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Equipment Discussions >> Binoviewers

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Eddgie
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Reged: 02/01/06

Followup to post on aperture reduction in EdgeHD 8
      #5753550 - 03/24/13 10:15 AM

As a quick recap, I measued the aperture loss imposed by using the Mark Vs in my EdgeHD 8" scope about 3 weeks ago, and while the back focus was under 200mm, the aperture of the scope was reduced to 7.5 inches, resulting in 14% light loss.

The configuration was the Baader Clicklock adapter (37mm),
the T2 Prism (38mm) and the Mark V (123mm) for a total of 198mm of back focus.

In an attempt to recover as much aperture as possible, I decided to try the Baader SCT to T2 adatper in place of the Baader Clicklock. I was hoping to get to 7.8 inches of aperure

The configuration is now shortened by 22mm to 176mm.

Sadly, I fell short of my goal of getting to 7.8mm.

The new measured aperture was 7.7 inches, so I got back .1 inch of aperture for each 11mm of light path reduction.

What this means is that even using a standard 2" diagonal in mono mode, the EdgeHD 8" would have a back focus of about 150mm, and in this configuration, the system would be just at the edge of aperture loss.

Extrapolating out for aperture loss of .1 inch for every 11 millimeters of aperture loss, it is clear why my result with the 2" Denkmeier supersystem was so disappointing. I did not measure it directly at the time (didn't know how) but I had commented that the Denk Supersystem had a very negative impact on image quality in both the EdgeHD 8" and C14.

Extrapolating back now, it would appear that the EdgeHD 8" had been reduced to 6.7" in aperture with the 2" based supersystem.

This would result is a starggering light collection loss of 45 square inches for the full aperture down to 30 squre inches of light collection for a 2" based Denk Supersystem!!! And this explained why the first time I put in the Denk Supersytem in the EdgeHD I immediatly knew that I made a mistake. The damage was horribly apparent.

And in reducer mode, I think the aperture was probably reduced to about 6.3 inches, turning the scope into a C6 with a 43% obstruction. Trust me when I say that it was like looking though a 4" scope. This is a configuration I would strongly caution people against using.

So it appears there is no possible binoviwer configuration that would allow the EdgeHD 8" to work at full aperture, though the Maxbright with the 10mm adapter woudld come very close at about 7.8"

I think the EdgeHD 8" is indeed somehow more tightly baffled than the older standard C8. These scopes would start to loose aperture at about 170mm of back focus, but it was very gradual so that even with 200mm of back focus, the system had only lost about .2 inches.

Just wanted to give EdgeHD 8" users a heads up. This scope does not tolerate back focus well at all, and over 150mm, you start to loose aperture pretty agressively as compared to the old C8 design.

I was really hoping to get a bit more aperture back with the Baader SCT to 2" adapter, but it was not enough to make a meaningful difference.

I will probably use the Baader SCT to T2 adatper because it also reduces the focal lenght about 65mm which combined with the extra aperture will hopefully give a bit more true field and a bit brighter image simply because of the very slightly increased exit pupil for a given size eyepiece, but it was not the gain I had hoped for. I am disappointed though that the difference only bought .2" of aperture at a cost of $30 per .1". That is like APO pricing

Hope this information is useful to someone out there.


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HTJ
journeyman


Reged: 10/12/12

Loc: Denmark
Re: Followup to post on aperture reduction in EdgeHD 8 new [Re: Eddgie]
      #5754446 - 03/24/13 04:53 PM

Hi Eddgie

Thanks for this report. I am currently looking for something with higher aperture and the 8" Edge was on the table, but this certainly reduces the chances of me acquiring it, as enough backfocus for a binoviewer is an important criteria for me.

Btw. did you managed to figure out a way to stop the Baader SCT adaptor from wigling around.

FWIW my former Intes Alter M500 (127 mm Mak-Cass) seemed to have a significant amount of backfocus, but they come with a heavy pricetag, especially when going over 6".


