Upgrading my diagonal - need help

[West83]

Hi all,

So I am searching for a diagonal to replace the very old stock diagonal on my celestron fs80.

I plan on upgrading the focuser as well, but i can only find 2" focusers in europe, that have 86mm flange. Considering that the cheapest i found is 190€, i think it will have to wait for now. So I am focusing on the diagonal.

Since my scope is f/11 i have read that prisms perform better than mirrors, but I tend to think that its true for higer end and more expensive prisms. I was looking at mirrors:
-Omegon Carbonline 1.25" which is 99€ (looks similar to williams optics)
-artesky dielectric 1.25" which is 79€
- auriga dielectric 1.25" twistlock (adding this one due to twistlock seeming quite handy)
(Both from astroshop)

Any thoughts on these?

Also a question once i manage to upgrade the focuser to 2", am i losing a lot by using a 1.25 diagonal, or is 2" diagonal much better despite using 1.25 EPs?

Thank you.

[Lady Fraktor]

Hello West83,

Any of the dielectrics should be good, if you want to save a few € get the Artesky.
Prisms work well with achromats but you need to star test your refractor first to see if a prism will help.
You have to remember prisms were in use a couple centuries before apos arrived

You will not be losing anything using a 1.25" diagonal with a 2" focuser.
Some eyepiece designs may require the shorter lightpath to achieve focus.

[Bigzmey]

Can you post some pics, model info on your old 1.25" diagonal? Old does not necessary mean bad. You may not notice any visible improvement replacing it with modern entry level dielectric.

I would suggest to spend a bit more and get a good quality 1.25" prism (or 2" diagonal when you eventually upgrade to 2" focuser).

For the prism I highly recommend Baader T2

https://www.astroshop.eu/diagonal-prism ... sm/p,10810

You will need T2 1.25" nosepiece and eyepiece holder

https://www.astroshop.eu/extension-redu ... 4-/p,10587
https://www.astroshop.eu/barrel-adaptor ... ce/p,33230

In US you can get the whole set, but I don't see that option on Astroshop

https://agenaastro.com/baader-t-2-90-pr ... 6005k.html

[West83]

Thank you for your replies, much appreciated! @Lady Fraktor How does a star test work in a refractor, similar to newtonian, and what exactly am I looking for to indicate it?

I am inclined to go for the artesky, mainly due to the price and as something to bridge the gap. I will likely be purchasing more expensive gear in the next few months, and the prism might be an option then. @Bigzmey Is there any particular benefit I would get from a prism, compared to for example the Artesky diagonal?

The existing diagonal is 20 years old and it's the stock diagonal. I would guess that upgrading should result in better views. I will take some photos of the diagonal and post them. One of the issues is also that it doesn't hold well in the current focuser, as there is a shaft in the nosepiece that was likely induced by the previous owner turning the diagonal while the focuser screw was tightened.

Btw, just a question, has anyone every try to refurbish the stock focuser? I was looking to buy a GSO86 focuser, but it's nowhere to be found

[Lady Fraktor]

The stock focuser is adequate but a very basic design, any refurbishing really just give it a bit of longevity rather than improving it.

I will send a PM on how to check if you require a prism, it has been discussed here before but I am not sure of the posting.

[Bigzmey]

I am inclined to go for the artesky, mainly due to the price and as something to bridge the gap. I will likely be purchasing more expensive gear in the next few months, and the prism might be an option then. @Bigzmey Is there any particular benefit I would get from a prism, compared to for example the Artesky diagonal?

I got my first prism a few years ago to mainly gain some back focus for my Onyx ED 80mm F6.3. When I have switched from mirror to prism diagonal I immediately noticed less light scatter on bright targets (Moon, planets, stars). This in turn resulted in more details been resolved on the Moon and planets, and splits of tight doubles which were not possible with mirror.

Eventually, I have replaced mirrors with prisms on all my slower scopes (F7 to F15).

The only potential drawback of prism is that it introduces some CA (in theory). So far I have not seen any noticeable CA added by prisms in all my scopes and in 150ST F5 achro I seem to get less CA when using 2" prism.

