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Is it worth replacing my diagonal?


OK Apricot

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Pretty sure I've got the standard piece supplied with the Skymax 127, it looks very tacky inside and I'm not really fussed about cleaning etc but feels like I've got an excuse to upgrade something. I was looking at the StellaMira 1.25" dielectric diagonal on FLO - am I really going to notice any improvement? 

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You probably wouldn't see as much difference as when you upgrade from stock eyepieces.

The stock diagonals reflect a little less light than dielectrics, and may not be as flat as a more expensive model.

I replaced my diagonal with a Tak 1.25" prism, and also a 2" dielectric (after I'd changed the visual back). I wasn't aware of a significant change, but your post has reminded me that I never did a side-by-side comparison in a single session. I'll try to remember to do that.

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The diagonal can be the weakest link in the optical system used. I've had stock diagonals, dielectric mirrors, prisms, 1.25" and 2", low end, high end and I think investing in a decent diagonal is important because there's no point in having a decent objective that's accurately collimated and good quality eyepieces, if the diagonal is not flat, or poorly coated, or out of collimation. I currently use a Baader prism and a Baader mirror diagonal in my refractors and they are both a big improvement on the cheaper stock items I've had in the past. I've not had a StellaMira but I have had an Altair dielectric 1.25" mirror which I believe is similar and it was very good.

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The stock diagonal likely needs cleaning.  I clean mine every time I clean the eyepieces, mostly by blowing out any dust.

And once a year, remove and clean the mirror, just as a newtonian telescope user dos.

Several years ago, with the same Maksutov, I compared about a dozen star diagonals.

I could not really detect differences in transmission, because you cannot see a difference less than about 10%, but I could definitely detect differences in image sharpness and light scatter.

There is really no reason to think about 2" eyepieces for that scope--the design is optimized for field stops of 27mm and smaller.

In 1.25" star diagonals, it was pretty much the "luck of the draw", with the best mirror award going to the TeleVue Enhanced Coated star diagonal.

A few runners up were the stock multi-coated Celestron 1.25" prism diagonal (but I couldn't get past the plastic housing) and a Lumicon 1.25" Enhanced Coated diagonal (no longer in production).

The differences in image sharpness were quite subtle, and required a LOT of time going back and forth.

However, the difference in scattered light was more noticeable, which is why I kept the TeleVue, which had the least light scatter of any of them.

I also noted the TeleVue diagonal had the largest clear aperture of all of them except the Celestron prism, which only mattered on a 24mm Panoptic or 32mm Plössl, both with the maximum field possible in 1.25".

 

By the way, some actual specs for the scope:

With the 1.25" visual back, the focal length is ~1540mm with the average 1.25" diagonal.

The clear aperture is actually 121mm because rays from the edge of the aperture miss the primary mirror.

That makes the scope f/12.7.

If the threads on the back of the visual back are removed, the focal length shrinks to 1518mm and the scope is f/12.5.

That makes the maximum field with 1.25" eyepieces around (depends on the eyepiece) 1.02°, which is pretty good for that f/ratio.

In fact, it's a decent scope to use a 40mm Plössl in, because it yields 38-39x, a decent low power for the aperture (if you can tolerate the narrow apparent field of ~40°).

The 40mm, and to some degree, 32mm are not good in the daytime due to the large secondary shadow.  For terrestrial use, I preferred a 24mm 68° eyepiece.

Edited by Don Pensack
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  • 2 weeks later...
On 29/03/2022 at 17:13, OK Apricot said:

I'd be most grateful of a "side-by-side" comparison. I noticed a few significant spots in the out-of-focus image of a star while aligning the other night and when packing up, noticed spots on the diagonal. 

At the start of my session last night I did just a very quick comparison between the Skywatcher stock diagonal and my Revelation quartz 2".
I was using an ES 6.7mm on a Skymax127, and looking at a double star in Gemini, so around 20° away from the first quarter moon.   

I would say that the difference was not dramatic, but it was noticeable. I thought that the Revelation gave both a slightly better contrast and a slightly brighter view, so that fainter background stars were more obvious. Overall, the view was more pleasing.

So for now I'll stick with my original thought: you'll notice a bigger difference upgrading a 10mm stock eyepiece to a Starguider, but the diagonal upgrade is worth considering further along, funds permitting.

 

    

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