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Eddgie
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Reged: 02/01/06

Re: Followup to post on aperture reduction in EdgeHD 8 new [Re: HTJ]
      #5754627 - 03/24/13 06:04 PM

I don't know if this should be a show stopper.

The EdgeHD 8" remains a fantasting scope, and for me, the main downside to the apeture loss was that I noticed that the planetary views were not as good as when I used the Maxbright/T2 in a 1.25" visual back. The difference was way subtle, but the image looked a bit dimmer than I would have expected when using the Mark Vs on Jupiter a couple of weeks ago.

If you can keep it short though you only loose .3 inches and this is not horrible.

The only time you will see it (if ever) is if you are a discrrning planetary observer with a lot of time on an EdgeHD 8" already. Otherwise, you might never know it.

I have not had a chance to use it much. I don't trust the configuration though.

My plan is to attempt to use a stippling punch to stipple the flat rear port of the EdgeHD 8" (just a couple of stillpes here) and stipple the heck out of the face of the ring inside the Baader SCT adapter ring so that there is some friction there.

The only time I used it, it actually seemed to hold the binoviewer in place fairly well. I had 24mm Hyperions in it, and there was so much weight out on the end, that when the scope is at an angle, I think the torque on the faces is pretty high.

I think greatest danger is when the scope is pointing closer to the horizon and the eyepeices are more straight up.

Worries the heck out of me though, so I am going to try the stippling approach. Just a couple in the back of the SCT port should be enough if I can get a lot of textrure on the face of the ring.

It is low priority though. Fact is, I don't hardly ever use my EdgeHD for planetary observing, but for someone that intends to when using binoviewwers, I thought this would be important information to have.

Baader SCT to T2/Baader standard Prism/and Maxbright binoviewrs might be the best "Planetary" configuration for the EdgeHD 8".

And I have to say, after going from the Denk Supersystem to the Maxbrights, the difference on planets was pretty huge to me. The Supersystem was likely working at well under 7" of aperture and the first time I put in the Maxbrights, I knew immediatly that I had made the right decision to go to the short light path of the Baader system for planetary observing.

I love the Mk 5s in the C14 though.

And for general viewing in the EdgeHD 8", the Mark Vs are great. It is hard to see the apeture loss on stellar targets so for clusters, I love the Mark Vs in the EdgHD 8"

Here is the problem though. We don't know if the other EdgeHD scopes have this same characteristic.

We do know that the C11 used to have the widest well illumianted field of any Celsetron scope (off axis in millimeters).

But a recent thread brought to light that the EdgeHD 11 now has the same illumination falloff of the C14.

The C14 starts to loose aperture at 200mm of back focus.

The C11 used to run well out to 300mm before apeture loss occurred.

If the new design is now limited to 200mm, it could be that people using a lot of back focus (2" diagoanl, Supersystem) that worked well in the old C11 might now suffer some meaningful apeture loss in the EdgeHD 11.

We just won't know until people start measuring it and reporting results.


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teskridg
Pooh-Bah


Reged: 01/15/08

Loc: PA
Re: Followup to post on aperture reduction in EdgeHD 8 new [Re: Eddgie]
      #5760048 - 03/27/13 11:36 AM

These posts would strongly suggest that for the likely small subset of us, like me, that the trade-off between a standard C11 and an Edge HD C11 isn't just at the edge of the field. That is, as I universally binoview with a power x switch, I might be better off in terms of aperture cut-off with a standard C11 than with the Edge HD, even though the correction for coma and flatness of field at the edge of the field is better with the Edge version. Since most users probably do not binoview as much as I do, the decision to get an Edge HD version would seem simpler- based on the prices of the two systems and perhaps on the easier and cheaper availability of a FF/Coma reducer for the standard C11. For extensive binoviewing, the standard C11 (or C14 for that matter) would seem advantageous. Tim

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Eddgie
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Re: Followup to post on aperture reduction in EdgeHD 8 new [Re: teskridg]
      #5760075 - 03/27/13 11:52 AM

I don't know if that is at all true.

