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Which diagonal when using small refractor as its own finder?

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#1 tgreene

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Posted 24 September 2021 - 05:11 PM

I'd appreciate thoughts & suggestions on what type of diagonal is best for a small refractor (80 - 100 mm diameter) when using it as its own finder. 

I have an 80 mm f/7 ED 'scope with no finder. I like to find objects by sighting along the tube and then looking for them in the 'scope's field using a low-power, 24 mm fl eyepiece. I am currently using a 1.25" dielectric diagonal, but I was wondering if others find a correct-image prism diagonal to be more intuitive when searching for objects via small moves in altitude or azimuth.

 

Any downsides to using a prism diagonal - would their lower throughput be noticeable and would one impact the optical quality?

 

Thanks in advance for any thoughts.



#2 ButterFly

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Posted 24 September 2021 - 05:18 PM

Not really for night use - things don't move that quickly, so just go left instead of right.  For daytime use, things can fly around quickly, so I prefer a 45 degree correct view prism.  I can sight along and peek in the eyepiece at the same time.  A 90 degree prism will be the exact same thing as a 90 degree mirror, except the view is left-right correct.

 

You can always add some iron sights to your scope using wire.


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#3 Mark9473

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Posted 24 September 2021 - 06:02 PM

I definitely prefer a correct image for starhopping. After many many years of dealing with the mirrored image of a normal diagonal, it felt instantly better when I got a correct-image diagonal. I think part of that is that I often also use binoculars on the night sky.


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#4 c2m2t

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Posted 24 September 2021 - 06:24 PM

Hi tgreene!

I am in total agreement with Mark, correct image observing is much more satisfying. I have two 2" WO correct image diagonals that satisfy my needs very nicely with one small exception. On bright objects the prism generates a horizontal spike through the complete field that can be detrimental to many but not all views of bright objects....not an issue for bright DSO's. It really depends on the target and personal tolerances. I always have on hand a mirror diagonal if a particular subject wants to be rendered without the spike.

 

Getting back to using it primarily as a finder, the correct image diagonal is ideal. I would suggest the reduced light through-put is negligible if even noticeable. The correct image makes star hopping as per a star chart much easier....I used to do the flip/mirror/rotate routine but once having a corrected view, there was no going back. I am at an age now that mental gymnastics becomes an unnecessary chore! grin.gif I'd rather save the effort for the subject matter.

 

The WO 2" correct image diagonals are no longer available. Baader makes a good one as well and there may be others. I would suggest, if at all possible, kick the tires before you purchase a correct image diagonal. The Baader is an investment.

 

Cheers, Chris.


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#5 tgreene

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Posted 24 September 2021 - 07:41 PM

Thanks all for your comments so far - very helpful!

 

I have a new small 'scope on order from Stellarvue, and I'll look into getting one of their 90 deg prism diagonals (1.25" is fine for now).



#6 Jon Isaacs

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Posted 24 September 2021 - 08:55 PM

Thanks all for your comments so far - very helpful!

 

I have a new small 'scope on order from Stellarvue, and I'll look into getting one of their 90 deg prism diagonals (1.25" is fine for now).

 

I use a 2 inch mirror diagonal with my short focal length diagonals.  As long as the diagonal is oriented so the eyepiece is vertical, rather than off to the side, the left-right reversal is not an issue. With refractors, I use star diagonals for everything, terrestrial, birding, star hopping. 

 

As far as a prism diagonal, there two kinds of prism diagonals, correct image and star diagonals.  It seems like you want a correct image diagonal.  

 

Jon


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#7 gwlee

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Posted 25 September 2021 - 12:24 AM

I prefer 2” eyepieces and a 90* AMICI prism diagonal with my 72mm refractor that provides a correct image. Depending on the EP, it gives a 5* FOV at 11x or a 4* FOV at 16x. 4* is the size of the largest ring in the Telrad finder that I use on my dob. When not being used for astronomy, the little refractor with its AMICI prism diagonal is a useful spotting scope. 
 

On the brightest objects on the darkest nights at 87x, I have seen a faint horizontal line with my 92mm scope, but not with my 72mm scope. Therefore, the AMICI normally lives in the 72mm scope, where it seems to work as well as my TV Everbrite mirror diagonal. 


Edited by gwlee, 25 September 2021 - 10:52 AM.

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#8 Sketcher

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Posted 25 September 2021 - 01:30 AM

For your purpose, I wouldn't be concerned about light transmission differences.  Besides, any such differences will likely not be noticeable enough to matter.

 

Some diagonals will have smaller clear apertures than others.  This is more common with some of the prism diagonals than it is with mirror diagonals;  but sometimes the reverse can be true.  This might be of greater concern than light transmission when using a 24mm (68 degree apparent field I presume) eyepiece -- where you'll likely want a more fully illuminated field.

 

The preference between mirror-reversed and correct-image diagonals, in my opinion, will depend on additional details that you've not mentioned.  If you're not matching the star fields in the scope with star fields on a chart, then I see no real reason to prefer one over the other.  But if you are matching star fields, then you'll want to match the field orientations of the diagonal to that of the charts you'll be using -- or, if using electronic charts, you'll want to toggle the chart orientation to match whichever diagonal you have in your scope.

