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Rocket Stars

Advice on diagonal and eyepieces/barlows

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I have a Celestron edge 8hd. It came with original diagonal end a plössel 40mm. 

I really don't like the diagonal. The screws docent hold well, small screws that makes cold fingers hurt. ect. 

So, I'm planning to replace the original with a decent one. Adding more eyepieces, and Barlows that also can be used for planetary imaging. New or good used ones. 

Regarding diagonals. Is there a real benefit with 2", compared to 1 1/2"?  Also, should one go for prism or mirror?  Ive read that prism is better, but seems like mirrors is the most popular? Baader click lock would be nice. But is pricy, and im probably mostly going to do astrophoto. So I believe comprssionrings is what I will get. 

Eyepeces and barlows. Its a djungle of types...I do get viewing angles. But what do I get that is good, but in resonable money? I have slight astaganism (English spelling on that one? lol) But wearing glasses only sometimes. I imagine that eyepieces with "bigger" aperture can be helpful? I do fit a bit to get my eyes in focus in the 40mm plössel. . 

Hope someone can help me to focus, the light in the end of the barrel! :)

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Hopefully someone more knowledgeable than myself will help but I have heard that using a 2" star-diagonal in some SCT's introduces spherical aberration and increases focal length. I figure an 8" 2000mm focal length scope is essentially optimized for narrow field, higher magnification performance, so a 2" diagonal might not make the best of sense. What I mean by this is that if one were wishing to get wider field views, it might be cheaper to buy an 8" f5 dob secondhand and use that rather than investing in a 2" diagonal, 2" eyepieces and adapters etc.

Again, I'm no expert but here is a nice overview of different kinds of star-diagonals. To be honest, I've used a few diagonals (Everbrite, TS, Tal) and unlike eyepieces my eye cannot discern a difference. For general usage you can't go wrong with Tak's prism, Baader's clicklock or TeleVue's Everbrite. In the few scopes I've owned that use a diagonal there's also been no visual difference between 1.25" & 2" diagonals. 

As you say, eyepieces and Barlows is a minefield :ohmy:.

Regarding Barlows, I figure for such a useful tool you ought to buy quality so in effect you buy once and cry once. In my own case, I've got a TV Barlow that I've had for donkey years. TV's Powermates are a step up from their Barlow - which is already quite impressive - but I've never had the pleasure to use one. However, in your f10 is a Barlow going to be much use :icon_scratch:? I mean, even comfortable, mid range focal length eyepieces (15mm - 10mm, for example) are still going to give you some pretty high power views. It's a relatively rare night I'm viewing over 200x, for example. 

On a similar theme, due to the narrower beam of light entering your eye in your f10 you don't need top quality glass eyepieces which were probably designed with faster scopes in mind. Nevertheless, following my belief that if you buy quality, you buy once and only cry once and extracting budget from the equation Explore Scientific ES80 and ES100 range, the Meade 5000 UWA range, the Tele Vue Naglers, Delos, Ethos, Delite, Panoptic range etc, Pentax XW range (less the 14mm, I think) William Optics UWAN's and Skywatcher Nirvanas come recommended time and time again regardless of the scope being used.

I hope this has helped a little and I hope someone more informative than me will be able to give a hand 🙂

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Posted (edited)

Thank you so much for the reply! 

That was an interesting white paper! 😮

It shows clearly, that you get what you pay for. And, seems to be a upper hand for prism. In long F numbers at least. Hm.. think a prism 1 1/2 should fit well. 

And, I have the celstron original pice that comes as a kit with the scope, works. But even docent gets on that list.. 

Thank for helping my knowledge forward! 😃👌



Edited by Rocket Stars
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51 minutes ago, Rocket Stars said:

It shows clearly, that you get what you pay for.

I think that's generally true coupled with one's own decent research and questioing in forums. Sometimes there are excellent options that don't cost and arm and leg but yes, in general, often buying cheap one ends up crying twice.

Personally, I wouldn't worry about the diagonals on that list. I forwarded it more to help inform you about the different options and differences between designs, rather than a purchase guide. I'm sure your Celestron diagonal is very good (have you read about it being bad?). I'd keep hold of it and not make any upgrade until you've put it through its paces.

Similar to you, I've also read that prisms are not always suitable in fast scopes for they introduce astigmatism and may give a less brighter image compared to a mirror diagonal. On the other hand, with focal ratios higher than f7, I've read that colour correction and light scatter is better in a prism. Needless to say, all this is anecdotal for I've never made any direct comparison 🙂

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You are absolutely right!

When, I upgrade in future. Maby a used one In good condition. Or new.. We will see. 

But for sure. I will keep the Celestron as I will not get much for it at the used market anyway. And I will compare it.  I think one of the big reasons, is that I would like one with compression rings. :) The original kit diagonal lacks a secure way of holding. Unless you tighten uncomfortable hard. Often with cold hands.  


Cheers! :) 


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