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Are we overlooking the obvious?

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#151 25585

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Posted 18 June 2019 - 12:36 PM

I do not want a straight through finder, even if it is correct image. RACI is good.

And so are high quality RACI main prisms.    

 

One advantage of refractors and Cassegrains for viewer-friendliness is that users can buy themselves a RACI prism diagonal and have normal eyesight orientation.

 

On Baader's site, they do not recommend their own cheap RACI prisms for high magnifications. 

 

TV do sell a 60° roof prism, but it's not CI, a strange omission.   


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#152 Mitrovarr

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Posted 18 June 2019 - 12:37 PM

For whatever it's worth the big mirror and prism comparison found it really didn't matter for deep sky. The small transmission differences were just not detectable.


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#153 Sarkikos

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Posted 18 June 2019 - 01:23 PM

For whatever it's worth the big mirror and prism comparison found it really didn't matter for deep sky. The small transmission differences were just not detectable.

The difference in weight would be detectable.  wink.gif

 

Mike


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#154 Astrojensen

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Posted 18 June 2019 - 02:59 PM

 

But, I would point out, AstroPhysics and TeleVue and StellarVue offer mirror diagonals, not prisms, with their superb refractors.

Even Takahashi's 2" diagonal is a mirror.

If prisms were demonstrably superior, this would not be the case.

This is not necessarily true. The reason A-P, TV and Stellarvue all offer mirror diagonals and not prisms is highly likely that they're worried about the spherical aberration prisms introduce in fast refractors. This is especially true of the sub-f/6 scopes from TV. This worry obviously (as it should, in this case) trump any worry about a small increase in scatter. 

 

 

Clear skies!
Thomas, Denmark


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#155 Astrojensen

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Posted 18 June 2019 - 03:05 PM

The difference in weight would be detectable.  wink.gif

 

Mike

True, but the difference isn't huge. My Baader Maxbright 2" dielectric diagonal weighs 538 grams and my 2" Baader-Zeiss prism diagonal with 2" Clicklock eyepiece holder weighs 624 grams, so not all that much of a difference. 

 

 

Clear skies!
Thomas, Denmark



#156 Starman1

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Posted 18 June 2019 - 03:51 PM

I do not want a straight through finder, even if it is correct image. RACI is good.

And so are high quality RACI main prisms.    

 

One advantage of refractors and Cassegrains for viewer-friendliness is that users can buy themselves a RACI prism diagonal and have normal eyesight orientation.

 

On Baader's site, they do not recommend their own cheap RACI prisms for high magnifications. 

 

TV do sell a 60° roof prism, but it's not CI, a strange omission.   

It was for birding, where L-R reversal is unimportant.


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#157 Redbetter

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Posted 18 June 2019 - 07:08 PM

For whatever it's worth the big mirror and prism comparison found it really didn't matter for deep sky. The small transmission differences were just not detectable.

The author is not particularly experienced in threshold or low surface brightness DSO observing based on what he has said on such subjects in the past.  This would be a difficult test and requires dark sky observing, again, not the author's bailiwick.  The tests were suburban and on bright DSO's.  I am not sure of the best way to do such a threshold DSO test of diagonals either, I am just saying that it would require different conditions and probably a tester experienced with that sort of observing.

 

I suspect it isn't the transmission differences that might matter so much as any scatter or issues with "glare" off of walls, etc.  It might be that dark sky tests of low surface brightness objects would track well with the suburban results.  I don't know, but I would be reluctant to draw conclusions about them at this point.


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#158 Jeff Morgan

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Posted 18 June 2019 - 10:44 PM

Why a mirror for deep sky?

 

Wouldn't less scatter also be advantegous for deep sky objects (at least in larger aperture greater 8" where even some deep sky objects are bright).

 

Scatter is a significant factor in planetary observing. Bright, small details, using your cone cells. Very high resolution (3x that of rod cells). And of course, color vision. Often a feature of almost identical color to its background. Think Ganymede transiting Jupiter. Scatter blurs small color differences.

 

Deep sky is a totally different game. Generally large objects (as compared to planets), low brightness, averted vision, lower resolution, no color. Scatter just not a player here.

 

Because of diverse equipment and requirements I may end up with both the Zeiss-spec T2 and the 2" BBHS. In that case I will give the T2 a spin on some DSO's. I suspect it will be very nice on open clusters with strong color tints. But I'm not expecting a lot outside of that.