[Lady Fraktor]

I have seen achromats and some apos which do not benefit from a prism so always better to test your optics for yourself.
While testing you will also learn a bit of your optics which is not a bad thing.
Test = € properly spent

[Bigzmey]

I have seen achromats and some apos which do not benefit from a prism so always better to test your optics for yourself.
While testing you will also learn a bit of your optics which is not a bad thing.
Test = € properly spent

I believe what you are referring to is how much CA is introduced by the prism and what is the net result of this CA added to the optical system, i.e. whether it adds or compensates CA already present in the scope. Your mileage may vary, but I approach it with purely practical point of view. If the prism does not add noticeable by eye CA at the highest power you use, then you are good to go. And as I mentioned in all my SCTs, Maks and fracs from F6.3 to F15 I don't see any noticeable by eye CA. So, I dare to claim that any scope with F10 or slower would be just fine with prism diagonal.

Irrespectively to CA, reduced light scatter of prisms compared to mirrors is a well known and appreciated. In fact I was shocked by how much less light scatter is there compared to a mid range ($100+) dielectric diagonal. The fainter secondary which is completely invisible with dielectric in the glow from the bright main in a tight double, becomes sharp and obvious dot using a prism under the same power. So, once again I am dare to claim that any scope F10 or slower will benefit from a prism compared to the similar tier mirror diagonal.

[Lady Fraktor]

I never said anything about CA.

A question I put to Herr Thomas Baader in a email a few years ago and his reply.

Q: "Do the Baader prism diagonals with BBHS coating work well with refractors faster than f/7 or f/8? I am wondering if chromatic aberration (or spherochromatism) would be a problem as the light propagates through the glass prism."

A: "We do not recommend a prism for refracting telescopes unless the optical calculation of the objective lens calls for using a prism instead of a mirror.
For 100 years - all Carl Zeiss Refractor telescopes were designed to compensate for the added glasspath of a prism.
And throughout our 25 years of offering this prism we repeatedly found refractor telescopes - especially of very short focal length - where the airy disc went colorless only when adding a prism into the beam of light - as was the case with the "long gone" Astro Physics Traveler.
So please check carefully with your existing mirror star diagonal if the first ring of the airy disc appears reddish when observing a bright star - or better when observing an artificial star at high magnification. if this is found to be the case then a prism will bring a noticeable improvement.
If the first diffraction ring appears white already than a prism would not lead to an improvement but will induce a color error."

If you want you can also go to the TEC website where Yuri discusses prisms as well.
He pretty much echoes what Thomas states.

[notFritzArgelander]

I never said anything about CA.

A question I put to Herr Thomas Baader in a email a few years ago and his reply.

Q: "Do the Baader prism diagonals with BBHS coating work well with refractors faster than f/7 or f/8? I am wondering if chromatic aberration (or spherochromatism) would be a problem as the light propagates through the glass prism."

A: "We do not recommend a prism for refracting telescopes unless the optical calculation of the objective lens calls for using a prism instead of a mirror.
For 100 years - all Carl Zeiss Refractor telescopes were designed to compensate for the added glasspath of a prism.
And throughout our 25 years of offering this prism we repeatedly found refractor telescopes - especially of very short focal length - where the airy disc went colorless only when adding a prism into the beam of light - as was the case with the "long gone" Astro Physics Traveler.
So please check carefully with your existing mirror star diagonal if the first ring of the airy disc appears reddish when observing a bright star - or better when observing an artificial star at high magnification. if this is found to be the case then a prism will bring a noticeable improvement.
If the first diffraction ring appears white already than a prism would not lead to an improvement but will induce a color error."

If you want you can also go to the TEC website where Yuri discusses prisms as well.
He pretty much echoes what Thomas states.

Yep.

[Bigzmey]

I never said anything about CA.

A question I put to Herr Thomas Baader in a email a few years ago and his reply.

Q: "Do the Baader prism diagonals with BBHS coating work well with refractors faster than f/7 or f/8? I am wondering if chromatic aberration (or spherochromatism) would be a problem as the light propagates through the glass prism."

