Three diagonals - which will triumph?

[Ags]

Yes, its the same one.

[Louis D]

3 hours ago, Millenium said:

Hi, I'm new to this hobby, I bought a telescope with a very cheap and poorly made diagnoal (?) that is made of plastic, it's the celestron dx102az. I was told I should upgrade it to get better result, someone tipped me about your post. Is the diagnoal you got similar to these ones?

Celestron 90° Star Diagonal (1.25") 94115-A B&H Photo Video (bhphotovideo.com)

Teleskop-Express: Celestron 1,25" 90° diagonal prism for refractors and cassegrains

I would get the GSO 1.25" 90-deg 99% Dielectric Mirror Diagonal based on my experience with the GSO 2" 99% diagonals.  I'm not a fan of my William Optics 1.25" 99% Dielectric Dura Bright Carbon Fiber Star Mirror Diagonal.  It has a 22mm restriction at the bottom of the eyepiece receiver which vignettes widest field eyepieces.

[Don Pensack]

7 hours ago, Millenium said:

Hi, I'm new to this hobby, I bought a telescope with a very cheap and poorly made diagnoal (?) that is made of plastic, it's the celestron dx102az. I was told I should upgrade it to get better result, someone tipped me about your post. Is the diagnoal you got similar to these ones?

Celestron 90° Star Diagonal (1.25") 94115-A B&H Photo Video (bhphotovideo.com)

Teleskop-Express: Celestron 1,25" 90° diagonal prism for refractors and cassegrains

If you're in the UK, look at the diagonals page on First Light Optics' website.

If you're in the US, I agree with Louis.

[Guest]

24 minutes ago, Don Pensack said:

If you're in the UK, look at the diagonals page on First Light Optics' website.

If you're in the US, I agree with Louis.

Thanks for taking the time to reply.

None of the above, I live in Norway. It doesn't make a huge difference in shipping cost if I order from the states (BHP), FLO or Telescope Express. I am slighly confused though, I thought OP specifically mentioned Celestron diagnoal as best value, while dielectric was worse in that example. Maybe I'm just missunderstanding due to lack of experience/understanding the whole thing.

Also, not sure if it makes a difference, I'm planning to use it on a celestron starsens explorer dx102az.

[Ags]

In my case, the dielectric was a poor example, while the Celestron prism performed well. If you read through my thread, you will see many people speak well of the prism. 

That's not a criticism of the dielectric - I am sure most perform superbly, and I was just unlucky.

[Zeta Reticulan]

Was the TS dielectric made by GSO?

[Ags]

Maybe, it says Made in Taiwan.

[chrispj]

Following up on Ags' test, on the last clear night here in Devon I did a similar completely subjective test, though given the results you might also call it an exercise in confirmation bias. Again looking at Epsilon Lyra. In this case the three diagonals are the StellaMira 1.25" 90° di-electric, the StellaMira 2" erecting prism and the Takahashi 1.25" prism. In all cases looking through the StellaMira 80mm f10 refractor. This was the first time out for the Takahashi prism so no idea what to expect here.

 

 

First up, I worked my way down through the focal lengths and all 3 diagonals split both pairs with a 7mm Pentax XW. In the case of the di-electric, both components would merge into lozenges intermittently with the seeing, but not an unpleasing result. The 2" erecting prism normally lives in the ST102 (along with a 35mm Panoptic) as the second scope on my SkyTee as a super-sized RACI finder so hasn't been used at other than low powers before (looking at the likes of the moon and Pleiades through this have always been crystal clear and just beautiful). Through the erecting prism, both pairs were cleanly split, colours were more blue than the warmer colour of the di-electric, a definite discernable difference between the mirror and the prism. Now for the Takahashi, and this may be where the confirmation bias steps in. Not a distinct step up over the StellaMira erecting prism, but back to back for my eyes the stars seemed slightly tighter in focus, the split was cleaner. No great difference in colour. If I didn't try them back to back then I would be very happy with either image and struggle to say there was any difference between them.

