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Hi. I will be looking to purchase a star diagonal either for a relatively inexpensive refractor (102/600 or 90/500) or potentially for and ED80/500 depending on how much my car MOT is going to set me back 😁. Any recommendations for each of these would be gratefully received! 

1.25" not sure I have a requirement at present for 2" unless someone here can persuade me otherwise 🤣🤣🤣

Guidance on dielectric, prism, quartz... All welcome! 

Edited by Marc1964
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The Svbony one is excellent - https://www.aliexpress.com/item/32881221402.html . Supposedly a dielectric mirror, metal body, with a compression ring to avoid marking your eyepiece barrels. At £20.50 (+VAT) it's a bargain, and they give excellent service  (normally about 8 days delivery time - beware it's Chinese new year this weekend, so deliveries may be delayed a week or two ! )

David

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I much prefer a 2" star diagonal to a 1.25" even when using 1.25" EPs, as the 2" have a much larger completely flat surface. More importantly, it allows use of 2" EPs which can give magnificent wide field views. I have an APM 80 mm F/6 triplet, which gives a massive 5.3 deg field of view with the 31 mm Nagler, and 3.75 deg with the 22 mm Nagler . The view of M33 in the latter from a dark site in France was one of the best views of that object I have ever seen. I have a fairly cheap SkyWatcher dielectric one, but also a Denkmeier Filter Switch diagonal, which looks like William Optics build them. This one I use most, as I can slide 2" filters in and out of the optical path quickly. The Denkmeier is supposed to be the better one, but you will be hard pressed to see the difference.

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Unless you have some specific requirement or desire such as very wide field (which probably will entail an expensive eyepiece to make the most of it) then I'd say 1.25" should suffice for now at least, especially if money is a bit tight.  The only reason I have a 2" diagonal is for one 2" SWA wide angle eyepiece, which weighs about the same as a bag of sugar so needs a strong mount and diagonal just to hold it, the views through it are magnificent though.

That Svbony one sounds almost too good to be true, probably worth a shot for the money but only if you can afford to lose £20, with the cheap Chinese knock-offs you may find it's pot luck as to whether you get something decent or a total POS, and I wouldn't count on any proper after-sales support or parts availability.  Consider also that if it spectacularly fails then it could damage your scope and/or your eyepiece. 

If you wanted peace of mind then go for William Optics or similar from a reputable astronomy retailer - https://www.firstlightoptics.com/diagonals/william-optics-125-dura-bright-dielectric-diagonal.html

 

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The Svbony diagonal is real, not at all 'too good to be true'.  It's not a 'cheap Chinese knock off' at all. You WILL get full after sales support, at least as good as any UK dealer. I have been dealing with Svbony for years (far longer than FLO) and have had nothing but excellent service. Even got given a pre-production sample of their SV225 camera for evaluation - not had that from dealers in this country!

Heres a picture of my diagonal, and it is brilliant.

David

ae_1596044256044.jpg

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A 2" Skywatcher dielectric mirror diagonal served me well, especially when used with a decent 2" 30mm ED eyepiece. It was bought cheap on special offer. They're regularly available used and sometimes retailers split them from apo refractor packs then sold separately at cut price.

However I sold the Startravel 120 and accessories thinking I'd not buy another refractor for quite a while. Here I am a year later with a 4" f7 apo... 🙄

This time around it's a Skywatcher 1.25" dielectric. The 2" is considerably heavier, and I did conclude swapping between 2" and 1.25" eps to be a faff.

The views through it with both the refractor and a Skymax 102 are great for not so massive outlay.

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1 hour ago, MrFreeze said:

The Svbony one is excellent - https://www.aliexpress.com/item/32881221402.html . Supposedly a dielectric mirror, metal body, with a compression ring to avoid marking your eyepiece barrels. At £20.50 (+VAT) it's a bargain, and they give excellent service  (normally about 8 days delivery time - beware it's Chinese new year this weekend, so deliveries may be delayed a week or two ! )

David

I can't see dielectric in that specification :icon_scratch:

Usually dielectric coatings give a reflectivity of 97%-99%. That one claims 90% so I would suspect standard coatings.

For the price though the spec is fine.

