114 replies to this topic

[triplemon]

I measures a bunch of commercial Star Diagonals years ago (wavefront, spectral transmission, alignment) and found that only 2/15 were premium performers, 4/15 acceptable performers and the other 9 deficent or unusable.

Tom,

these diagonals sit pretty close to the focal plane, so the relevant light beam diameter that gets combined to a single point at the focal plane is MUCH smaller than the full aperture of the mirror. Does your strehl calculation accunt for that ? Would your measurement pick up such small scale surface deviation in the first place ?

 

For numbers, these diagonals are localted some 50-60mm from the focal plane. A good scope using such a diagonal is likely around f/7 or slower (mind you, these are not used with newtonians), making the relavant spot on the diagonal just under 8mm diameter. Only the relative surface deviation across any such diameter would matter. Its what primary mirror folks would call "surface roughness". The measurement itself would need to pick up such high order deviations across less than 1/6th of the full aperture to be meaningful.

 

I only worry about strehl close to the center of that diagonal, the part in use for high power observations. If the diagonal mirrors edge is turned or has zones - say even the full outer half of the diameter, that is only relevant for stars near the field stop in a low power, wide FOV eyepiece. And even the best eyepices are far from being diffraction limited out there, even at f/7.

Edited by triplemon, 19 May 2025 - 01:31 PM.

[TOMDEY]

Tom,

these diagonals sit pretty close to the focal plane, so the relevant light beam diameter that gets combined to a single point at the focal plane is MUCH smaller than the full aperture of the mirror. Does your strehl calculation accunt for that ? Would your measurement pick up such small scale surface deviation in the first place ?

 

For numbers, these diagonals are localted some 50-60mm from the focal plane. A good scope using such a diagonal is likely around f/7 or slower (mind you, these are not used with newtonians), making the relavant spot on the diagonal just under 8mm diameter. Only the relative surface deviation across any such diameter would matter. Its what primary mirror folks would call "surface roughness". The measurement itself would need to pick up such high order deviations across less than 1/6th of the full aperture to be meaningful.

 

I only worry about strehl close to the center of that diagonal, the part in use for high power observations. If the diagonal mirrors edge is turned or has zones - say even the full outer half of the diameter, that is only relevant for stars near the field stop in a low power, wide FOV eyepiece. And even the best eyepices are far from being diffraction limited out there, even at f/7.

Sure, you can most often get away with that sensible realization. There are scaling factors available for all of the Zernike Circle polynomials to transform my "full aperture" wavefronts to your known footprint diameter as you would use it (called "cookies" in the optical test community). Some applications actually do use all or most the entire folding flat for each field point. e.g. my Solar Projector here >>> Because of that, I used three cascaded superb Star Diagonals with my measured full-aperture Strehls of 0.99, 0.97, and 0.93. Casual rule of thumb multiplies these for an operational Strehl of roughly 89%. In that "multi use" sense, it's useful to at least know which of your Star Diagonals deliver great wavefront... even for occasional use where big cookies are involved.    Tom

 

[For our aerospace builds we always certified system wavefront contribution over all cookies on each and every surface and element for 5 fields minimum, scores of fields (more common)].    Tom

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[MrsM75]

I got the Celestron Prism today, it looks plastic and right off the bat I already hear a moving sound inside when holding it. 

I do not think it will beat the GSO dielectric, let alone the William Optics Durabright one. But I will try it tonight.

[betacygni]

24panoptic has a wider FOV than the 32plossl.

The true field of view are the same with the 24mm panoptic and 32mm plossl. Both have a 27mm field stop, you just get more apparent field of view and smaller exit pupil with the panoptic, which may or may not be preferable.

[Mike W]

The true field of view are the same with the 24mm panoptic and 32mm plossl. Both have a 27mm field stop, you just get more apparent field of view and smaller exit pupil with the panoptic, which may or may not be preferable.

I just did a poor job of stating AFOV also magnification difference (darker background sky).

Edited by Mike W, 19 May 2025 - 06:17 PM.

[T1R2]

I got the Celestron Prism today, it looks plastic and right off the bat I already hear a moving sound inside when holding it. 

I do not think it will beat the GSO dielectric, let alone the William Optics Durabright one. But I will try it tonight.

Oh sorry you have problems with it, I love mine, in post #61 I tell you how to address any problems you might have with it, I would try to tighten the small 1.5mm hex set screws on the side of the diagonal, they hold the prism inside without letting it fall out when/ if the back plate comes off either when removing it for cleaning or by accident. And if the back plate is loose it will also allow the prism to rattle. But I think you should tighten the small hex screws in the side of the diagonal .

[MrsM75]

So my $44 Celestron Prism come from Amazon it rattling inside, the prism is loose inside. Should I still try it tonight?

 

Good thing I can get a return easy with Amazon. It quite rattling inside the prism inside it moves.

 

You mean the 4 screws on the bottom plate of the diagonal, it is super tight already. The prism inside it loose.

 

It does not look used at all, no marking on the barrel. It a new diagonal, it just bad QC quality control in made in China $44 cheap item.