 

I stepped up to a 5mm StellaLyra LER and compared the two 1.25" diagonals. A clean split for both diagonals, but again a more pleasing image through the prism.

 

Finally I swapped to the OVL binoviewer. I haven't used this  on anything except the moon before and only for quite low power viewing. With a pair of Astro Essentials 20mm plossls, I swapped through first a 2x then 3x ES focal extender, splitting both pairs with the 3x extender (so ~6 2/3mm equivalent focal length). Merging the 2 views did become a challenge with the stars much off centre, which doesn't seem to be an issue looking at the moon. To finish I stepped up again to an ancient TeleVue 2.5x barlow, which seems to magnify slightly greater than 4x with the binoviewer so with the 20mm plossls would give <5mm equivalent or slightly higher than 160x magnification. Again, through the prism no problems to bring the stars to focus (I didn't try the mirror for this).

 

Edit - I was wrong with this next paragraph - there is a safety stop, it's so slight that I'd discounted it but it is there.

One thing to note with the Takahashi prism - unlike the StellaMira 1.25" mirror diagonal, there is no stop at the end of the eyepiece holder to stop an over-long barrel going in too far and impacting the prism. The ES 2x focal extender does have an exceptionally long barrel so I will need to fit a par-focal ring otherwise there's potential that I'll mark the prism over time.

 

      

So to conclude, all 3 diagonals give a perfectly serviceable image, with the two prism diagonals for me giving a discernibly sharper and subjectively more pleasing image, the Takahashi prism being a marginal step up again on the SM prism.

 

Edited July 26, 2022 by chrispj
I was wrong...

[Pixies]

Accepted wisdom is a prism is better in slow scopes and a mirror in fast ones, I think?

[Ags]

Kindof... prisms can also be used in in fast scopes that are undercorrected in red. My Zenithstar is f6 and the prism noticably reduces CA at focus, and inside and outside of focus. I specifically bought the Celestron prism to try out this effect and I'm surprised by the strong effect I see.

I have since tried the Celestron prism with my C6 (functioning at f6.3), and preferred the cheap SkyWatcher mirror diagonal in that scope - stars just seemed a bit sharper.... So I think a prism can can enhance views, but it depends on the scope.

Edited July 25, 2022 by Ags

[Zeta Reticulan]

On 24/07/2022 at 07:53, Ags said:

Maybe, it says Made in Taiwan.

They look like Guan Sheng. Although I believe Long Perng are also based in Taiwan. A lot of TS products are GSO.

[Zeta Reticulan]

6 hours ago, chrispj said:

Following up on Ags' test, on the last clear night here in Devon I did a similar completely subjective test, though given the results you might also call it an exercise in confirmation bias. Again looking at Epsilon Lyra. In this case the three diagonals are the StellaMira 1.25" 90° di-electric, the StellaMira 2" erecting prism and the Takahashi 1.25" prism. In all cases looking through the StellaMira 80mm f10 refractor. This was the first time out for the Takahashi prism so no idea what to expect here.

 

 

First up, I worked my way down through the focal lengths and all 3 diagonals split both pairs with a 7mm Pentax XW. In the case of the di-electric, both components would merge into lozenges intermittently with the seeing, but not an unpleasing result. The 2" erecting prism normally lives in the ST102 (along with a 35mm Panoptic) as the second scope on my SkyTee as a super-sized RACI finder so hasn't been used at other than low powers before (looking at the likes of the moon and Pleiades through this have always been crystal clear and just beautiful). Through the erecting prism, both pairs were cleanly split, colours were more blue than the warmer colour of the di-electric, a definite discernable difference between the mirror and the prism. Now for the Takahashi, and this may be where the confirmation bias steps in. Not a distinct step up over the StellaMira erecting prism, but back to back for my eyes the stars seemed slightly tighter in focus, the split was cleaner. No great difference in colour. If I didn't try them back to back then I would be very happy with either image and struggle to say there was any difference between them.

