Prism or Mirror diagonal at f7.7?

[johninderby]

2 minutes ago, JeremyS said:

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

And book an apointment at your local chiropractor. 😁

[Andrew_B]

48 minutes ago, Captain Magenta said:

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

 

A 2" prism is apparently more likely to cause noticeable false colour than a 1.25" one, simply because of the extra glass the light has to travel through. If you want to minimise scatter while also keeping false colour to a minimum then a 1.25" prism is the way to go but if you want a 2" model then a good quality mirror is probably the better choice.

[JeremyS]

4 minutes ago, johninderby said:

And book an apointment at your local chiropractor. 😁

[jetstream]

8 minutes ago, JeremyS said:

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

Yup, its the best lol!

Actually I test and compare my frac stuff to straight through views. My old Zeiss T* coated prism is superb- in triplet APOs...both which star test perfectly.

[Deadlake]

8 minutes ago, jetstream said:

Yup, its the best lol!

Actually I test and compare my frac stuff to straight through views. My old Zeiss T* coated prism is superb- in triplet APOs...both which star test perfectly.

Funny I actually think the prism diagonal gives better views in the Vixen then straight thru.

[Deadlake]

Just found this guidance:

I managed to find the question put to Thomas Baader, here is the question 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."

[JeremyS]

5 hours ago, vlaiv said:

Is BBHS Baader prism diagonal just marketing trick?

Prism diagonals don't really need silver coated surfaces to do their thing, so why put expensive BBHS (and it must be expensive because mirror treated that way is more expensive than others) coating if it does nothing?

But isn’t the point that BBHS is silver, which has a higher reflectivity across a wider wavelength than other metals? Silver used to be used in Newtonian mirrors and had a high reflectivity for a short time, but it rapidly tarnished. The BBHS is coated and thus doesn’t tarnish.  (or in the case of a prism is directly protected by the fact of being deposited on the prism)

[vlaiv]

4 hours ago, Stu said:

@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.

I think I remember reading something like that - but I have no idea how might it work. I will look to see if I can find anything on spherical aberration introduced by prism. It needs to add either positive or negative spherical - which would then add up or cancel with any spherical that scope has.

[vlaiv]

6 minutes ago, JeremyS said:

But isn’t the point that BBHS is silver, which has a higher reflectivity across a wider wavelength than other metals? Silver used to be used in Newtonian mirrors and had a high reflectivity for a short time, but it rapidly tarnished. The BBHS is coated and thus doesn’t tarnish.  (or in the case of a prism is directly protected by the fact of being deposited on the prism)

For mirror it has point, but for prism - there is no point. Total internal reflection is responsible for - well reflection in prism. There is no reflective layer of any sort - just interplay of angles and refractive indices.

Light never even reaches silvered side of prism for it to have any effect. If there was any effect - we could take black paint (or cloth not to be destructive) and cover reflective side of prism and it would "loose" its power (at least to a degree) - but that does not happen.

If you take prism and put it on black surface - it will still reflect perfectly fine - as long as angles are right.

[vlaiv]

@Stu

According to this thread:

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

over at CN - it looks like spherical itself is not very important - it is spherochromatism that changes with use of prism diagonal - because it is glass and affects different wavelengths differently.

 

[Stu]

16 minutes ago, vlaiv said:

@Stu

According to this thread:

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

over at CN - it looks like spherical itself is not very important - it is spherochromatism that changes with use of prism diagonal - because it is glass and affects different wavelengths differently.

 

Thanks Vlaiv. So that is changes in sphero chromatism rather than just changes in longitudinal CA? Trying to wrap my head around all this 🤪🤪

The curves don’t seem to change profile, they just seem shifted overall which seems just like a shift in Longitudinal CA to me, no?

[vlaiv]

6 minutes ago, Stu said:

The curves don’t seem to change profile, they just seem shifted overall which seems just like a shift in Longitudinal CA to me, no?

Indeed - changing profile of any single curve would be to alter spherical aberration at that wavelength.

But changing of their position also changes "properties" of their spherical aberration at point of best focus which is tied to green.

Imagine that you have single wavelength of light and it has some curve like in the graph. Straight vertical curve is easy - no aberration and there is single point of best focus - one that puts vertical line at horizontal 0.

But if line is curved - you need to find place of best focus - where point is smallest at focus plane / most concentrated light.

We tend to do that for green light as our eyes are most sensitive to green. If we keep green fixed - and prism changes position of blue and red - it changes shape of their spots - they can get more focused or less focused, or rather their spot diagram changes.

I know that this sounds like focus shift - but it's a bit different:

Top would be pure focus shift without spherical aberration. There is point of perfect focus and as you move away from it - spot grows linearly in size.

But what happens is at bottom image - spot diagram changes and morphs as you move its focus position (and there is no perfect focus point).

 

[IB20]

3 hours ago, JeremyS said:

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

I did do a straight through v Tak prism test on the Starbase. Target sample was v small (n=2) but couldn’t tell an iota of difference in those particular conditions. I plan to repeat the experiment on more targets and differing conditions but it’s easier at a particular altitude and a camping chair without me becoming a contortionist!

[jetstream]

6 hours ago, Deadlake said:

Funny I actually think the prism diagonal gives better views in the Vixen then straight thru.

Its the interaction between scope and prism that causes things or no things.

My TSA120 is not fussy, offering superb views with a mirror diag, prism diag and straight through. My "cheap" chinese optic SV90 also loves the prism, straight through or mirror diag and but Vic Maris did star test these scopes (hes vg at it) before sending them out.

Whats also interesting is that these not "fussy" scopes go extremely high mag and maintain razor sharp views. To me the test of a refractor is the ability to take high mag regardless of the diag used.

 

[Ian McCallum]

On 27/10/2021 at 17:37, JeremyS said:

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

Me too!🤣

[Ian McCallum]

On 27/10/2021 at 17:39, johninderby said:

And book an apointment at your local chiropractor. 😁

That too!🤣