The GSO diagonals that I've used have combined good build quality and good optical performance with a reasonable price eg: ones like this:

https://www.firstlightoptics.com/diagonals/stellamira-1-25-90-di-electric-diagonal.html

https://www.firstlightoptics.com/diagonals/stellamira-2-90-di-electric-diagonal.html

 

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24 minutes ago, John said:

The GSO diagonals that I've used have combined good build quality and good optical performance with a reasonable price eg: ones like this:

https://www.firstlightoptics.com/diagonals/stellamira-1-25-90-di-electric-diagonal.html

https://www.firstlightoptics.com/diagonals/stellamira-2-90-di-electric-diagonal.html

Those look more like Synta diagonals, such as the WOs that have the nasty constriction ring in the bottom you noted:

GSO diagonals look like these:

GSO 1.25" 90-deg 99% Dielectric Mirror Diagonal with Compression Ring:

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GSO 2" 90-deg 99% Dielectric Mirror Diagonal for Refractors:

spacer.png

Notice also the 2" to 1.25" adapter.  This is a hallmark of many GSO 2" products.

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1 hour ago, MrFreeze said:

The Svbony diagonal is real, not at all 'too good to be true'.  It's not a 'cheap Chinese knock off' at all. You WILL get full after sales support, at least as good as any UK dealer. I have been dealing with Svbony for years (far longer than FLO) and have had nothing but excellent service. Even got given a pre-production sample of their SV225 camera for evaluation - not had that from dealers in this country!

Heres a picture of my diagonal, and it is brilliant.

Can you confirm the Svbony doesn't have the WO style constrictor ring at the bottom of the eyepiece holder as seen below:

dielectric125_04.jpg

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15 minutes ago, Louis D said:

Those look more like Synta diagonals, such as the WOs that have the nasty constriction ring in the bottom you noted:

GSO diagonals look like these:

GSO 1.25" 90-deg 99% Dielectric Mirror Diagonal with Compression Ring:

spacer.png

GSO 2" 90-deg 99% Dielectric Mirror Diagonal for Refractors:

spacer.png

Notice also the 2" to 1.25" adapter.  This is a hallmark of many GSO 2" products.

I agree Louis.

I'll withdraw my recommendation on the 1.25" Stellamira diagonal on that basis. The 2 inch one is OK though.

I've owned diagonals in the style that you post above and they are pretty good, especially the 2 inch GSO type. You can get those under a number of different brands, usually, although stock levels are very scarce just now, like many astro items.

 

 

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24 minutes ago, Louis D said:

Can you confirm the Svbony doesn't have the WO style constrictor ring at the bottom of the eyepiece holder as seen below:

dielectric125_04.jpg

Nope - there is no constrictor ring in there. I have a 45 degree diagonal with one and they are a pain as they can prevent you from rotating an eyepiece with polarising filter fitted.

David

16130776778531653375885.jpg

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In your situation I would get a 2" diagonal but not the dielectric versions, just do not appear a great necessity to your equipment. RVO do a Bresser 2" diagonal and isn't overly costly. It is one I intend/need to purchase for myself. I have a 2" diagonal, but really do need a second one, and I identified this as my likely best option. Costs £65, and states 93% reflectivity, but to my thinking it should be a good idea.

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As usual good advice and a spot of controversy as well 🤔. Thank you all for your thoughtful contributions as ever! 

The number of choices are quite boggling and some of the differences sometimes seem quite marginal, so good to be able to narrow choices down and learn from other people's experiences. 

I personally don't want to spend more than £75 on the diagonal so pleasing to see some of the choices within my mental budget 😁

Edited by Marc1964
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3 hours ago, MrFreeze said:

The Svbony diagonal is real, not at all 'too good to be true'.  It's not a 'cheap Chinese knock off' at all. You WILL get full after sales support, at least as good as any UK dealer. I have been dealing with Svbony for years (far longer than FLO) and have had nothing but excellent service. Even got given a pre-production sample of their SV225 camera for evaluation - not had that from dealers in this country!

...

I stand corrected, good sir!  Still always a good idea to be wary of cheap items and get opinions from people who've actually bought and used them (I haven't had that particular diagonal but I've experienced other cheap stuff from China, it's been... so so.  Some has been utter carp).

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