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#159 Magnetic Field

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Posted 19 June 2019 - 03:26 AM

The author is not particularly experienced in threshold or low surface brightness DSO observing based on what he has said on such subjects in the past.  This would be a difficult test and requires dark sky observing, again, not the author's bailiwick.  The tests were suburban and on bright DSO's.  I am not sure of the best way to do such a threshold DSO test of diagonals either, I am just saying that it would require different conditions and probably a tester experienced with that sort of observing.

 

I suspect it isn't the transmission differences that might matter so much as any scatter or issues with "glare" off of walls, etc.  It might be that dark sky tests of low surface brightness objects would track well with the suburban results.  I don't know, but I would be reluctant to draw conclusions about them at this point.

Your post reads like an insurance policy. I don't understand a single world.

 

Who is that author and what did he claim that is not supported by evidence.

 

It is still not clear to me if a prism would also have advantages for deep sky observing.

 

Can we conclude a prism is the Jack of all trades?


Edited by Magnetic Field, 19 June 2019 - 03:28 AM.


#160 Redbetter

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Posted 19 June 2019 - 04:11 AM

Your post reads like an insurance policy. I don't understand a single world.

 

Who is that author and what did he claim that is not supported by evidence.

 

It is still not clear to me if a prism would also have advantages for deep sky observing.

 

Can we conclude a prism is the Jack of all trades?

Read the thread, follow the link that was posted earlier. 

 

I didn't say he claimed anything unsupported by evidence, hence to wording I chose that went completely over hour head.   I merely pointed out the circumstances of the testing employed and his stated level of experience as it applies to fainter DSO's.  I consider those potentially major factors in determining whether the testing is applicable to the observing I do.  However, that doesn't mean the conclusions were actually wrong, only that the test isn't likely to be conclusive for DSO's (good, bad, or indifferent.)  A more critical test performed by a tester with the appropriate experience could better establish that.

 

As for your last question the simple answer is "no."  That has also been covered in the thread.  If I were to select a jack-of-all trades it would be a mirror since it can handle both short and long ratios while prisms introduce substantial aberrations at short ratios.  Being a jack-of-all trades often implies that there are things that something/someone does that are acceptable, but they might not be the best at any or all of them.



#161 Redbetter

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Posted 19 June 2019 - 04:38 AM

A bit of misalignment really is a very minor issue, since most refractors have very large diffraction limited fields.

Thomas, I disagree with this statement.  I am finding refractors very sensitive to misalignment and the place to start is by ruling out the diagonal and focuser.  It is enough of a problem that unless the alignment is true, I wouldn't even bother trying to evaluate the optical performance of the objective.  It could be that misalignment manifests as some sort of wedge error.   Whatever the nature, it is painfully apparent to me visually.  I would like to say it is confined to short ratios, but I see it at f/12 and f/15 as well.  Short ratios likely magnify the effect.

 

Case in point:  I have been trying to get some decent seeing to properly evaluate a short ratio refractor straight through, as well as for evaluating some diagonals and other small refractors.  The f/5 refractor I have been testing at first appeared to have a substantial optical defect resembling coma/flaring in one axis.  I thought it might be tilt of the eyepiece, but could not null it that way...initially.  A refractor collimation tube showed the focuser and objective collimation to be good once I squared things up carefully.. 

 

So I rechecked with an eyepiece and carefully nulled everything square with the three set screws and some spacers to facilitate that.  The result was that the aberration disappeared when pointed to planets and stars.  The next time I went to use it, I slipped in an eyepiece and the problem was back, just like before.  But I again found that I could fully null it through judicious use of the set screws.  Physically this might be a small misalignment, but in this particular very short focal length scope it has an outsized impact on the image.    


Edited by Redbetter, 19 June 2019 - 01:00 PM.


#162 Sarkikos

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Posted 19 June 2019 - 06:15 AM

A little anecdotal evidence in favor of the Baader 2” BBHS Sitall Mirror Diagonal.  A few weeks ago I spent a night at a yellow zone site, viewing DSO with my NP-101is and the BBHS.  The Leica Zoom was in the BBHS most of the time.  The sky was very transparent (for a change).  I saw 131 DSO that night.  A number of galaxies were 11th magnitude, one was 12th magnitude.  Not bad for a 4" refractor at a yellow zone site.

