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Should I keep my 2 inch Diagonals?

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#1 Tropobob


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Posted 20 April 2019 - 07:56 PM

I am thinking of dispersing with my 2inch EP collection and diagonals, as I do not use them often and I would rather have things being small and simple, both when observing and when storing my equipment.


But, I am concerned that when I use an 1.25inch EP with a wider field (say a 24mm Panoptic or 32mm Plossl), that I may have some reduction of light in the outer field of view. 


Is my concern valid?        (I have various refractors from F5.7 to F9, most of which can take a 2 inch diagonal.)  




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Posted 20 April 2019 - 08:10 PM

I just did a quick computation here... Well, yes... in the sense that a 1.25 Star diagonal would definitely vignette the 24mm Panoptic, with a F/5.7 feed. Whether you will notice that as annoying or not... not sure. I'm using those eyepieces with a 1.25 Star Diagonals and F/4.5 feed and not bothered over it. But I do my ~more serious~ viewing with smaller field stop eyepieces... Consider the 24mm Pano just for finding stuff, then go to higher powers...


To divest all of your 2-inch seems a bit extreme. Why not at least keep just one 2-inch Star Diagonal... even if you rarely use it?    Tom

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#3 Ken Sturrock

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Posted 20 April 2019 - 08:11 PM

I'd keep the diagonals. Aside from the size, they are usually more secure and (right or wrong) traditional wisdom has it that they perform better, even with smaller eyepieces, because any distortion tends to be near the edges.

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#4 tony_spina



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Posted 20 April 2019 - 08:27 PM

I would keep them. I have learned the hard way that your preferences change over time. You may want small today but then a few years from now you decide ultra wide field is what you want.

So unless your strapped for cash or storage space, save the 2" stuff. Cheaper in the long run

Edited by tony_spina, 21 April 2019 - 01:55 AM.

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#5 Redbetter



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Posted 20 April 2019 - 09:55 PM

Nothing wrong with a 24 Pan or 32 Plossl, but I consider a 2" diagonal and 2" eyepieces with larger field stops desirable with refractors.  Being able to achieve very wide true fields of view and see larger objects is one of a refractor's key strengths to my way of thinking, but we all have our own viewing preferences. 


Some of this may depend on what 2" eyepieces you have (and if you have some 2" nebula filters.)  If you have ones that are well corrected in the outer field and have field stops greater than 40mm then you can't really replace those views with 1.25" eyepieces with only 27mm field stops.  At f/9 I would be using a 55 Plossl for widest field and largest exit pupil, and I use a 41 Pan at f/6 for the same.  I use a 31T5, 26T5. and 20T5 for scale and field size in between.


If you have 2" eyepieces with poorer edge correction then you might not find the outer field as useful/pleasant to take it in.  In that case it might make more sense to keep your best 2" diagonal, get rid of poorer performing 2" eyepieces, and pick up a 2" eyepiece or two that are more effective with your scopes.


At any rate, I would keep at least one 2" diagonal.  That provides flexibility for the future.

#6 Erik Bakker

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Posted 21 April 2019 - 03:05 AM

A 1 1/4" star diagonal will be fine with a 24 PanOptic. But having both size diagonals, I like the way a 2" diagonal solidly sits and the end of the focuser and find it doesn't ad much bulk to the OTA from 3" of aperture and up. It allows me to use my 2" wide field eyepieces in my refractors. Wide and well illuminated visual fields are perhaps one of the refractors' most important assets, sometimes overlooked by focus on glass types and color correction at higher powers wink.gif


Only on 50-60mm refractor OTA's a 1 1/4" quality diagonal would be my choice. From 70mm of aperture and up, I prefer the 2" versions for the reasons mentioned.

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#7 AxelB



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Posted 21 April 2019 - 04:52 AM

If you really feel like thinning the herd, keep your best 2" diagonal and your best 2" eyepiece.


Observing wide open clusters is one of the strengths of refractors and you need 2" to do that. There’s also some wide nebulas like the Veil that require low power wide field (with Oiii filter).

#8 Jon Isaacs

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Posted 21 April 2019 - 05:46 AM

A 1 1/4" star diagonal will be fine with a 24 PanOptic.


I think that depends on the diagonal as well as the scope. I see it with some combinations with maximum field 1.25 inch eyepieces like a 32mm Plossl and a 24mm SWA.  A 1.25 inch diagonal generally has a optical path length of about 75mm so at F/7.5, the light cone converges 10mm from the from of the diagonal to the rear/top where the focal plane is located.  That seems like it's enough to cause vignetting.  The diagonal I measured had a 29mm entrance. 


One thing also is that with a 1.25 inch filter, there will be very noticeable vignetting with eyepieces like the 24mm SWA and a 32mm Plossl.  The outer portion of the field is very dark.  Just measured a couple of 1.25 inch filters, a UHC and an H-Beta, the clear aperture is about 23.5mm.  


Using a 2 inch diagonal with 2 inch filters on the 2 inch-1.25 inch adapter fully illuminates the field.  


Anyway, I think we are all in agreement, keep at least one 2 inch diagonal.  



#9 Tropobob


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Posted 21 April 2019 - 07:29 PM

Thanks everyone for the good advice. 

I have decided to keep at least one quality, 2" diagonal for a few more years.

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