First, we don't know if the other scopes have the same constraints on back focus, though many SCTs do loose back focus at more than 200mm or so. It is pretty easy to come in under 200mm even with Mark Vs.

You just can't use a 2" based system and do it.

The reducer type systems (powerswitch and such) will almost always cause an aperture loss though.


Also, even if you lost a couple of millimeters in a larger scope, it would be a smaller percenage of total aperture.

The EdgeHD 8" may be a rather unique case. Until we hear other reports from EdgeHD users, we won't really know, and I hope more people test and check.

But a few millimeter loss in a C11 for example (which I doubt would happen if the back focus were kept under 200mm) would not be visible to anyone.

Even a half inch loss may go unnoticed by most in a very large aperture.

But some configurations will indeed cut the aperture.

And finally this.

The EdgeHD 8" is not a scope I normally use for planets. This is the only time the aperture loss would otherwise bother me, and I am content to suffer a bit of aperture loss for general use.

But if planetary is the main use, then once again, my message has for the last year been "Keep the back focus as short as possiblle to avoid as much aperture loss as possible."

My bet is that under 200mm, and most of the larger EdgeHDs will work at full or near full aperture.

I am hoping more people will measure and post though, so we have the facts.

I do believe the EdgeHD 8" may be the extreme case, but I thought it was important to post my findings.

Edited by panhard (03/27/13 09:19 PM)


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Patrick
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Reged: 05/16/03

Loc: Franklin, Ohio
Re: Followup to post on aperture reduction in EdgeHD 8 new [Re: Eddgie]
      #5762079 - 03/28/13 09:57 AM

Quote:

I am hoping more people will measure and post though, so we have the facts.




Edggie...what's the procedure for checking?

Also, what impact do you suspect the new 0.7x Celestron focal reducer will have on your results?

Patrick


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Eddgie
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Reged: 02/01/06

Re: Followup to post on aperture reduction in EdgeHD 8 new [Re: Patrick]
      #5762122 - 03/28/13 10:30 AM

Easy answer first.

Even the standard C8 will loose apeture if used with a binoviewer and the standard focal reducer. This configuration will drop the aperture below 7 inches in a C8 and there is no SCT configuration possible using binoviewers and a reducer that does not incur serious aperture loss.

I suspect the EdgeHDs will be worse than the standard scope for apeture reduction based on my own experience with the EdgeHD 8".

The Alan Glee Mk II is probably the very best you can do in a C8, an that reduces the apeture by .5", which is not all that terrible for general use, but should be avoided for planets of course.

The proceedure to measure... A green laser works best, but a single emitter LED light might work OK.

Next time you use the telescope with binoviewers, bring it to focus with the configuration you want to test on a a star or planet with an eyepeice that gives about a 1mm exit pupil (infinity focus).

Do not touch the focuser knob until after you have made the measurment. It is critical that the telescope be in focus for the configuration you want to test.

When you are ready to do the test, position the telescope facing square to a wall, maybe 1 foot from the wall. Get it as square as possible.

Now, holding your light source above the same eyepeice you used earlier, shine it into the eyepeice. It will take some trial and error, but when you have aligned the laser, you will see a green projection on the wall.

This circle will contain the secondary shadow.

You can measure the apeture directly by simply marking the edges of the circle with a piece of tape (it would help to have somone do this part for you).

Once you have the edges marked, you can measure the distance between the marks and you have your effective aperture.

You can also use a Finder scope as a beam collimator. Place the finder with the objective pointing toward the laser and the eyepiece pointing into the binoviewer.

One person mounted the laser on a camera tripod so he could make tiny adjustments. He rotated the binoviwer to one side so the laser could be horizontal. This may help because it is very difficult to get the laser exactly over the exit pupil of the eyepiiece.