 

I stepped up to a 5mm StellaLyra LER and compared the two 1.25" diagonals. A clean split for both diagonals, but again a more pleasing image through the prism.

 

Finally I swapped to the OVL binoviewer. I haven't used this  on anything except the moon before and only for quite low power viewing. With a pair of Astro Essentials 20mm plossls, I swapped through first a 2x then 3x ES focal extender, splitting both pairs with the 3x extender (so ~6 2/3mm equivalent focal length). Merging the 2 views did become a challenge with the stars much off centre, which doesn't seem to be an issue looking at the moon. To finish I stepped up again to an ancient TeleVue 2.5x barlow, which seems to magnify slightly greater than 4x with the binoviewer so with the 20mm plossls would give <5mm equivalent or slightly higher than 160x magnification. Again, through the prism no problems to bring the stars to focus (I didn't try the mirror for this).

 

One thing to note with the Takahashi prism - unlike the StellaMira 1.25" mirror diagonal, there is no stop at the end of the eyepiece holder to stop an over-long barrel going in too far and impacting the prism. The ES 2x focal extender does have an exceptionally long barrel so I will need to fit a par-focal ring otherwise there's potential that I'll mark the prism over time.

 

      

So to conclude, all 3 diagonals give a perfectly serviceable image, with the two prism diagonals for me giving a discernibly sharper and subjectively more pleasing image, the Takahashi prism being a marginal step up again on the SM prism.

 

I thought the Tak' prism actually had a safety shoulder but it was not that obvious. Although it wouldn't surprise me if I was seeing things. I really like the Tak' TKA00547 prism for a variety of reasons. Although it does have its shortcomings, the collet in the twistlock can have problems with undercuts and there is no filter thread. 

I've also recently directly compared the TKA00547 with a Long Perng Amici (mine says Omegon on it) for some lunar viewing. I prefer Amicis for the Moon, although not generally for planets. I usually use a Baader Amici for dedicated lunar sessions. The Long Perng is very good but I thought the TKA00547 just edged it out for overall acuity and clarity. This wasn't surprising considering how Amici prisms work.

Above: William Optics Amici prism shown in housing. I think these WO's are made by Kunming United Optics. 

I'm just not that sure I can deal with a mirror-reversed Moon lol.

 

Edited July 25, 2022 by Zeta Reticulan

[MalcolmM]

44 minutes ago, Zeta Reticulan said:

thought the Tak' prism actually had a safety shoulder but it was not that obvious.

I think this is correct. It is quite small but for example will prevent a Nagler Zoom (40mm barrel length)  hitting the prism. Shoulder on a quick measurement is about 33mm deep. Prism itself being about 38 mm deep

Malcolm 

[Zeta Reticulan]

8 minutes ago, MalcolmM said:

I think this is correct. It is quite small but for example will prevent a Nagler Zoom (40mm barrel length)  hitting the prism. Shoulder on a quick measurement is about 33mm deep. Prism itself being about 38 mm deep

Malcolm 

Thanks for the verification. I feel better using my 2x TV Barlow in it now lol.

[MalcolmM]

17 minutes ago, Zeta Reticulan said:

Thanks for the verification. I feel better using my 2x TV Barlow in it now lol.

I think the Tak is a fabulous wee diagonal. It's very light and has a short light path. I happen like the compression system for securing the eyepiece though I know not everyone does.

Malcolm 

[Louis D]

2 hours ago, Zeta Reticulan said:

I'm just not that sure I can deal with a mirror-reversed Moon lol.

Don't ever get a Newt/Dob, then.  You'll have to lean over the scope from the side opposite the focuser to right the image (180 degree rotation).  It's not very comfortable and finders tend to get in the way.

Since I started on a Dob, I guess I just never let the whole image rotation issue bother me.  The left/right flip of a mirror diagonal makes using printed star charts more difficult than a 180 rotation.  For Newts/Dobs, you just rotate the chart 180 degrees.  For the image flip case, you need to try to look through the page from behind.  I suppose with electronic charts today, you just specify the preferred orientation to the software.

