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EdgeHD 8" Aperture reduction with Mark V

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#1 Eddgie


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Posted 10 March 2013 - 10:48 PM

Ok, been meaning to measure this and finally got to it tonight.

I thought I might be loosing a bit of aperture in my EdgeHD 8" with the Mark Vs.

Maybe I have the wrong numbers but the aperture reduction is a bit worse than I thought, though the ray trace I have for Celestron scopes is for the standard scopes and not the EdgeHD design.

I calculated the back focus to be about 196mm. About 38mm for the Baader SCT Clicklock, about 38mm for the Baader Prism, and about 120mm for the Mark V.

Is that right for the Mark V? Is it about 120mm?

According to Ken Hutchinson's SCT vignetting analysis, 196mm of back focus on a standard C8 should give about 7.8" of aperture.

On the EdgeHD 8", I measured an aperture of 7.5" with this configuration.

I had a suspicion that the aperture was reduced a bit more than a couple of millimeters. In particular, Jupiter seemed dim when using even 15mm Plossls. Still bright enough to observe, but enough that I felt that I was seeing it if I tried to go any higher in magnification.

So, a 15% difference in brightness.

I looked at going to the Televue Short visual back, but this only buys me about 12mm of in-focus, and according to Ken's chart, this only gets me 1/8th inch of aperture back.

The Baader SCT adapter is only 15mm, but that would only buy me 20mm, and that would only give me about 1/4th inch back, but the cost would be to loose the ability to easily rotate the diagonal.

So, sadly, it is what it is.

Good news (for me) is that I do not use this telescope a lot for planets, and this is where I first noticed the loss of aperture.

So, sad news. Does not look like it is possible to configure an EdgeHD 8" for binoviewers without accepting a bit of aperture loss.

I am going to order a Televue Short adapter this week. Will post when I have had a chance to measure.

#2 GlennLeDrew


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Posted 10 March 2013 - 11:16 PM

Are you saying you can look into a scope, without doing a side-by-side comparison, and tell on the basis of nothing else that the image is 15% dimmer than expected? This would be the eyeball equivalent of perfect pitch.

At any rate, you should rig up some kind of length-adjustable system so that you can actually test for aperture reduction at any focus location desired. This need be no more complicated than segmented or sliding tubes for straight through viewing. In such way you could empirically determine the maximal back focus distance before aperture reduction commences.

#3 Eddgie


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Posted 10 March 2013 - 11:58 PM

Well, the view seems a bit dimmer with 15mm Plossls than the view in the C14 using the same 15mm Plossls. While the C8 is working at a bit slower focal ratio in this configuration than the C14 in the same configuration, I feel like the image is dimmer than in the C14.

And as compared to the Maxbrights with the same 15mm eyepeices in a 1.25" visual back, I think it was a bit brighter that way. I just could not get sufficient clamping in a 1.25" diagonal to feel comfortable though, so I went to a 2" nose.

I noticed the same thing only to a greater extent when I went from a 2" diagonal based binoviewer to the Maxbrights. That was a big jump in brightnes. Maybe 20%.

The best I will do though is the Televue Short back. This still allows easy diagonal rotation. The Baader 15mm connector is like the old SCT diagonals I think. You have to unscrew a collar to rotate the diagonal. I didn't care for that even for SCT diagonals. I think I would like it even less holding binoviewers against rotation.

And people on the Binoviwer forum routinely report seeing differences of 6% to 8%, so I don't know why seeing 15% should be a surprise.

But I feel like I can see it. Not saying that everyone would, but I do a lot of planetary observing, and I feel like I am very sensitive to illumination level changes which I think become more easily perceived at smaller exit pupils.

#4 junomike



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Posted 11 March 2013 - 01:26 PM

Ed, Would this work?

I used to do this with a 2" GSO Diagonal on my old C8 so It would pass through the SE Mount (One Armed Bandit)


#5 Paul G

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Posted 11 March 2013 - 02:27 PM

And people on the Binoviwer forum routinely report seeing differences of 6% to 8%, so I don't know why seeing 15% should be a surprise.

People report a lot of things, doesn't mean they are true. I worked in a neurophysiology lab and we measured things like this. Nobody was able to differentiate between two targets that differed in brightness by less than about 8%. And that is with two targets in the same field of view so they can be directly compared. To expect someone to be able to look through an eyepiece or binoviewer, then another and compare it to their short term memory of the first and be able to discern even smaller differences is, well, you know. AAVSO crunched a very large number of observations and came up with about 10%.

