Star Diagonal upgrade

[pellwood]

Hi Everyone,

I am looking to upgrade my star diagonal from the basic one shipped with the scope. Should i go for an erecting prism or di-electric? looking to find the pros cons of either.

My observations are a broad church a little bit of everything really, lunar, planetary and the brighter objects in the Messier catalog. I observe primarily from my back garden with a 80mm refractor, fl 900mm.

I would welcome any thoughts,opinions or advice.

Regards

Pete

[John]

I'd not recommend an erecting prism for astronomical viewing. A dielectric diagonal has a harder wearing reflective surface than normal coatings so that can make sense. Are you restricted to the 1.25" fitting ?

Don't expect an earth shattering performance improvement though. Very subtle if anything I'd suspect with an 80mm F/11.1 refractor.

[pellwood]

Yes sorry, should have mentioned that   its a 1.25" fitting. With such a 'slow' scope i guess i would only be getting 5-10% max additional light transmission with a dielectric diagonal.

Thanks for the feedback regarding the prism, could you explain why you wouldn't recommend? I'm just interested to know (from a newbies point of view)

Kind Regards

Pete

[John]

Hi again Pete,

The correcting type diagonals use prisms to produce an image thats the same as we see with our eyes. This is fine for terrestrial viewing but the additional glass and reflections cause light loss and scatter and sometimes other issues that are not going to help with the quality of astronomy viewing.

The astro mirror diagonal produces an upright image but with left and right reversed.

You can get astronomical diagonals that use a prism but it's a simpler optical design than the erecting ones use and not detrimental to image quality.

[Knighty2112]

Before upgrading the diagonal, I would get one or two better quality and different sizes of EP's from any of the stock ones that came with your telescope. I have 3 different diagonals from three separate scopes, and to be honest I'd be hard pressed to see any difference with the image brightness or image quality between them all. If you want to view some DSO's better then get a good Plossl EP between 25 to 32mm.

[CodnorPaul]

Before upgrading the diagonal, I would get one or two better quality and different sizes of EP's from any of the stock ones that came with your telescope. I have 3 different diagonals from three separate scopes, and to be honest I'd be hard pressed to see any difference with the image brightness or image quality between them all. If you want to view some DSO's better then get a good Plossl EP between 25 to 32mm.

Yes agreed - I changed my stock diagonal with the XLT 2" version and see no difference in quality or brightness, but 2" EPs are worth the upgrade for me

[rwilkey]

The dielectric diagonal is a good investment in my opinion, I bought my wife the William Optics 1.25" and I have to say it does have a marked improvement over the old one.  Yes, a little subtle as John says but both my wife and I have a good eye for quality, and I was surprised at the difference it made. Used on an 90mm f/9 refractor by the way.  Also I have a good one (Sky Watcher 2") on my 100mm ED.  Quality of view is key to my viewing pleasure.

[pellwood]

Thanks for the advice guys

John - thanks for the explanation regarding the erecting prisms

Knights, CodnorPaul - great advice and something I am already undertaking. I just need a 12mm to complete my collection (I'm saving up for a Vixen SLV

Rwilkey - The WO is on my short list so this could well be the one I go for.

Thanks again guys

Regards

Pete

[Dave Lloyd]

The dielectric diagonal is a good investment in my opinion, I bought my wife the William Optics 1.25" and I have to say it does have a marked improvement over the old one. Yes, a little subtle as John says but both my wife and I have a good eye for quality, and I was surprised at the difference it made. Used on an 90mm f/9 refractor by the way. Also I have a good one (Sky Watcher 2") on my 100mm ED. Quality of view is key to my viewing pleasure.

Ye gods! Your wife shares your astro passion? Who stops your house being overrun with astro goodies?

[John]

The best 1.25" diagonal I've owned was the Tele Vue Everbright. Superbly made but priced accordingly of course !. I use a couple of those in the 2" size now.

I've also owned the William Optics Dielectric 1.25" which was very nice but it has a thick ring inside the eyepiece barrel where it joins with the diagonal body. This ring vignettes eyepieces that have the maximium field stop size. I think Skywatcher have a 1.25" dielectric which is more or less a clone of the WO one.

The Revelation 1.25" Dielectric was also a decent diagonal for it's cost but the version I was supplied with had a set screw rather than compression ring fitting.

