Help needed re effective aperture of 1.25" diagonals

[Paz]

I've decided to get a couple of better quality 1.25" diagonals for my small scopes - a Baader RACI diagonal and an Altair dielectric mirror.

The Altair has arrived. It looks very good in terms of build quality but I could see when I opened it that there is a smaller aperture within the diagonal on the eyepiece side of the mirror and I wondered if that would restrict the field of view or effective aperture. This post is looking for advice on whether this restriction does restrict the view (I've tried to test this which I explain below), whether such restrictions are par for the course and are a common feature on other diagonals, why manufactures do this, and whether I should send this back and look for another one that is not restricted or accept this is par for the course and just get on with enjoying what looks to me to be a fine diagonal.

To try to measure the issue I set up my ST80 just over 5 meters from a wall and put a ruler on the wall. The aim was to simply measure the distance along the ruler as a rough measure of the maximum true field of view using my 32mm plossl. I also got out my mainstay sky watcher RACI diagonal, Tak prism, and Celestron mirror diagonal (that comes with a C8). This involved ratcheting the focus right back.

I noticed that my Skywatcher RACI diagonal has the same kind of reduced aperture in the same place as the Altair diagonal but the Tak prism and Celestron diagonals are wide open throughout. At the end of this post are some photos looking straight down the eyepiece end. In the first two pictures you can see the same kind of aperture restriction (Altair and Skywatcher) and in the last 2 pictures you an see no such restriction (Tak Prism and Celestron Mirror).

I measured the following fields of view with a 32mm plossl and ST80....

Altair Dielectric - 12.3cm

Skywatcher RACI - 11.7cm

Tak Prism - 12.7cm

Celestron Mirror - 12.7cm

Is this a valid test to do? If not what would be. If it is then it does therefore seem to be clipping the field of view a bit in the Altair diagonal but not as much as much as my RACI diagonal (which I never realised was doing this). 12.3 is 97% of 12.7 by diameter and 94% by area. Measuring effective aperture by shining light through the eyepiece and out of the objective showed no measurable difference between diagonals but I didn't have a means of measuring this very accurately.

Other notes from this test - I could see what I think was the metal rim of the filter thread of the 32mm eyepiece in the Tak and Celestron diagonals, but I could not see it in the Altair or Skywatcher diagonals.

I then tried a 25mm SLV and all of the diagonals showed the same 9.6cm field of view. It therefore looks like it would only make a difference with my 32mm plossl, and it would make no difference with every other eyepiece I've got which are all field stops smaller than the 25mm SLV.

I also had a quick look at an artificial star to compare scatter and how small/tight the point of light was with a 4mm SLV (I know this is not the ideal test but if all other things are equal except the diagonal it may show up something). I would say I thought the Altair, Celestron, and Tak all looked sharp/similar but I think the limiting factor for testing was my ability/luck with getting an accurate focus with the ST80 each time so I could not really distinguish between those. I do however feel I could discern enough to say the Skywatcher RACI prism showed a bigger point/spread and was not a match for the others.

Any comments would be gratefully received.

Altair Dielectric

Skywatcher RACI Prism

Tak Prism

Celestron C8 Mirror Diagonal

[John]

I've owned a couple of William Optics 1.25" diagonals (one was a clone) and noticed that the thick retaining ring down inside the eyepiece holder tube did effectively restrict the AFoV of the widest 1.25" eyepieces a bit. The ones that did not do this included the Tele Vue Everbright 1.25" and a few others that I can't recall now. I guess the Altair is a clone of the William Optics ?

 

[MarsG76]

I imagine it would depend on the focal length of the telescope used to see much restriction in the real world...

In the past when I bought my first 2" eyepiece, the Televue 31mm Nagler Type 5, I did not have a 2" Diagonal to use on my 8" SCT, so I got a 2" to 1.25" adapter and used that instead.... It worked great... later then I ended up getting a 2" quartz diagonal and compared the views through the adapter and straight 2" diagonal, the only difference I saw were slight darkening on the edges of the FOV, and really mostly noticeable when looking at the moon or something bright, like during day time. The 2" was sharp without any drop off to the edge. I imagine this would be due to the fact that the objective/Mirror focus the gathered light to quite a tight spot.

Now this is quite a large step down so in your case I don't imagine there would be any difference in the real life observing with the slight obstruction in your diagonal. 

[parallaxerr]

I'd agree with MarsG76. The light cone will be very narrow by the time it gets to the diagonal and only 10-20mm or so away from the focus point (where it enters the EP) when it hits the mirror.

Thus, the focal length may dictate whether it's an issue or not due to the angle of the light cone and it potentially being wider at the diagonal in a faster scope. You could always ray trace it to be sure!

Having said that, I doubt it would be noticeable at all in use if there was a little clipping going on.

[YKSE]

As I understand it, RACI diagonals do have smaller clear aperture than other diagonals (mirror or none-RACI prism).

Hers's what Baader says on their 1.25" RACI diagonal with 24mm clear aperture, quote "Many Erect image prisms have only 19 mm free passage opening". note that 1.25" other diagonals should deliver 27mm image circle to your 32mm plossl's field stop.

http://www.baader-planetarium.com/en/accessories/optical-accessories/prism-and-mirror-star-diagonals/baader1¼"-amici-erecting-prism-45°-with-24mm-free-aperture.html?___SID=U

I am not sure if the smaller clear aperture in the RACI diagonal results in hard vignetting or not, if you use the 32mm plossl, focus on a far-away  daytime target, can you see the dark ring in the edge?

 

[John]

I should have been clearer with my post. I did actually see vignetting of the field edge with the WO diagonal clones when using eyepieces with close to the max field stop size for a 1.25" barrel. I was not theorizing.

It's been a long time since I used such diagonals (I'm 2" only now) but I suspect the eyepieces concerned were things like the 35mm Ultima, 32mm plossl, 24mm 68 and the KK Widescan 20mm 82 which need the max field stop aperture to deliver their full AFoV. The positioning of the thick ring at the bottom of the eyepiece barrel led to these issues and a 1.25" diagonal with no such ring (ie: the Tele Vue Everbright 1.25") does not cause them.

You can see this thick ring down the eyepiece tube in this photo "down the neck" of a WO 1.25" diagonal. Otherwise they are good diagonals though so it's only an issue if you want to max out the AFoV in the 1.25" format:

 

 

[Paz]

Thanks very much for the replies. I understand now it is just par for the course and it will only make a minor difference with my longest eyepiece which won't be noticed in the field. For completeness I ran the same test with my f15 maksutov and found the same effect.

I don't want to go down the 2" diagonal route on grounds of weight size and cost, or the higher cost 1.25" route and am happy with the Altair diagonal for what it is, I'll be keeping it and look forward to lots of good times.