Diagonals & Eye pieces 1.25" vs 2"

Benik (Ben) · #1 25-10-2014, 01:00 PM

Hi

I am purchasing a C9.25 SCT and i am looking at my eye piece & diagonal selection.

A Common complaint is that the standard Celestron diagonal is not great, so i will upgrade it.

so whilst upgrading it should i replace it with a 1.25" or a 2" diagonal?

What is the advantage of a 2" over a 1.25"? if any.

I am looking at Celestron X-Cel 1.25"(60deg FOV) or Celestron Luminous 2" (80deg FOV)

Is there any advantage of the different FOV in the eye pieces?

Any advice or recommendations are welcome

Cheers Ben

)

Renato1 (Renato) · #2 25-10-2014, 01:46 PM

I bought a 1.25" Televue diagonal, and was darned if I ever saw any real difference between it and the Celestron diagonal that came with my C8.

I have an ultra-wide 23mm Luminos and it is a really heavy eyepiece that delivers a very nice wide field of view. But its eye-relief is necessarily shallow compared to a narrower field eyepiece (could be a nuisance if you wear glasses).

As far as wide angle eyepieces go, you see pretty much the same field down in power to around a 25mm eyepiece, regardless of whether it's a 1.25" or a 2" eyepiece (though the image at the very edge of the field may be better for the same price in the 2" eyepiece).

If you want a wide angle view at say 27mm, 32mm or 40mm, then you need a 2" eyepiece.

2" diagonals are more awkward to use, because you keep having to screw and unscrew the thread in the visual back to move it around, and do it up tight, and especially very tight when you have a very heavy eyepiece.

In contrast, a 1.25" diagonal with a lighter eyepiece is simple to reposition just by using its screw.

I happily did all my galaxy and planetary nebula hunting with the more convenient 1.25" diagonal and 20mm wide angle eyepiece. Though for looking at star clusters and other rich regions of the sky, I do enjoy using the 2" diagonal with my 23mm and 30mm ultra-wide angle eyepieces.

Regards,
Renato

The_bluester (Paul) · #3 25-10-2014, 01:52 PM

Ok. I have a 925, there is benefit in a good 2" diagonal, to get the best of low power, wide field 2" eyepieces you need the 2" diagonal. All the ones I have seen have a 1.25" adapter with them so you can still use the smaller format eyepieces.

If you buy a 2" diagonal, get a refractor type and a screw on adapter in place of the original visual back. The SCT diagonals screw straight on in place of the original visual back but are difficult to rotate when you want the eyepiece in a better spot.

Edited to add as the previous post was made while I was writing, a straight SCT one is a real pain for exactly the reasons above. When you want to move it you can't undo the screw thread, when you don't want it to it slips undone and suddenly the ep is facing down.

Wavytone · #4 25-10-2014, 06:27 PM

Ben, what matters is the true field of view of the scope, not the apparent field of view of the eyepiece. The C9.25 is the smallest Celestron that can usefully take 2" eyepieces and certainly I would say go for a 2" back and 2" diagonal because at f/10 your scope can fill large low power eyepieces such as the panoptic 41, vixen lv 50mm (50 degree afov) or lvw 42mm (65 degrees afov), or the ES 30mm 82-degree afov.

Renato1 (Renato) · #5 25-10-2014, 06:38 PM

Quote:

Originally Posted by Wavytone

The C9.25 is the smallest Celestron that can usefully take 2" eyepieces a.

You have me at a loss. What makes 2" eyepieces not useful in smaller telescopes?
Regards,
Renato

The_bluester (Paul) · #6 25-10-2014, 08:17 PM

I am with Renato there, depends on the EP as much as anything else. I just sold some 2" Celestron Ep's to another IISer with a C8 and I am expecting him to get a nice result out of them. the only one where he may see a little vignetting would be the 40mm which has a field stop which may be a little bigger than is useful in his scope.

The_bluester (Paul) · #7 25-10-2014, 08:21 PM

For the OP, I am using one of these

http://www.bintel.com.au/Accessories...oductview.aspx

With one of these

http://www.bintel.com.au/Accessories...oductview.aspx

You just need to check the threads to ensure the adapter is the right one. I actually started out with the SCT diagonal and as posted before it was a royal PITA so I got the adapter and a 2" nose for the diagonal to convert it to the refractor style. One of those occasions where I was given advice and did not take it! Bintel did reckon that the way I am not set up is the easiest way to go.

Camelopardalis (Dunk) · #8 25-10-2014, 08:56 PM

I'd recommend going for a Baader Clicklock visual back if you're using an equatorial mount as it makes it really easy to change the angle of approach to something more comfortable

2" eyepieces can give very satisfying FOV in a SCT. If you like or regularly observe away from light pollution I'd suggest looking out for a 40mm Paragon or clone, as they seem to work really well with the scopes.

Wavytone · #9 25-10-2014, 10:17 PM

Ben with SCT's and narks it's a matter of the clear aperture of the back of the mirror cell being large enough to fill the field of a 2" eyepiece which may have a field stop of 47mm diameter.

On a C8 - or smaller SCT's - the back is too small to fill such eyepieces.

If you don't believe me, I'd be happy to show you what happens if I plop the 42mm LVW on a small C5 or Meade 6".

rrussell1962 · #10 26-10-2014, 08:36 AM

My understanding is that the baffle diameter limits the clear aperture at the rear of the cell and therefore the usefulness of 2 inch diagonals on the smaller Celestron SCT's. Per the site below the C8 has a baffle 38mm in diameter and the C9.25 has a 48mm baffle.

http://osastrolog.blogspot.com.au/20...ffle-size.html

Also, just to clarify an earlier post, the 42mm Vixen LVW has a 72 degree FOV, all the other LVW's have a 65 degree field.

Camelopardalis (Dunk) · #11 26-10-2014, 09:50 AM

None of this bleating about narrow baffle tubes affects the OP who is interested in a C9.25, which Celestron lists as having a 46mm baffle tube. IMO the C9.25 absolutely benefits from 2" eyepieces for DSO viewing.

As for the C8...for the casual observer, like me....I've not noticed the vignetting when out pushing the FOV with my Pentax XW40, which has about the widest field stop you can get. I'm sure it's there just as theory predicts, and more experienced observers could point it out, be bothered/unhappy about it, etc...but when the subject is in the centre of the FOV and extends toward the edges, maybe I'm just distracted?

rrussell1962 · #12 26-10-2014, 10:02 AM

Actually Dunk, I agree with you. I use a 2 inch diagonal on my Nexstar 8 SE. First plus is that it screws straight on the back of the OTA or focal reducer and feels a lot more secure. Second plus is that I can use all my eyepieces. The vignetting has never really bothered me, in fact I've never really noticed it. It only takes a second to rotate it when the tube is on a GEM. I've never had a problem with balance either.

I hadn't thought about a Baader clicklock, nice idea though. I may investigate.

The_bluester (Paul) · #13 26-10-2014, 10:09 AM

Having followed this thread I will have to look into the clicklock, the adapter and refractor style diagonal work nicely but that would be better again.

I have been through and corrected some oddities in my posts, I really should know better than to post from an iphone. Autocorrect would have to be the bane of modern life.

Camelopardalis (Dunk) · #14 26-10-2014, 10:16 AM

The great thing with the Baader Clicklock diagonal is that the nose unscrews to reveal a female SCT thread - which threads right on! However the Baader Nexstar locking ring is required to lock it into place. Solid as

The other thing about this is that it keeps the focal length as short as possible (close to using a 1.25" diagonal), and physically it's also very short and I can reach zenith on the SE mount with the scope dovetail shoved all the way up the clamp