I usually use a lower power eyepecie and I have been told that it can affect the accuracy, but I would think that if anything, it would overstate the amount of aperture, but I think it is still pretty accurate to within a millimeter or two.

Easy test to do.

The problem of course is that the baffles start to cut into the light cone as the mirror moves forward the necessary distance required to move the focal plane back.

This is an absolutly unavoidable conseguence of using a lot of back focus in a moving mirror cassegrain telescope.

You could of course correct for it by making the baffles larger at the expense of a bigger central obstruction, but that seems to be a poor tradeoff.

Anyway, if you measure some different configurations, please let us know what you find.


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Patrick
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Reged: 05/16/03

Loc: Franklin, Ohio
Re: Followup to post on aperture reduction in EdgeHD 8 new [Re: Eddgie]
      #5762344 - 03/28/13 12:49 PM

Quote:

I suspect the EdgeHDs will be worse than the standard scope for apeture reduction based on my own experience with the EdgeHD 8".





You might be right. I believe the available backfocus on the EdgeHD 8" will only be 105mm with the new focal reducer.

Patrick


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bcuddihee
Carpal Tunnel
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Reged: 11/04/06

Loc: Cincinnati Ohio
Re: Followup to post on aperture reduction in EdgeHD 8 new [Re: Patrick]
      #5762611 - 03/28/13 03:39 PM

Just some thoughts on the subject of aperture reduction in a bino'd c8. First of all I did the light test and I cuncur with all that has been written to this point, however, I would hate to think that in the attempt to bring this issue to
the forefront..we may indeed lead folks to put off binoviewing at all. IMO, bioviewing never was about trying to squeeze every single photo into ones eyes but rather how the image presents itself. For me i would rather view with a c8 with binos than a c11 without.


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Eddgie
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Re: Followup to post on aperture reduction in EdgeHD 8 new [Re: bcuddihee]
      #5762675 - 03/28/13 04:30 PM

Well, for general viewing, it is not really an issue I don't think. I mean it doesn't help any because in essence, the scope is turned into something like 7.5" f/11 scope with a 38% obstruction.

Where I would be concerned though is for planetary observations.

I found the dimming even using a pair of 15mm Plossls to be enough that I was having trouble with the view..

For me it isn't much of an issue because I use my C14 for the very vast majority of my planetary observing.

Anyone that considers their EdgeHD 8" their best planetary scope may indeed want to consider the consequence of keeping the back focus well under control.

Again, I was using a fairly short light path with the Mark Vs and standard T2 Prism, and many people are not using the T2, so if they are using something with a standard diagonal, it is unlikley that they will get even 7.5" of apeture.

And that was the point of my message. It is going to be difficult to get the maximum performance out of a binoviwer in the EdgeHD 8".

People that use their EdgeHD 8" as their primary planetary scope should be very very careful about back focus.

Again, I have a C14 so for me, it isn't a major issue, but I don't think I got a good a view of planets with the Clicklock/T2/Mark V as I did when using a 1.25" Visual back/T2 prism, and Maxbright.

And this clearly explains why I almost threw up when I first viewed though a 2" based supersystem. I am sure now that the apeture was reduced to maybe 6.5 inches in that configuration, and perhaps 6" with the low power arm in place. One look and I knew immeditatly that something was terribly wrong.

So, not saying one should not binoviewr ane EdgeHD 8" at all.

But one can ideed damage the view by allowing the light path to get to long.


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Mark9473
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Reged: 07/21/05

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Re: Followup to post on aperture reduction in EdgeHD 8 new [Re: Eddgie]
      #5762713 - 03/28/13 04:56 PM

Quote:

Well, for general viewing, it is not really an issue I don't think. I mean it doesn't help any because in essence, the scope is turned into something like 7.5" f/11 scope with a 38% obstruction.

Where I would be concerned though is for planetary observations.