Edited by MrsM75, 19 May 2025 - 10:07 PM.

[quilty]

it shouldn't rattle. None of mine do. the prism uses to be fixed by some foam rubber. You can fix it easily on our own or send it back.
I predicted you wouldn't like it.
Screw off the diagonal tin plate and insert some 2 mm foam plastic or rubber and the rattling is sorted out.
unscrew the baffle going into the focusser and glue it fix. This is a safety measure which avoids any unintended unscrew and ep drop. (Two turns of aluminium into plastic and it's off, quite a flimsy attachment which needs a help)
Then it probably does better than your dielectrics

Edited by quilty, 19 May 2025 - 10:22 PM.

[T1R2]

So my $44 Celestron Prism come from Amazon it rattling inside, the prism is loose inside. Should I still try it tonight?

 

Good thing I can get a return easy with Amazon. It quite rattling inside the prism inside it moves.

 

You mean the 4 screws on the bottom plate of the diagonal, it is super tight already. The prism inside it loose.

 

It does not look used at all, no marking on the barrel. It a new diagonal, it just bad QC quality control in made in China $44 cheap item.

Tighten the 1.5mm hex screws on the side in the center, they require a 1.5mm Allen wrench or one of those small interchangeable screw driver bits

The small screws my finger is pointing to. 

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[T1R2]

First back the screws out and gently thread them back in until they touch the prism, and only add a very small amount of tension so they don't back out 

[MrsM75]

Tighten the 1.5mm hex screws on the side in the center, they require a 1.5mm Allen wrench or one of those small interchangeable screw driver bits

The small screws my finger is pointing to. 

ah that Hex screw, got it, I thought you mean the 4 screws on that bottom plate where it say Celestron on it.

 

 I do have alot of Hex screws in all sizes, as I use it to collimate my Maks.

 

Thank you Sir, go try that now.

[quilty]

I'd fix the diagonal face, see above

[Levant]

ah that Hex screw, got it, I thought you mean the 4 screws on that bottom plate where it say Celestron on it.

 

 I do have alot of Hex screws in all sizes, as I use it to collimate my Maks.

 

Thank you Sir, go try that now.

I would never try to fix something brand new. A refund may be difficult if you scratch it. I would just return it as soon as possible.

[T1R2]

I would never try to fix something brand new. A refund may be difficult if you scratch it. I would just return it as soon as possible.

Well, we're not fixing anything we're tightening a couple screws, there isn't any way to scratch it by what she was doing, the set screws touch the frosted sides of the prism. and as long as you don't use 100kg's of torque, you cant really do any damage. 

[Procyon]

Amazon would be happy if a customer repaired some of the junk they send instead of returning it. If it doesn't work out send it back. Break a 40 dollar warranty? Are you kidding lol.

Edited by Procyon, 20 May 2025 - 02:36 AM.

[quilty]

She needs to fix it anyway. The barrel into plastic thread (2 turns) is unsafe. I'd never insert a larger ep into like a Baader Hyperion.
Part of the deal, flimsy mechanically but optically decent and cheap.

any of my 5 semi plastic diagonals went off non intendedly. All needed a gluing fix, one was rattling.
Now they're fine all

Edited by quilty, 20 May 2025 - 03:12 AM.

[quilty]

Well, we're not fixing anything we're tightening a couple screws, there isn't any way to scratch it by what she was doing, the set screws touch the frosted sides of the prism. and as long as you don't use 100kg's of torque, you cant really do any damage.

You're not even fixing a possibly maladjusted prism that way. Rattling means the prism is loose, right?
Are you sure to get it into the correct position by a side fix?

You can send it back for a replacement part which needs a fix as well. And produce another ton of CO2. Or just fix it. Easier than wrap and send and wait...

Too much talk about such a simple device

Edited by quilty, 20 May 2025 - 03:10 AM.

[T1R2]

You're not even fixing a possibly maladjusted prism that way. Rattling means the prism is loose, right?
Are you sure to get it into the correct position by a side fix?

You can send it back for a replacement part which needs a fix as well. And produce another ton of CO2. Or just fix it. Easier than wrap and send and wait...

Too much talk about such a simple device

She doesn't even know if it is misaligned yet,  they are easy to align as I detailed in post #61 either by taking the back off and placing a square of craft felt in place of the leaf spring and screwing the back back onto the diagonal and allowing the back to hold the prism in place. Or by putting the diagonal in a refractor with a laser collimator in the EP holder and placing a paper mask over the front of the lens with a center + on the front, and placing a mirror in front of the scope so you can see the front while standing at the diagonal then loosen the side hex screws and manipulating the back plate screws until the laser hits the center of the mask, then once align you retighten the side hex screws. But at f12-15 I don't think a small misalignment of that diagonal will have much effect on the image. 

[quilty]

But I do know it's missaligned when it rattles. And I think a side fix doesn't make sure of a correct position
your No. 61 I'd do at once. Most easily as said in 83.
Everyone is free to do how he wants. But not everything makes sense the same.

By the way, these cheap diagonals use to push the ep out of central position. Do you consider this by any means?

