
Coating: Multi-Coated
Housing material & finish: Aluminum & Anodizing
Standard Accessories: 1.25" adapter
Image orientation is right-side up and left-to-right correct

[Starman1]

I swear that link changed between the time I posted it and now.

It did not say Amici, and it did not say image is correct left-to-right. 

It said Erect Image only and added a note that the image was correct up and down but reversed left to right.

I also notice that now Agena Astro's website has two different links for the same diagonal at two different prices.

 

Someone at Agena Astro must be noting the link and noticed incorrect information for the diagonal and changed it, not noting there was already a link for the Amici prism diagonal.

As proof:

https://agenaastro.c...m-diagonal.html

https://agenaastro.c...al-k2t-e01.html

Same item, two different prices.

[Starman1]

If the $199 price is good, it would be a "grab now" item.

[Spikey131]

 

 

APM's offering has a 46mm clear aperture and sells for $295. It probably doesn't have a Zeiss prism, so I might have the dreaded "spike" to deal with in the FOV. 

!    

One thing you need to know is that all Amici prisms will show the “dreaded spike” at high magnification on bright objects.  
 

The other thing you need to know is that the only reason to choose an Amici prism is because you want an upright and left-right correct image as the highest priority.  There is no other advantage and there are many disadvantages to Amici prisms.

 

I have a Baader Zeiss Amici that I use in fast refractors.  But only during the day.

[Starman1]

A high quality multi-coated 90° prism can be excellent in a longer focal ratio scope.

I draw the line at f/8, though, since my f/7 triple is not color free with a prism diagonal but is with a mirror.

[ABQJeff]

2” Amici:

I have both the APM 2” Amici (three of them actually) and the Baader 2” Astro Amici (DX-2).  I like the APM better.  It is brighter, more true color and has a larger clear aperture.  The Baader does have a thinner “line”, doesn’t have as much light scatter on bright stars and planets, and a bit shorter optical path length but has smaller clear aperture and a slightly yellow tinted view.

 

If you (like me) aren’t using it for high mag planetary work (which you shouldn’t with an amici) and using for fuzzy DSO, lunar or terrestrial viewing, the line doesn’t show and some increase in light scatter on bright planets is a non-issue.  APM hands down especially for the price.

 

Now for T2 diagonals (which can take 2” eyepiece clamps) that have ~30mm clear aperture, the T2 astro amici (~$350) is fantastic.  I have two of them.

[ABQJeff]

A question though, why stop at 27mm Pan?  With an F/6, a Nagler 31, 35mm Pan or even Pentax 40XW would be usable and give a huge field of view and large exit pupil (great for nebula).  I mention this as then you would want to use the APM diagonal for the large clear aperture.

[brightsky]

I have used a Baader T2 Astro amici in my F7.4 exclusively now for over five years.  The spike is just a non-issue and performance for both deep sky and terrestrial is excellent.  I have thought about the 2” Baader but the price is high and the benefit not so much.  I am fairly content with the FOV that I get with 1.25” format .  The good reports on the APM 2” amici are tempting but my experience with the T2 Baader was makes me hesitant to change brands.

For a fast 6” refractor like yours, any prism would likely yield unacceptable chromatic aberration, so you may have to go with a mirror or get an F 7-8 scope.

[TheBigEye]

2” Amici:

I have both the APM 2” Amici (three of them actually) and the Baader 2” Astro Amici (DX-2).  I like the APM better.  It is brighter, more true color and has a larger clear aperture.  The Baader does have a thinner “line”, doesn’t have as much light scatter on bright stars and planets, and a bit shorter optical path length but has smaller clear aperture and a slightly yellow tinted view.

 

If you (like me) aren’t using it for high mag planetary work (which you shouldn’t with an amici) and using for fuzzy DSO, lunar or terrestrial viewing, the line doesn’t show and some increase in light scatter on bright planets is a non-issue.  APM hands down especially for the price.

 

Now for T2 diagonals (which can take 2” eyepiece clamps) that have ~30mm clear aperture, the T2 astro amici (~$350) is fantastic.  I have two of them.

Wow - great info - thanks Jeff! Very surprising to hear about your viewing preference being in a diagonal costing half as much. I ended up ordering the Baader 2" 90° Erecting Amici Prism Diagonal with 1.25" Eyepiece Adapter # AMICI-2 2956152 from Agena Astro.Hopefully it's made by the same company that makes APM's version, or of comparable quality. I went with Agena's offering in case I need to return it - or possibly exchange it, for the DX-2. Return shipping to Germany would probably be quite expensive. It would be nice to do a side-by-side comparison, but I guess I'll see if the Amici "line" bothers me and then decide. Really, as long as the line isn't visible in wide field, low-to-medium power "deep-sky sweeping," it'll be fine. For less than the $400 saved, I can always buy a high quality mirror diagonal for high power viewing. Really appreciate your input. Thanks!    

