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Prism or Mirror diagonal at f7.7?


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5 minutes ago, Stu said:

Actually, from reading that it seems that their prisms have always had this coating, does anyone know if that’s correct? I have one which is not labelled as BBHS so have always assumed it doesn’t have it.

My Max Bright does not, however the BBHS obvious does. The BBHS will also reflect into the near infra red while the Max Bright will not. Hence for NV usage it's BBHS mirror diagonal only. According to Baader the human eye is a little more sensitive in that region than people think....

Edited by Deadlake
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55 minutes ago, Deadlake said:

That seems to be the effect of a prism on my LZOS scope looking at Jupiter, it makes the belts look blurry.

The f6  is most likely the culprit, but who knows.  So its the red colour it doesnt get along with? what about barges, festoons white ovals etc? Your scope has good figure so it must be the speed? or an issue with the prism?

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18 hours ago, Franklin said:

I have an f/11 102ED refractor and I was advised to use a prism diagonal with it. So I got one of the Baader T2 32mm prism diagonals with the fine focus attachment and I think they work great together. I may have the opportunity to get my hands on a Vixen SD103S, which will be a huge upgrade for me if I do. At f/7.7 will the Baader prism be ok or should I look into getting a mirror diagonal? I've read that mirror diagonals are more suited to fast scopes and prism diagonals to slower scopes. At f/7.7 the Vixen is kind of in the middle. Will I see a difference?  👍

The Vixen should be perfectly ok with your existing prism. I used a 100mm F7.4 along with a Tak prism for years without seeing a hint of CA. Often, an observer will introduce more CA into an optical system by the use of complex wide angle eyepieces for lunar and planetary observing, than a prism would ever introduce. 

Edited by mikeDnight
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3 hours ago, jetstream said:

The f6  is most likely the culprit, but who knows.  So its the red colour it doesnt get along with? what about barges, festoons white ovals etc? Your scope has good figure so it must be the speed? or an issue with the prism?

Using a BBHS prism and the belts go blurry, lose of detail. Such back to the BBHS mirror and suddenly the belts acts are visible. 

Re-reading what BillP reported I'll go back and check the Vixen SD103S with the T2 BBHS prism and mirror diagonal.  

 

Edited by Deadlake
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@vlaiv, or anyone that knows this stuff. Does the light passing through a prism change a well corrected scope into an over corrected scope, or an under corrected scope into a well corrected scope? Not saying it doesn’t happen I’d just be interested to know if it’s true and what mechanism is in play?

Deadlake’s suggestion may be correct if that’s the case but I haven’t seen it discussed in any of the extensive reviews on these diagonals, only the CA issue with fast scopes.

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1 hour ago, Deadlake said:

The Mewlon performance is a quote from CN, the point being about the Mewlon being a slow scope and viewing with a mirror diagonal gives reportedly better result. Given the Mewlon is F11.5 I would not of expected that.

I'm not disagreeing about a fast scope having a steeper light cone, I'm trying to make the point that a prism will also effect the light path. If the scope is well corrected you can end up overcorrecting the observed image. That seems to be the effect of a prism on my LZOS scope looking at Jupiter, it makes the belts look blurry.

Dogma, didn't know that was an offensive term. Just making a choice on prism or mirror diagonal based on speed of scope may not end up with the best results. I suspect that F7 slower triplet might be better with a mirror diagonal than a prism. 


Regards to BillP, I was thinking of his report here, section 3E on Jupiter:

https://www.cloudynights.com/articles/cat/user-reviews/the-baader-bbhs-sitall-silver-diagonal-r3038

The fact that the BBHS mirror was showing more detail on Jupiter was a surprise to me, his 152 mm APO (F7.7??) should be better with a prism, but he is reporting this is not the case.

 

So your experience with a prism seems to confirm that a mirror diagonal is better with a fast scope?

Being told one is dogmatic is not normally known as a compliment. It was not dogma, just physics.

What you are likely highlighting is that there are many interconnecting factors involved in these results, and the outcomes are not always obvious. They change with observer, seeing, scope and no doubt other factor.

I am often wary of bold statements like ‘far better than’ or ‘dramatically’ this or that because in my experience the reality is much more subtle. If you read everything on CN you would think that viewing through a Televue Radian led to a coffee coloured, nay, brown image! They were jolly good little eyepieces in my experience with nothing more than a different tint to the view at most.

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3 minutes ago, Stu said:

there are many interconnecting factors involved in these results, and the outcomes are not always obvious. They change with observer, seeing, scope and no doubt other factor.

Totally agree, espc the effects of atmospheric dispersion- this made my TSA120 look like a cheap walmart scope one night on Jupiter. AD can be a huge factor when the planets are low IMHE.

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You can find something on CN to support any point of view if you look for it. I’ve read reviews on CN on equipment that I have had a chance to test myself and have been left wondering what the reviewer was on when they reviewed the equipment. 