 

Mike


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#163 turtle86

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Posted 19 June 2019 - 09:44 AM

I think if you're willing/able to pony up for one of the higher end diagonals (A-P, TV, Baader, etc.) there's probably not too much to worry about.
I never get the sense that my A-P Maxbright degrades the view in any meaningful way, whether with bright objects or dim DSO's.

The challenge, as with most other gear, is to find comparable performance at a less expensive price point.

Edited by turtle86, 19 June 2019 - 11:45 AM.

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#164 Magnetic Field

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Posted 19 June 2019 - 02:52 PM

Read the thread, follow the link that was posted earlier. 

 

I didn't say he claimed anything unsupported by evidence, hence to wording I chose that went completely over hour head.   I merely pointed out the circumstances of the testing employed and his stated level of experience as it applies to fainter DSO's.  I consider those potentially major factors in determining whether the testing is applicable to the observing I do.  However, that doesn't mean the conclusions were actually wrong, only that the test isn't likely to be conclusive for DSO's (good, bad, or indifferent.)  A more critical test performed by a tester with the appropriate experience could better establish that.

 

As for your last question the simple answer is "no."  That has also been covered in the thread.  If I were to select a jack-of-all trades it would be a mirror since it can handle both short and long ratios while prisms introduce substantial aberrations at short ratios.  Being a jack-of-all trades often implies that there are things that something/someone does that are acceptable, but they might not be the best at any or all of them.

Can anyone post that link?

 

I found the following link in this thread by back scrolling: http://llopt.com/astronomy-services/

 

Is this the link you are talking about?

 

It would really often help if people would make a reference.



#165 Starman1

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Posted 19 June 2019 - 02:57 PM

Can anyone post that link?

 

I found the following link in this thread by back scrolling: http://llopt.com/astronomy-services/

 

Is this the link you are talking about?

 

It would really often help if people would make a reference.

See post #5 and the link in post #77.


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#166 starman876

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Posted 20 June 2019 - 07:09 AM

This thread is great.  Looks like it is time to start using my AP diagonal all the time.   Any information on Takahashi diagonals?

 

Thanks for all the great information.  


Edited by starman876, 20 June 2019 - 07:09 AM.

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#167 25585

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Posted 20 June 2019 - 07:41 AM

My AP was my first dielectric, though the TV aluminised are not at all disappointing. I just wanted an AP product, and a near-mint pre-owned reduced enough came along. Likewise, later, a TV Everbrite. To be honest, there is very little difference, if any, between the 3 models for what I look at, and the magnifications used. I also have a SW dielectric as an OE kit piece, and visually, nothing much different either.

 

Strength of construction and rigidity, where the AP & TV excel, is important for the big heavy eyepieces I use. There its focuser strength that is most important, perhaps another "obvious" that is overlooked too often. 

 

My 2 correct image prisms were chosen and bought intentionally. Both APM roof and the Baader Amici astro model are excellent. I bought the APM in their re-location sale, and the Baader just in time before Brexit kicked the £:€ rate in its teeth again. Both these prisms give bright beautiful views, no vignetting. I have used each in a F5 refractor and had no noticable issues. APM wins on value for money, Baader for perfectionists.

 

Why skimp on quality when it could compromise your telescope and eyepiece investments?  

 

 


Edited by 25585, 20 June 2019 - 08:11 AM.

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#168 Sarkikos

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Posted 20 June 2019 - 09:11 AM

A Baader T-2 Amici Prism is in my 70mm RACI finder.  Works great.  I've never seen the line.

 

Mike


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#169 Starman1

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Posted 20 June 2019 - 09:40 AM

A Baader T-2 Amici Prism is in my 70mm RACI finder.  Works great.  I've never seen the line.

 

Mike

It's unlikely you will.  Especially at low power in a finder scope (nice finder, by the way!).

But if you do want to see it, point the finder at a daytime sky and stand back from the focuser (no eyepiece present)

and look into the diagonal.  It will be a hair-thin gray line running from top to bottom in the image.

Of course, that doesn't correspond to a situation of use, but I have yet to see a correct-image prism I couldn't see the line in when doing that.


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