[TheBigEye]

I have used a Baader T2 Astro amici in my F7.4 exclusively now for over five years.  The spike is just a non-issue and performance for both deep sky and terrestrial is excellent.  I have thought about the 2” Baader but the price is high and the benefit not so much.  I am fairly content with the FOV that I get with 1.25” format .  The good reports on the APM 2” amici are tempting but my experience with the T2 Baader was makes me hesitant to change brands.

For a fast 6” refractor like yours, any prism would likely yield unacceptable chromatic aberration, so you may have to go with a mirror or get an F 7-8 scope.

Hi! Thanks for weighing in! I guess I don't understand the difference between the "T2" and the regular Amici. My understanding is that the T2 has a smaller prism than the regular Astro Amici, and the housing accepts different nose and tailpieces. Also that the clear aperture of the nose/tailpieces are more restricted. I want to be able to use eyepieces with a 30mm+ clear aperture. So, not sure how well the T2 would play with a 17" f/4.5 reflector.

 

I think your observations about CA in this fast refractor are valid. Certainly on bright planets and the moon. I think wide field deep-sky stuff,  [the reason I bought the scope in the first place] viewed at low-to-medium powers, will be less affected by CA than say high power views of the moon and planets. Perhaps I can run a CA-reducing filter to bring the CA under control.  Thank you for your comments! 

[brightsky]

Baader makes two upscale or ‘astronomical ‘ amici diagonals with Zeiss specs, the 1.25” T2 astro-amici and the 2” astro-amici.  The clear apertures are 31mm and 44mm respectively.

Baader T2 components all share 42 x .75 mm threads and are used with 1.25” eyepieces and accessories.

Diagonals are not used with dobs.

[DRodrigues]

Those looking for erecting the image, but straight-through, can have a look at http://www.pt-ducks.....htm#Omegon Pro - a Nagler 22 doesn't vignette...

[mazen]

Wow - great info - thanks Jeff! Very surprising to hear about your viewing preference being in a diagonal costing half as much. I ended up ordering the Baader 2" 90° Erecting Amici Prism Diagonal with 1.25" Eyepiece Adapter # AMICI-2 2956152 from Agena Astro.Hopefully it's made by the same company that makes APM's version, or of comparable quality. I went with Agena's offering in case I need to return it - or possibly exchange it, for the DX-2. Return shipping to Germany would probably be quite expensive. It would be nice to do a side-by-side comparison, but I guess I'll see if the Amici "line" bothers me and then decide. Really, as long as the line isn't visible in wide field, low-to-medium power "deep-sky sweeping," it'll be fine. For less than the $400 saved, I can always buy a high quality mirror diagonal for high power viewing. Really appreciate your input. Thanks!    

Could you report back on your thoughts on the Baader Amici Prism diagonal once you've had an opportunity to use it? I'm interested!

Edited by mazen, 04 March 2025 - 12:34 PM.

[TheBigEye]

Could you report back on your thoughts on the Baader Amici Prism diagonal once you've had an opportunity to use it? I'm interested!

Hi mazen:

 

Sure, I'll be glad to weigh in on that. I plan to do more detailed reviews on the 152mm f/5.9 refractor, the 2" RACI prism diagonal, and a direct comparison of the AZ8 and SkyTee 2 alt-azimuth mounts after I've had time to do a thorough performance evaluation of this new (to me) equipment. 

 

Meanwhile, I did receive the Baader 2" 90° Erecting Amici Prism Diagonal (AMICI-2 2956152) and have done some limited viewing with it. When looking at Venus, precisely centered in the field at low power, the "roof prism spike" is very obvious. Move the scope to take Venus just slightly off-center, and the spike disappears. Less of a problem with Jupiter, and zero issues with the moon and terrestrial viewing.

 

There is quite a bit of CA on bright objects, which is to be expected in an f/5.9 achromatic refractor. I'm sure passing that light through another piece of glass (the prism), exacerbates the effect. I tried an SVBONY Color Correction filter which all but eliminated the CA, but introduced a slightly yellowish cast to Venus, Jupiter, and the moon.  I'm sure that using an enhanced mirror diagonal, instead of the prism-type, would improve contrast and reduce CA as well.

 

I bought this scope primarily for wide-field, deep sky "sweeping," and it provides awesome views with a 26mm Nagler. No evidence of vignetting and the RACI image orientation is more aesthetically pleasing for that type of observing. I'm quite pleased with the diagonal - it's a keeper!

 

- Chuck