So while there are many extremely knowledgeable people on CN there are also those that aren’t. Sorting the wheat from the chaff comes to mind.

Edited by johninderby
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I just found this post on CN which may be relevant and might confirm @Deadlakes point. Seems to suggest that prisms correct for under correction, or bring over correction to well corrected scopes.

https://www.cloudynights.com/topic/579355-prisms-and-under-correction/

I’ve not tried my prism in my LZOS basically because I assumed a mirror would be better due to induced CA in faster scopes. I’ll give it a go next time out to see whether the over correction thing is a thing. As mentioned earlier, I noticed a benefit to prisms in my Tak. Ultimately all it would do is confirm that the mirror is best in faster scopes I guess.

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11 minutes ago, Deadlake said:

BillP review linked above suggest the BBHS mirror was better then the prism in a F7.7 Lunt doublet….

That wasn’t the point. I was talking about faster scopes. There may well be something in terms of over correcting well corrected slower scopes (meaning mirrors are better there too) but I was trying to separate things out.

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You may find this comprehensive head to head field test of a large number of prism and mirror diagonals by author Bill Paolini of interest: https://www.cloudyni...omparison-r2877

"Moving to Jupiter, the Celestron 80mm APO provided very pleasing views, with the NEB, SEB, NTB, and STB belts being readily visible, and a rich amount of detail and structure visible within both NEB and SEB, including a nice storm within one of these belts.  The polar regions were also richly portrayed with gradations of shading and with the NNTB peaking-into-view ethereally when atmospheric seeing was most stable.  All the prism diagonals were showing Jupiter's details exceedingly well. The Takahashi and the two Baader prisms were fairly on-par in the amount of details and crispness of the details they showed.  The Celestron and Vixen were not quite as defined as the others, lacking the more finely etched views from the other prisms, but still quite good.

After completing observations of Jupiter in the Celestron 80mm APO with the prisms, the various aluminum, silver, and dielectric mirror diagonals were tested.  First impression when moving from observing with the prisms to the mirrors was, "wow...more scatter!"  I felt this was a rather obvious tell.  I was also surprised that the details on Jupiter were definitely softer through the mirror diagonals than they were when using the prism diagonals. As example, the NNTB was not showing though the mirror diagonals, and any structure within NEB and SEB was only hinted at as a nondescript albedo differences.  Changing out the mirror diagonals to prism diagonals, and all the NEB and SEB crisp definition and structure reappeared, as well as the ethereal NNTB.

As more and more field observations were conducted with the 80mm APO on Jupiter, it became apparent that the prisms were providing another level of performance that the mirrors were not....."

This was using an f/6.25 scope and Bill went on to state:

"While most manufacturer's seem to advertise f/7 or slower as a focal ratio for optimum prism performance, based on the observations of this comparison I feel that f/6 would also be perfectly adequate as very little color was generated even at this focal ratio."

 

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You may find this comprehensive head to head field test of a large number of prism and mirror diagonals by author Bill Paolini of interest: https://www.cloudyni...omparison-r2877

"Moving to Jupiter, the Celestron 80mm APO provided very pleasing views, with the NEB, SEB, NTB, and STB belts being readily visible, and a rich amount of detail and structure visible within both NEB and SEB, including a nice storm within one of these belts.  The polar regions were also richly portrayed with gradations of shading and with the NNTB peaking-into-view ethereally when atmospheric seeing was most stable.  All the prism diagonals were showing Jupiter's details exceedingly well. The Takahashi and the two Baader prisms were fairly on-par in the amount of details and crispness of the details they showed.  The Celestron and Vixen were not quite as defined as the others, lacking the more finely etched views from the other prisms, but still quite good.

After completing observations of Jupiter in the Celestron 80mm APO with the prisms, the various aluminum, silver, and dielectric mirror diagonals were tested.  First impression when moving from observing with the prisms to the mirrors was, "wow...more scatter!"  I felt this was a rather obvious tell.  I was also surprised that the details on Jupiter were definitely softer through the mirror diagonals than they were when using the prism diagonals. As example, the NNTB was not showing though the mirror diagonals, and any structure within NEB and SEB was only hinted at as a nondescript albedo differences.  Changing out the mirror diagonals to prism diagonals, and all the NEB and SEB crisp definition and structure reappeared, as well as the ethereal NNTB.

As more and more field observations were conducted with the 80mm APO on Jupiter, it became apparent that the prisms were providing another level of performance that the mirrors were not....."

This was using an f/6.25 scope and Bill went on to state:

"While most manufacturer's seem to advertise f/7 or slower as a focal ratio for optimum prism performance, based on the observations of this comparison I feel that f/6 would also be perfectly adequate as very little color was generated even at this focal ratio."