[rwilkey]

Ye gods! Your wife shares your astro passion? Who stops your house being overrun with astro goodies?

I started by buying my wife a telescope for Christmas and it all started from there, she now does more stargazing than me, often getting up before dawn to view the night sky.  It's good that we are both interested as this saves a lot of hassle about where to go of a night-time.

[mikeDnight]

Hi Everyone,

I am looking to upgrade my star diagonal from the basic one shipped with the scope. Should i go for an erecting prism or di-electric? looking to find the pros cons of either.

My observations are a broad church a little bit of everything really, lunar, planetary and the brighter objects in the Messier catalog. I observe primarily from my back garden with a 80mm refractor, fl 900mm.

I would welcome any thoughts,opinions or advice.

Regards

Pete

Hi Pete,

It's worth being aware that just because a diagonal has a dielectric mirror, its no guarantee of optical quality. I've used many diagonals most of which use indifferent mirrors, as well as some prism diagonals that are still offered by some well known manufacturers such as Celestron that were, and still are, not fit for purpose. If you want the best performance from your 80mm scope buy a top end diagonal, it will be a sound investment, even if you change your scope in the future.

Johns already mentioned Televue dielectric diagonals which offer optically superb mirrors. A slightly cheaper option but still optically first class and perfect for your refractor, is Takahashi's 1.25" Prism diagonal costing around £68 from TrueTechnology Ltd. I use this diagonal on my 4" refractor and it allows the use of extreme high power while providing the purest image. It is not an erecting prism so doesn't introduce diffraction spikes. The difference in performance between a standard mass produced cheap end diagonal and a Televue or Takahashi diagonal is very noticeable when they are used alongside each other.

Mike

[Roy Challen]

Hi Pete,

It's worth being aware that just because a diagonal has a dielectric mirror, its no guarantee of optical quality. I've used many diagonals most of which use indifferent mirrors, as well as some prism diagonals that are still offered by some well known manufacturers such as Celestron that were, and still are, not fit for purpose. If you want the best performance from your 80mm scope buy a top end diagonal, it will be a sound investment, even if you change your scope in the future.

Johns already mentioned Televue dielectric diagonals which offer optically superb mirrors. A slightly cheaper option but still optically first class and perfect for your refractor, is Takahashi's 1.25" Prism diagonal costing around £68 from TrueTechnology Ltd. I use this diagonal on my 4" refractor and it allows the use of extreme high power while providing the purest image. It is not an erecting prism so doesn't introduce diffraction spikes. The difference in performance between a standard mass produced cheap end diagonal and a Televue or Takahashi diagonal is very noticeable when they are used alongside each other.

Mike

I agree with the quality aspect. The Takahashi prism is indeed of excellent optical quality. The Tal mirror diagonal, although not dielectric, is also very good.

With regards to erect image prisms I agree with what John and others have said - generally​. ​​The Baader Amici prism however, is of observation quality, at least I haven't noticed any real difference between this and the Tak prism. I haven't noticed any diffraction spikes either. Is this a different design to other erect image prisms? Either way, this prism makes relating to what you see on Moon/star charts and what you actually much easier, and is useful for daytime observations (birding etc)

[jetstream]

My stock 2" Skywatcher is very good, well worth having. I know from personal experience that a poor diagonal wrecks the view... very bad scatter, astig at times etc.

Sometimes getting a bigger scope like a 150p Dob http://www.firstlightoptics.com/dobsonians/skywatcher-skyliner-150p-dobsonian.html

ends up giving better views and can be cheaper than adding accy's to ones existing scope.

[mikeDnight]

It's interesting what you say about your Baader prism Roy. Theres a thread on cloudy nights that discusses various diagonals and how they compare. The Takahashi and Baader received a great review and were highly regarded. Almost on par with Zeiss.

[John]

The slight snag for the OP is that some of the diagonals being discussed probably cost more than the scope that it would be used in has !

[Roy Challen]

Yes, very true John! The Baader costs more than any of my scopes too, I just happened to get it with one of said scopes (worth more than that scope IMO). Getting back to the point then, a dielectric mirror diagonal is a very good upgrade to the stock diagonal, and needn't cost too much. But don't buy a cheap erecting prism​. ​