 

Beware, field orientation will rotate when either type of diagonal is rotated.  With the way I use a refractor, the diagonal gets rotated to whatever position is most comfortable for me, and this will depend on the height of the eyepiece (which will vary with the altitude of the object) and whether I'm sitting (usually the case for me) or standing -- a situation where it's more tiresome to make prolonged, and more detailed observations.

 

Another comfort issue is the choice between a 90 degree and a 45 degree diagonal.  (Correct-image diagonals are available in either format.)  For me, for astronomical observations, I find a 90 degree diagonal to be far more comfortable to use.  For higher altitude objects, a 45 degree diagonal can end up being more awkward to look through -- for me.

 

To answer your other question:  I usually prefer using a simple, 90 degree (not correct-image) diagonal.  But I have, and have made use of, other diagonals for special projects that are outside the realm of this thread.  Sometimes it can be a good thing to have access to a variety of different diagonals.  So if you get one that you find doesn't work out for you, you might want to consider holding on to it.  You never know what the future might hold.


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#9 LDW47

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Posted 25 September 2021 - 04:27 AM

Good range of advice regarding diagonals but as a bit more 

 

I'd appreciate thoughts & suggestions on what type of diagonal is best for a small refractor (80 - 100 mm diameter) when using it as its own finder. 

I have an 80 mm f/7 ED 'scope with no finder. I like to find objects by sighting along the tube and then looking for them in the 'scope's field using a low-power, 24 mm fl eyepiece. I am currently using a 1.25" dielectric diagonal, but I was wondering if others find a correct-image prism diagonal to be more intuitive when searching for objects via small moves in altitude or azimuth.

 

Any downsides to using a prism diagonal - would their lower throughput be noticeable and would one impact the optical quality?

 

Thanks in advance for any thoughts.

Just stick with the dielectric diagonal that you have, it won't get any better  I tried a finder, got rid of them all years ago, I have a box full of them of all kinds. One step up if your scope can handle them is a 2" dielectric and a 30-40mm 2" widefield ep for finding / star hopping and then go up in mag from there  For most thats the ultimate way to whet your finding skills  Think about it when you are scanning the vast reaches of space for dso's or even the moon / planets how do you find the time or even discern right from left or top from bottom, does it really matter, does it make what easier in the vastness of .......... Terestrial viewing is a different story, so have a backup diagonal for that, they aren't that expensive


Edited by LDW47, 25 September 2021 - 04:42 AM.


#10 brightsky

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Posted 25 September 2021 - 10:17 AM

I'd appreciate thoughts & suggestions on what type of diagonal is best for a small refractor (80 - 100 mm diameter) when using it as its own finder. 

I have an 80 mm f/7 ED 'scope with no finder. I like to find objects by sighting along the tube and then looking for them in the 'scope's field using a low-power, 24 mm fl eyepiece. I am currently using a 1.25" dielectric diagonal, but I was wondering if others find a correct-image prism diagonal to be more intuitive when searching for objects via small moves in altitude or azimuth.

 

Any downsides to using a prism diagonal - would their lower throughput be noticeable and would one impact the optical quality?

 

Thanks in advance for any thoughts.

I used a 1.25" Tak prism for a year then changed to the Baader T-2 90° Deluxe Astro-Grade Amici prism diagonal on my refractor. I use a 24 mm eyepiece for spotting with the scope as you do.  So far, switching back and forth between diagonals for comparison has raised no concerns. This is not to say that someday I may encounter a fatal flaw in the Amici, but thus far in viewing DSOs and planets I have adopted and enjoy the ease of correct image viewing and see no reason to change back.  I will still drag out both diagonals on excellent seeing nights and compare, but so far trouble with diffraction spikes or clarity is not appreciated.  The only downside was the cost of this particular Amici diagonal, but I now feel it was well worth the cost for me. So I remain a skeptical user of the Baader diagonal, but am very much enjoying the experience. 


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#11 tgreene

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Posted 25 September 2021 - 11:03 AM

For your purpose, I wouldn't be concerned about light transmission differences.  Besides, any such differences will likely not be noticeable enough to matter.

 

Some diagonals will have smaller clear apertures than others.  This is more common with some of the prism diagonals than it is with mirror diagonals;  but sometimes the reverse can be true.  This might be of greater concern than light transmission when using a 24mm (68 degree apparent field I presume) eyepiece -- where you'll likely want a more fully illuminated field.

 

The preference between mirror-reversed and correct-image diagonals, in my opinion, will depend on additional details that you've not mentioned.  If you're not matching the star fields in the scope with star fields on a chart, then I see no real reason to prefer one over the other.  But if you are matching star fields, then you'll want to match the field orientations of the diagonal to that of the charts you'll be using -- or, if using electronic charts, you'll want to toggle the chart orientation to match whichever diagonal you have in your scope.