A: "We do not recommend a prism for refracting telescopes unless the optical calculation of the objective lens calls for using a prism instead of a mirror.
For 100 years - all Carl Zeiss Refractor telescopes were designed to compensate for the added glasspath of a prism.
And throughout our 25 years of offering this prism we repeatedly found refractor telescopes - especially of very short focal length - where the airy disc went colorless only when adding a prism into the beam of light - as was the case with the "long gone" Astro Physics Traveler.
So please check carefully with your existing mirror star diagonal if the first ring of the airy disc appears reddish when observing a bright star - or better when observing an artificial star at high magnification. if this is found to be the case then a prism will bring a noticeable improvement.
If the first diffraction ring appears white already than a prism would not lead to an improvement but will induce a color error."

If you want you can also go to the TEC website where Yuri discusses prisms as well.
He pretty much echoes what Thomas states.

Would color error be another word for chromatic aberration (CA), though? In any case this comment does not say anything about light scatter reduction by prisms compared to dielectrics. As we are well aware any optical device (scope, diagonal, EP) comes with benefits and drawbacks. You look at the net result and see if benefits outweigh the negatives.

I like what I see with prisms in my 3 fracs, 2 SCTs and 2 Maks. Unfortunately, pretty much for any piece of the equipment you can always find as many cons as pros on the net, just look at "5 reasons to buy a DOB" post. Ultimately, one have to try it themselves to make validated conclusion.

[notFritzArgelander]

I never said anything about CA.

A question I put to Herr Thomas Baader in a email a few years ago and his reply.

Q: "Do the Baader prism diagonals with BBHS coating work well with refractors faster than f/7 or f/8? I am wondering if chromatic aberration (or spherochromatism) would be a problem as the light propagates through the glass prism."

A: "We do not recommend a prism for refracting telescopes unless the optical calculation of the objective lens calls for using a prism instead of a mirror.
For 100 years - all Carl Zeiss Refractor telescopes were designed to compensate for the added glasspath of a prism.
And throughout our 25 years of offering this prism we repeatedly found refractor telescopes - especially of very short focal length - where the airy disc went colorless only when adding a prism into the beam of light - as was the case with the "long gone" Astro Physics Traveler.
So please check carefully with your existing mirror star diagonal if the first ring of the airy disc appears reddish when observing a bright star - or better when observing an artificial star at high magnification. if this is found to be the case then a prism will bring a noticeable improvement.
If the first diffraction ring appears white already than a prism would not lead to an improvement but will induce a color error."

If you want you can also go to the TEC website where Yuri discusses prisms as well.
He pretty much echoes what Thomas states.

Would color error be another word for chromatic aberration (CA), though? In any case this comment does not say anything about light scatter reduction by prisms compared to dielectrics. As we are well aware any optical device (scope, diagonal, EP) comes with benefits and drawbacks. You look at the net result and see if benefits outweigh the negatives.

I like what I see with prisms in my 3 fracs, 2 SCTs and 2 Maks. Unfortunately, pretty much for any piece of the equipment you can always find as many cons as pros on the net, just look at "5 reasons to buy a DOB" post. Ultimately, one have to try it themselves to make validated conclusion.

No. Color error is much more broad a term than chromatic aberration. The two terms are not synonymous but they are related. What is usually referred to as chromatic aberration is more precisely longitudinal chromatic aberration. The focus point for different wavelengths is different. Then there is spherochromatism where there are errors that vary by color with zonal aperture.

For instance SCTs and Maks don't have longitudinal chromatic aberration (your CA) but DO have spherochromatism. Apochromats also don't have longitudinal chromatic aberration but MAY have some spherochromatism. Achromats are famous for longitudinal chromatic aberration and also have spherochromatism.

CA aka longitudinal chromatic aberration is often described primary spectrum error.

https://www.telescope-optics.net/chromatic.htm

While spherochromatism (secondary spectrum error) is here:

https://www.telescope-optics.net/second ... matism.htm

The color error referred to by Mr Baader above is spherochromatism and the test is whether the out of focus Airy disk shows color differences.

[Lady Fraktor]

Back to the point, though I use both mirrors and prisms I still advocate to spend 10 minutes to test instead of just buying and hoping that the prism is needed.
When starting out a mirror is a good place to start with and then upgrade in the future if wanted.

[davesellars]

I never said anything about CA.

A question I put to Herr Thomas Baader in a email a few years ago and his reply.

Q: "Do the Baader prism diagonals with BBHS coating work well with refractors faster than f/7 or f/8? I am wondering if chromatic aberration (or spherochromatism) would be a problem as the light propagates through the glass prism."