[Zeta Reticulan]

10 minutes ago, Louis D said:

Don't ever get a Newt/Dob, then.  You'll have to lean over the scope from the side opposite the focuser to right the image (180 degree rotation).  It's not very comfortable and finders tend to get in the way.

Since I started on a Dob, I guess I just never let the whole image rotation issue bother me.  The left/right flip of a mirror diagonal makes using printed star charts more difficult than a 180 rotation.  For Newts/Dobs, you just rotate the chart 180 degrees.  For the image flip case, you need to try to look through the page from behind.  I suppose with electronic charts today, you just specify the preferred orientation to the software.

 

Oddly, I prefer the upside-down world of Newtonians. I just can't get my head around a mirror Moon. Who knows, maybe I'll adapt one day.

[Zeta Reticulan]

1 hour ago, MalcolmM said:

I think the Tak is a fabulous wee diagonal. It's very light and has a short light path. I happen like the compression system for securing the eyepiece though I know not everyone does.

Malcolm 

Yeah, it's a great diagonal, even if it feels like it came out of a Christmas cracker. I actually have two. One holds my DeLites properly and the other doesn't. So I'm guessing there's some tolerance variation. Its lightness is its main strength I think. Seriously helps with vibration return issues.

Works well with other Tak' stuff.

[MalcolmM]

4 minutes ago, Zeta Reticulan said:

Yeah, it's a great diagonal, even if it feels like it came out of a Christmas cracker. I actually have two. One holds my DeLites properly and the other doesn't. So I'm guessing there's some tolerance variation. Its lightness is its main strength I think. Seriously helps with vibration return issues.

Works well with other Tak' stuff.

Have you tried cutting a couple of mm off the grey plastic compression insert? This might help your 'other' diagonal hold the delites?

Malcolm 

[Zeta Reticulan]

2 minutes ago, MalcolmM said:

Have you tried cutting a couple of mm off the grey plastic compression insert? This might help your 'other' diagonal hold the delites?

Malcolm 

Not yet. I did disassemble it to have look though. TBH I prefer something a bit more substantial with the DeLites. 

[Zeta Reticulan]

On 28/06/2022 at 10:49, happy-kat said:

@ags does the eyepiece holder end of the celestron erect diagonal unscrew please? 

 

Most of them do, and are compatible with other Amicis.

 

 

However, the Long Perng Amici eyepiece holders are brazed on.

[chrispj]

17 hours ago, Zeta Reticulan said:

I thought the Tak' prism actually had a safety shoulder but it was not that obvious. 

 

16 hours ago, MalcolmM said:

I think this is correct. It is quite small but for example will prevent a Nagler Zoom (40mm barrel length)  hitting the prism.

I looked again, and again, then tried the focal extender, then drew a line on the barrel with a Sharpie, took another look and then checked again...

 

It is so slight I'd discounted it but yes the shoulder is there, as it should be. It did amaze me to think that a company with Takahashi's reputation might have missed such a feature!

[Zeta Reticulan]

2 hours ago, chrispj said:

I looked again, and again, then tried the focal extender, then drew a line on the barrel with a Sharpie, took another look and then checked again...

 

It is so slight I'd discounted it but yes the shoulder is there, as it should be. It did amaze me to think that a company with Takahashi's reputation might have missed such a feature!

For quite a while I was convinced there wasn't one as well. It's not obvious. 

[Ags]

I discovered today that the 1.25" nosepiece from the otherwise worthless TS diagonal fits in the Skywatcher diagonal, so now I have a Skywatcher freebie diagonal with a brass compression ring!

[Gabby76]

In a email with Thomas Baader a few years ago, he stated that you should test to see if you need a prism or not.

Using a mirror diagonal, check the colour of the first airy disk on infocus. If it is red/ orange a prism will be a benefit.

If the ring is white a prism will be no benefit and may actually add aberrations in the view.