 

Beware, field orientation will rotate when either type of diagonal is rotated.  With the way I use a refractor, the diagonal gets rotated to whatever position is most comfortable for me, and this will depend on the height of the eyepiece (which will vary with the altitude of the object) and whether I'm sitting (usually the case for me) or standing -- a situation where it's more tiresome to make prolonged, and more detailed observations.

 

Another comfort issue is the choice between a 90 degree and a 45 degree diagonal.  (Correct-image diagonals are available in either format.)  For me, for astronomical observations, I find a 90 degree diagonal to be far more comfortable to use.  For higher altitude objects, a 45 degree diagonal can end up being more awkward to look through -- for me.

 

To answer your other question:  I usually prefer using a simple, 90 degree (not correct-image) diagonal.  But I have, and have made use of, other diagonals for special projects that are outside the realm of this thread.  Sometimes it can be a good thing to have access to a variety of different diagonals.  So if you get one that you find doesn't work out for you, you might want to consider holding on to it.  You never know what the future might hold.

Thanks for your thoughtful advice in these many areas. I hadn't thought about how the field orientation will rotate when the diagonal is rotated. I also prefer 90 deg diagonals over 45 degree ones with refractors.



#12 tgreene

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Posted 25 September 2021 - 11:07 AM

I used a 1.25" Tak prism for a year then changed to the Baader T-2 90° Deluxe Astro-Grade Amici prism diagonal on my refractor. I use a 24 mm eyepiece for spotting with the scope as you do.  So far, switching back and forth between diagonals for comparison has raised no concerns. This is not to say that someday I may encounter a fatal flaw in the Amici, but thus far in viewing DSOs and planets I have adopted and enjoy the ease of correct image viewing and see no reason to change back.  I will still drag out both diagonals on excellent seeing nights and compare, but so far trouble with diffraction spikes or clarity is not appreciated.  The only downside was the cost of this particular Amici diagonal, but I now feel it was well worth the cost for me. So I remain a skeptical user of the Baader diagonal, but am very much enjoying the experience. 

Thank you for relaying your experience and thoughts. Why did you switch to the Baader, and why do you prefer it over the Tak - is it mostly because of its wider field and ability to use 2" eyepieces, its better optical quality (less diffraction?), or something else? I assume that the Tak prism also produced "correctly" oriented images, but please correct me if that is not true.


Edited by tgreene, 25 September 2021 - 11:25 AM.


#13 brightsky

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Posted 27 September 2021 - 03:00 PM

Thank you for relaying your experience and thoughts. Why did you switch to the Baader, and why do you prefer it over the Tak - is it mostly because of its wider field and ability to use 2" eyepieces, its better optical quality (less diffraction?), or something else? I assume that the Tak prism also produced "correctly" oriented images, but please correct me if that is not true.

The Baader is a 1.25" amici prism diagonal and gives a correctly oriented image.  The Tak prism is a standard 1.25" prism diagonal that gives a reversed, flipped image.  I use the Baader because it gives a oriented image and because of its optical quality.  Amici prisms have inherent problems with a diffraction spike on bright objects and some decreased 'clarity' at higher magnifications, and have traditionally been reserved for daytime use.  The Baader amici (Baader T-2 90° Deluxe Astro-Grade Amici prism diagonal) is supposed to be optically superior and therefore not be as subject to these problems in practical use.  I have found these claims to be true, and now use the Baader amici prism instead of the Tak prism.



#14 tgreene

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Posted 27 September 2021 - 07:31 PM

The Baader is a 1.25" amici prism diagonal and gives a correctly oriented image.  The Tak prism is a standard 1.25" prism diagonal that gives a reversed, flipped image.  I use the Baader because it gives a oriented image and because of its optical quality.  Amici prisms have inherent problems with a diffraction spike on bright objects and some decreased 'clarity' at higher magnifications, and have traditionally been reserved for daytime use.  The Baader amici (Baader T-2 90° Deluxe Astro-Grade Amici prism diagonal) is supposed to be optically superior and therefore not be as subject to these problems in practical use.  I have found these claims to be true, and now use the Baader amici prism instead of the Tak prism.

Ah, thanks for the explanation!

I had assumed that the Tak and other "prism" diagonals all gave correctly-oriented images, and I appreciate you clearing that up.



#15 Mitrovarr

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Posted 28 September 2021 - 12:43 AM

2 inch dielectric.

You want 2 inches for the largest possible field and then a mirror because 2 inch prisms are impractically expensive and rather heavy.

#16 Sarkikos

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Posted 30 September 2021 - 02:11 PM

I definitely prefer a correct image for starhopping. After many many years of dealing with the mirrored image of a normal diagonal, it felt instantly better when I got a correct-image diagonal. I think part of that is that I often also use binoculars on the night sky.

Not bad though if you have SkySafari installed on your smart phone or tablet.  You can switch the orientation of the charts anytime you want.  So you can star hop with your RACI finder or your refractor telescope.

 

Could be less expensive than buying a good amici diagonal.

 

Mike


Edited by Sarkikos, 30 September 2021 - 02:12 PM.



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