A: "We do not recommend a prism for refracting telescopes unless the optical calculation of the objective lens calls for using a prism instead of a mirror.
For 100 years - all Carl Zeiss Refractor telescopes were designed to compensate for the added glasspath of a prism.
And throughout our 25 years of offering this prism we repeatedly found refractor telescopes - especially of very short focal length - where the airy disc went colorless only when adding a prism into the beam of light - as was the case with the "long gone" Astro Physics Traveler.
So please check carefully with your existing mirror star diagonal if the first ring of the airy disc appears reddish when observing a bright star - or better when observing an artificial star at high magnification. if this is found to be the case then a prism will bring a noticeable improvement.
If the first diffraction ring appears white already than a prism would not lead to an improvement but will induce a color error."

If you want you can also go to the TEC website where Yuri discusses prisms as well.
He pretty much echoes what Thomas states.

That was an interesting post and comment from Baader regarding the first airy disc ring colour. I tested my new 4" refractor last night on a bright star and indeed the first ring going off-focus inwards was clearly orange/red with the remainder being white. I'll repeat the test again with a different bright star to ensure that the colour of the star was not causing it or other factor but according to your information this refractor would indeed benefit from using a prism. BTW, I do not see any Chromatic Abberation at focus - but as you say this is a completely different thing.

[Bino-Handle]

The prism correct image is my choice but not cheap in 2"

[notFritzArgelander]

I never said anything about CA.

A question I put to Herr Thomas Baader in a email a few years ago and his reply.

Q: "Do the Baader prism diagonals with BBHS coating work well with refractors faster than f/7 or f/8? I am wondering if chromatic aberration (or spherochromatism) would be a problem as the light propagates through the glass prism."

A: "We do not recommend a prism for refracting telescopes unless the optical calculation of the objective lens calls for using a prism instead of a mirror.
For 100 years - all Carl Zeiss Refractor telescopes were designed to compensate for the added glasspath of a prism.
And throughout our 25 years of offering this prism we repeatedly found refractor telescopes - especially of very short focal length - where the airy disc went colorless only when adding a prism into the beam of light - as was the case with the "long gone" Astro Physics Traveler.
So please check carefully with your existing mirror star diagonal if the first ring of the airy disc appears reddish when observing a bright star - or better when observing an artificial star at high magnification. if this is found to be the case then a prism will bring a noticeable improvement.
If the first diffraction ring appears white already than a prism would not lead to an improvement but will induce a color error."

If you want you can also go to the TEC website where Yuri discusses prisms as well.
He pretty much echoes what Thomas states.

That was an interesting post and comment from Baader regarding the first airy disc ring colour. I tested my new 4" refractor last night on a bright star and indeed the first ring going off-focus inwards was clearly orange/red with the remainder being white. I'll repeat the test again with a different bright star to ensure that the colour of the star was not causing it or other factor but according to your information this refractor would indeed benefit from using a prism. BTW, I do not see any Chromatic Abberation at focus - but as you say this is a completely different thing.

If there's a quick and easy test to see if there's a benefit, why not?

[PeterD]

So please check carefully with your existing mirror star diagonal if the first ring of the airy disc appears reddish when observing a bright star - or better when observing an artificial star at high magnification. if this is found to be the case then a prism will bring a noticeable improvement.
If the first diffraction ring appears white already than a prism would not lead to an improvement but will induce a color error."

This is an interesting piece of information. I am wondering, is it necessary to start this test with a mirror diagonal in place? I would thought no diagonal at all to start with and then see if the the prism does introduce a color correction. I will do this test with my Scopetech f12.5 and Baader T2 prism and will see what the results are.
I am wondering, do you have a contact to someone at Baader who is keen to answer technical questions?

[Lady Fraktor]

You will need to test with a mirror first to see if a prism is required.
You can just email or call Baader as they are usually quite good at providing information.

[Richard]

Me honestly have not found a great difference between the very expensive dielectric and normal quality diagonals , but my eyes are not the best I wear glasses and I am in my 60's
I always thought that the dielectric was made for the army so it would not scratch when sand got in and one just wiped it? but perhaps things have changed since then.

[PeterD]

My understanding is that the dielectric ones deteriorate at much slower rate than normal diagonal mirrors. Might be wrong though.