 

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You may find this comprehensive head to head field test of a large number of prism and mirror diagonals by author Bill Paolini of interest: https://www.cloudynights.com/articles/cat/articles/mirror-vs-dielectric-vs-prism-diagonal-comparison-r2877

"Moving to Jupiter, the Celestron 80mm APO provided very pleasing views, with the NEB, SEB, NTB, and STB belts being readily visible, and a rich amount of detail and structure visible within both NEB and SEB, including a nice storm within one of these belts.  The polar regions were also richly portrayed with gradations of shading and with the NNTB peaking-into-view ethereally when atmospheric seeing was most stable.  All the prism diagonals were showing Jupiter's details exceedingly well. The Takahashi and the two Baader prisms were fairly on-par in the amount of details and crispness of the details they showed.  The Celestron and Vixen were not quite as defined as the others, lacking the more finely etched views from the other prisms, but still quite good.

After completing observations of Jupiter in the Celestron 80mm APO with the prisms, the various aluminum, silver, and dielectric mirror diagonals were tested.  First impression when moving from observing with the prisms to the mirrors was, "wow...more scatter!"  I felt this was a rather obvious tell.  I was also surprised that the details on Jupiter were definitely softer through the mirror diagonals than they were when using the prism diagonals. As example, the NNTB was not showing though the mirror diagonals, and any structure within NEB and SEB was only hinted at as a nondescript albedo differences.  Changing out the mirror diagonals to prism diagonals, and all the NEB and SEB crisp definition and structure reappeared, as well as the ethereal NNTB.

As more and more field observations were conducted with the 80mm APO on Jupiter, it became apparent that the prisms were providing another level of performance that the mirrors were not....."

This was using an f/6.25 scope and Bill went on to state:

"While most manufacturer's seem to advertise f/7 or slower as a focal ratio for optimum prism performance, based on the observations of this comparison I feel that f/6 would also be perfectly adequate as very little color was generated even at this focal ratio."

Edit note: corrected link.

 

Edited by Second Time Around
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1 hour ago, Stu said:

That wasn’t the point.

Apologies. One thing out of this is maybe a BBHS mirror will be suitable for both fast and slow scopes. i.e. To get best performance you don't need to have prism diagonal for slow scopes unless they are under corrected.

From BillP's report, just for ease of access:

3e. Rendition of Colors

 

Initially I had not planned for any testing of how colors were portrayed by any of the diagonals.  My presumption was that they should all be relatively the same so I doubted there would be any notable differences to warrant a test.  This all changed when I began my planetary testing observing Jupiter.  On my first evening out observing Jupiter, I used the Astro-Physics dielectric diagonal first. The observation was excellent with Jupiter giving up a ton of details through the Lunt 152 Apo.  The GRS was also well in view in all its swirling glory.  When I switched to the BBHS diagonal, I was shocked as the view was obviously different, and the GRS appeared markedly more saturated displaying a beautifully bright and rich red-pink color!  This richer color of the GRS was not slight at all, making it appear surprisingly more contrasted and colorful against its surroundings.  In comparison through the dielectric, the GRS looked lackluster and pale in comparison and much less contrasted to its surroundings.   I switched diagonals several times, and repeated the observations on other evenings and the difference remained.  The BBHS silver mirror was definitely showing colors more vividly than the dielectric, and even a little better than the prism as well. 

1 hour ago, Stu said:

The last post on the thread suggests using a Baader GPC which corrects for binoviewer prisms.  If you have a x1.25 GPC, pop it in front of the prism and see if it helps.

I have a 2.6 GPC, I can try if that helps with the prism. 

Edited by Deadlake
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Shortly after acquiring my LZOS 105/650 f/6.2 scope I tried a direct comparison between my Baader 2” BBHS prism diagonal, and a Revelation 2” dielectric mirror diagonal. I can’t recall the eyepiece I used, but it was the same for both.

I looked at lunar features. With the prism diagonal I could easily see colour-fringes, with the mirror diagonal I could not discern any colour aberration.

Just an extra data-point.

Magnus

 

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3 hours ago, jetstream said:

Totally agree, espc the effects of atmospheric dispersion- this made my TSA120 look like a cheap walmart scope one night on Jupiter. AD can be a huge factor when the planets are low IMHE.

I think I've been able to use the scope over a range of seeing condition. to rule out seeing in this case between the prism and mirror diagonals. 

I did actually have a night of good seeing and was using a GEM mount, which is why the difference between the two diagonals stood out, didn't have to think about tracking.

This CN thread shows the effect of a prism effect on the longitudinal shift of colours caused by changes in spherochromatism. 

https://www.cloudynights.com/topic/544060-spherical-abberation-correction-and-diagonal-prism-size/

Edited by Deadlake
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3 hours ago, wookie1965 said:

From reading this I should get prism diagonals for my slow scopes i.e two f10s and a 9.3 going cost me a fortune. 

From reading this, I’m going to go Japanese and do straight through viewing 🤣

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