That said, 15% should be noticeable but not dramatic.

#6 Eddgie


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Posted 11 March 2013 - 04:42 PM

Not at all dramatic. In fact, quite subtle.

The only way I can see it is that using the same configuration on my C14 (same binoviewer, same eyepeices) I should have about the same brightness when using 13mm eyepeices.

In the C14, the image is just at the brightness level that I can still see the image with contrast loss due to illumination falloff starting to assert itself, but this power is still useful.

In the C8, it seems like I have to step down to the 15mms to keep the illumination near the same level.

With the binoviewrs, the C8 is working at a bit more than f/11, but so is the C14, so the brightness should be about the same.

But hey, I am only reporting my findings, that the EdgeHD 8" will likely be reduced in aperture using even a 1.25" prism with a larger aperture binoviewer.

Anyone considering a binoviewer configuration with a longer light path can expect more aperture loss in the EdgeHD 8"

At which point the problem becomes serious enough for some other observer is really I suppose up to them.

My primary reason for posting was to just share the data.

#7 Ava


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Posted 19 March 2013 - 09:24 AM

Hi Eddgie,

According to the Celestron EdgeHD white paper, the 8" has only 84% illumination at the edge of a 28mm field (incidentally an APS-C sensor or the 1.25" barrel size), while the 14" has 83% at a 42mm field, so likely much brighter at 28mm. The white paper also states that the fully illuminated area extends to a 16mm field in the 14", so likely a lot less in the 8" (it doesn't state numbers for the 8", although it does for all other sizes, hmm).

What I am getting at, maybe the difference you are seeing is related to worse field illumination in the 8" rather than actual aperture reduction, or a combination thereof, meaning the 8" EdgeHD is no worse than a regular 8". Without a ray trace or other direct measurement I don't think we can be sure what's what.

Glenn, how would I go about building such a rig? I'm curious and would like to have a go at measuring this but can't currently visualize how to do that, and how to separate effects (other than noting an increase in vignetting).

#8 Eddgie


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Posted 19 March 2013 - 10:54 AM

I did measure the aperture directly.

The telescope aperture is being reduced to 7.5".

This means that the illumination at the center of the field is 15% below what it would be if the scope was working at 8".

The illumination at the edge of the field hardly matters because with the binoviewer you can't get more than 14mm off axis anyway, but the off axis illumination at the edge of a 24mm eyepiece is likley only about 80% of what it would be if I was not loosing aperture.

Anyway, there is no question that the telescope's aperture is being reduced. I measured it myself by projecting the aperture on to a wall.

I could see immediatyly when I projected it that the aperture was reduced.

With 196mm of back focus, the standard C8 would loose about 3mm or 4mm of aperture, so the EdgeHD 8" clearly has less tolerance to excessive back focus than the old design.

I have a Baader 15mm SCT to T2 adatper on the way. This will save me about 30mm of back focus. I think this will put me at about 7.8" of aperture (fingers crossed).

Will report the measurment when I have a chance to make it.

#9 GlennLeDrew


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Posted 19 March 2013 - 02:28 PM

I first introduced the 'flashlight aperture test' a couple or more years ago in the Binocular Forum, and today a goodly number of folks who hang out there know about it and use it. It's abundantly apparent that Cat users could make use of this simple test, too. A Search for "flashlight test for aperture" among thread titles in the Binocular Forum should get a result (?).

A test rig for determining effective aperture at different focus distances can be a simple series of tube sections or sliding tubes, to test over a range of distance behind the scope from, say, 80mm up to 250-300mm. Even paper tubing is fine, as long as the eyepiece can reliably be supported laterally (most important) and axially. (It's more important to not have the eyepiece laterally displaced off the optical axis than it is to precisely control eyepiece tilt. And so the tube system must not sag or otherwise be skewed.)

At each position for the eyepiece, set focus for infinity, or at least nearly so. Then conduct the flashlight test. Better yet, given the typically large aperture and consequent dimming of the light coming out the front end, use a green laser. Install an eyepiece giving a 1-2mm exit pupil; the laser beam will then fill the exit pupil and hence the entrance pupil.

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