Advantages/Disadvantages of 1.25" v 2/" Diagonals?

[leebca]

Basic, probably dumb, question.

I have a large scope (14" Meade on LX850 German Eq Mt) en route to me.  It comes with a 1.25" star diagonal.  I want to eventually get into imaging.  Should I stick with this size or go with a 2" diagonal?  What are the advantages/disadvantages of either/both.  Either one better than the other for imaging?

Thanks.

[rowan46]

Not an imager but I would hazard a guess that their main advantage is the increased strength that 2" tubing and connectors give over 11/4" I would imagine that would be very handy in taking the weight of cameras, filter wheels etc. The main disadvantage is cost I would imagine. I don't think you would use a diagonal for imaging all of the rigs I have seen don't use them although flip mirrors are used for off axis guiders. A 2" diagonal is a visual tool really for 2" eyepieces. these can give a wider view than 11/4" eyepieces. I believe that 32mm is the largest eyepiece that can use the whole field before requiring a 2"

[emadmoussa]

Advantages: stronger, wider field, accepts both 1.25" & 2" eyepieces.

Disadvantages: heavy and more expensive.

Personally, I never use or will use a 1.25" diagonal.

[Naemeth]

You don't need a diagonal in imaging, you usually use extension tubes instead to get focus so you don't lose light (every glass surface loses you 1% of light, minimum).

For visual, Imad has highlighted the main strengths/weaknesses of each. 1.25" diagonals better suit traveling light where weight is an issue, 2" diagonals are used when weight isn't an issue (or much of one).

[leebca]

Thanks all.

My plan would be to image using prime focus, so you're all right about the diagonal not being used at all.  Dumb on my part! 

I'll be doing mostly observing this year and not imaging until next.  So the 2" will be the way to go.  Thanks all.  Much appreciated.

[emadmoussa]

One thing though, avoid cheap diagonals simply because even if they provide acceptable image they won't be good enough to do your good telescope optics justice, and because since they're cheap they're not as well coated and reflectivity will disintegrate over time.

Having said that, in my experience good quality diagonals with 91% reflectivity and diagonals with 99% reflectivity didn't show much difference in my eyes. Unless you're super duper fussy about this barely noticeable photon, I'd suggest you stick with a good quality diagonal with 91% reflectivity, it will save you money. 

i had the Sky-watcher 2" diagonal that came with the 80ED Apo, and for months I thought it was 99% reflective considering it's great performance, but it turns out it was only a 91%. Now I have an Altair Astro 99% and Teleskop Service 91% (both 2"),. The Altair Astro version is slightly lighter and better built, however, comparing it with the TS version I did not see a lot of difference.

And, like Naemeth said, diagonals not necessary for imaging (they help increase the focal length and achieve focus). but you better use an extension tube 65mm or more (no glass between the camera and the telescope optics).  You'll find one second hand here  - count six rows from the top and it's the first one on the right  ( 62mm T thread ext ). By the way, I used a diagonal on several occasions while imaging planets, worked very well. Then again, this was not the best way to do it.

[Northern Soul man]

Hi Ya Leebca and welcome to the SGL, the 2" seems to me to be a lot more robust, as said, nice scope and mount, that thing will do some damage under the stars!!  When I upgraded from 1.25 to 2" I also opened up the back of the scope to accept the larger diameter diagonal, I'm not sure of the back that's on the 14" Meade, I bought the Baader Click lock and installed it on the back end of the scope - there seems to be a little bit of a debate as to wether it really makes any difference regarding the amount of light through the back - a lot say that the light cone - by the time it reaches through the back - is obstructed, but I'm not entirely sure of this - but as said, the 2" diagonal "feels" really secure with larger ep's and binoviewers hangin off the back of it and once tightened, it makes a very secure fitting with no damage to the diagonal that you get with the standard knurled screws that dig into it - the amount you have to pay for these scopes - you would think that the manufacturers would provide this as standard.    Paul.

[leebca]

Thank you Emad and Paul.

I felt the same Paul.  With the cost of this scope, you'd think Meade would have, by default, attached the 2" diagonal to it.  Probably the game of keeping costs down to offer a less expensive retail price (or make more profit!).  It was, more or less, the fact they didn't that had me wondering which would be the best for this scope.  Thanks again.

[emadmoussa]

When I got my Celestron C11, my first impression was....''this diagonal goes to the bin"...sold it right away

[Northern Soul man]

Nice 1, just goes to show you that the initial investment in the scope, I think, is forgotten - depending on what you're going to do with it - ep's, filters, guiders, diagonals, finders, cameras and subtantial mounts just to mention a few need to be "inserted" into the hobby, the high end add ons are definate bank emptiers - and that's just for visual - don't get me started on the imaging "wants" - oh and Emad - you don't really want to part with the 11 - would be a shame mate?????.   Paul

[ollypenrice]

The diagonal doesn't increase the focal length. It has no effect on it. I just takes up backfocus.

In order to get a wide(ish) FOV in a scope with such an immense focal length you will want at least one ultrawide eyepiece, perhaps a TeleVue Nagler 31 or 26. These are always 2 inch. You cannot reach the wider fields in 1.25 eyepieces. Be aware that the 6.3 focal reducer is not the best way to get a wider field visually because the field stop is determined by the baffle tube, so the widest field you can have is without reducer and with a wide Nagler or copy.

This will be a great scope for planetary imaging. For deep sky you would need the focal reducer because the F ratio is far too slow for most DS imaging purposes. Also the long focal length will be tricky to guide, but if you get it all to work it could be spectacular.

You may come across a 3.3 reducer. Ignore them, they are only of use on the tiniest of webcam chips.

Olly

[bingevader]

You already have the wide angle EP, (ES 82^o 30mm)! 

So, if you're going to get one, you'll need the 2" diagonal.

Cheers

[leebca]

Thanks to all who have provided their experience and opinions.  Much appreciated!

[Adrian]

The diagonal doesn't increase the focal length. It has no effect on it. I just takes up backfocus.

Olly

Not quite true, Olly - as the OP will have a 14" SCT the increased backfocus of a 2" diagonal will increase the focal length of the optical system because of the way the SCT focuses.

On the 10" SCT that I used to have, adding a 1.25" diagonal increased the focal length by about 10% compared to no diagonal. For a 2" diagonal this was about a 15% increase compared to no diagonal - it gave me around a 2840mm focal length instead of 2460mm without.

To the OP - As Bingevader says, you need a 2" diagonal to use your 30mm ES 82 - that should give you amazing views. The 2" diagonal will feel much more substantial too. I had the Baader clicklock SCT adapter that fits on the 3.25" screw thread and opens out the back of the scope to the full 2" aperture. With a 2" diagonal this gives a well illuminated, very solid system.