Edited by quilty, 20 May 2025 - 03:48 AM.

[T1R2]

But I do know it's missaligned when it rattles. And I think a side fix doesn't make sure of a correct position
your No. 61 I'd do at once. Most easily as said in 83.
Everyone is free to do how he wants. But not everything makes sense the same.

By the way, these cheap diagonals use to push the ep out of central position. Do you consider that in any way?

At long focal ratios it doesn't really matter, it can be off a little, there is not very much space in the body for it to get to far out of alignment, and like I said at f12-15 you wont be able to see any major effects of that misalignment... if any. Diagonal's that are misaligned can induce astigmatism in short focal length/ short focal ratio scopes, but in long focal ratio scopes that misalignment isn't as damaging as it would be in a f5 refractor.

 

No I don't care if the cheap set screws pushes the EP out of alignment, because its not noticeable, it has virtually no effect at f12-15, it doesn't make any difference.   

[Jon Isaacs]

Some applications actually do use all or most the entire folding flat for each field point.

 

 

In the telescope world, that would be the Newtonian.. 

 

A diagonal that rattles, that is not promising.  

 

Jon

[quilty]

At long focal ratios it doesn't really matter, it can be off a little, there is not very much space in the body for it to get to far out of alignment, and like I said at f12-15 you wont be able to see any major effects of that misalignment... if any. Diagonal's that are misaligned can induce astigmatism in short focal length/ short focal ratio scopes, but in long focal ratio scopes that misalignment isn't as damaging as it would be in a f5 refractor.
 
No I don't care if the cheap set screws pushes the EP out of alignment, because its not noticeable, it has virtually no effect at f12-15, it doesn't make any difference.

Well, I do.

Fixes anywhere

1. prism, if rattling
2. gluing barrel (the one into focusser)
(3. ep centering)
4. longer clutch screws for oversized eyepieces (as Baader Hyperion, have you ever tried these with that C prism?), see pic.
5. one clutching M3 thread was stripped soon. Warranty case? Don't know, don't bother, drilled a new M4 thread into, quite probably more to follow



That's why I don't think Mrs will like it.

Edited by quilty, 20 May 2025 - 04:28 AM.

[T1R2]

Well, I do.

Fixes anywhere

1. prism, if rattling
2. gluing barrel (the one into focusser)
(3. ep centering)
4. longer clutch screws for oversized eyepieces (as Baader Hyperion, have you ever tried these with that C prism?), see pic.
5. one clutching M3 thread was stripped soon. Warranty case? Don't know, don't bother, drilled a new M4 thread into, quite probably more to follow

epsw.JPG

That's why I don't think Mrs will like it.

Nope, I didn't have to do any of that with my Celestron Prism, the nosepiece was thread locked in place and was very hard to get off, my EP holder works just fine, it doesn't move any EP out of alignment, but I use mine with Svbony 66* Goldlines and occasionally Meade HD60's or ES 20/68. My prism is perfectly aligned because I removed the flat leaf spring permanently thats used for collimation and cut a rectangle piece of black craft felt the size of the black piece of cardboard prism backing that's in the back of the diagonal and placed that in the back against the backplate in its place and it holds the prism in perfect alignment, and I never have to adjust it, I just thread the back plate on all the way and presto..its perfect. 

[quilty]

Let's say I seem to use them more intensely. Don't use any 2" diagonals. Hypers are at moment my new standard.
Goldlines did before but now replaced by Hypers, except the 20mm. Waiting still for the 21 and 24 mm Hyper to occur.
The extended 17 is now 13.2 mm, the 10 is 8.2 mm. the 21 will be about 18 and the 24 can't be extended. Oh yes, and the 8 mm extended by 20 mm is now 5.5 mm (at 100°+ AFOV)
Specially this one brings quite a torque to the focusser barrel at 8 to 10 o clock position which none of my 5 prisms stood.

The goldlines (=Svbony) are hard to replace, impossible at their price. Well, the reduced extended Hypers do. (RER Hypers, reduced price, extended length, reduced fl)
And the C (SW and Meade, all the same) semi plastic prisms are supposed to do (after fixing 1-5)

Edited by quilty, 20 May 2025 - 05:19 AM.

[T1R2]

Let's say I seem to use them more intensely. Don't use any 2" diagonals. Hypers are at moment my new standard.
Goldlines did before but now replaced by Hypers, except the 20mm. Waiting still for the 21 and 24 mm Hyper to occur.
The extended 17 is now 13.2 mm, the 10 is 8.2 mm. the 21 will be about 18 and the 24 can't be extended. Oh yes, and the 8 mm extended by 20 mm is now 5.5 mm

The goldlines (=Svbony) are hard to replace, impossible at their price. Well, the reduced extended Hypers do.
And the C (SW and Meade, all the same) semi plastic prisms are supposed to do

I use my diagonal for double stars and lunar/ planetary, and I also use the diagonal to move the telescope around, is that the "Intensely" you're referring to? or heavy ep's?

 

But lets no hijack the thread on this. 

Edited by T1R2, 20 May 2025 - 05:18 AM.