I found the dimming even using a pair of 15mm Plossls to be enough that I was having trouble with the view.




You've said this so often now but I still don't understand. How can light loss on a bright target be worse than light loss on a fainter target?


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Derwin Skotch
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Reged: 03/11/05

Loc: PA, USA
Re: Followup to post on aperture reduction in EdgeHD 8 new [Re: Eddgie]
      #5762732 - 03/28/13 05:06 PM

I keep wondering if an OCA like device could be built to work with SCT's to completely eliminate this issue. Siebert Optics makes a variety of systems to push the focal plane back for refractors and newts that add little or no magnification.

http://www.siebertoptics.com/SiebertOptics-OCA.html


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Eddgie
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Re: Followup to post on aperture reduction in EdgeHD 8 new [Re: Mark9473]
      #5764591 - 03/29/13 12:53 PM

Well, it is never a good thing to have light loss.

For a planet though, the issue is illumination. The illumination of the target assists in seeing the most low contrast detail, but the magnification makes the detail large enough to be seen.

With deep sky though, you only loose a small amount of limiting magnitude, so you only see the difference on very rich stellar targets, and even then, only if you really look hard for it.

For extended objects, the answer is much simpler. You just step down a notch to make the exit pupil a bit bigger. You loose a little scale, but it is not enough that it should be a concern.

For small details though, where the magnification is required to get the detail large enough to resolve, trading a bit of magnification for a bit of brightness really works against you. That is why I think it is more of an issue for planets.

So, aperture loss is never a good thing, but I think the main worry is the loss of illumination for high power work.


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Eddgie
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Re: Followup to post on aperture reduction in EdgeHD 8 new [Re: Derwin Skotch]
      #5764611 - 03/29/13 12:58 PM

Yes, I think that for planetary work, using a powerful OCS may indeed be the way to go.

I estimate that my EdgeHD 8" would need about 30mm more of in-focus to work at full aperture. Using an OCS or GPC would likely provide the equivalent back focus relief.

Again, for me, it is not an issue. I don't really ever use this scope for planets because I use my C14 for that task.

But for others that want to binoview an EdgeHD 8" for planets, the best approach may be to keep the light path less than about 180mm and use an OCS/GPC to allow the mirror to move far enough back so the light cone can clear the baffles.


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teskridg
Pooh-Bah


Reged: 01/15/08

Loc: PA
Re: Followup to post on aperture reduction in EdgeHD 8 new [Re: Eddgie]
      #5764910 - 03/29/13 02:42 PM

So in the case of a C11 vs. a C11 Edge HD, is the former more accepting of back focus without as much aperture loss compared to the latter? Tim

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Derwin Skotch
super member


Reged: 03/11/05

Loc: PA, USA
Re: Followup to post on aperture reduction in EdgeHD 8 new [Re: teskridg]
      #5765187 - 03/29/13 04:36 PM

I have ordered a lens with a relatively long focal length of -850 mm to experiment with. The plan is to mount it in a tube that is inserted into the baffle tube a couple of inches to act as a low power Barlow to move the focal plane. It is an inexpensive non-achromatic lens so I don't expect it will be put into permanent use. If this works I plan on finding a high quality optic to use.

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Eddgie
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Reged: 02/01/06

Re: Followup to post on aperture reduction in EdgeHD 8 new [Re: teskridg]
      #5765391 - 03/29/13 05:40 PM

Quote:

So in the case of a C11 vs. a C11 Edge HD, is the former more accepting of back focus without as much aperture loss compared to the latter?




This is a very good question.

According to the ray trace I have, the C11 used to have the best off axis (measured in millimeters away from the center of field) illumination of all of the Celestron scopes. It could work at well over 250mm without loosing illumination.

Now however, Celestron says that the fully illuminated field is only the same as the C14 when both are used with a focal reducer.

In the past, the focal reducer cutoff point for the C11 was a whopping 150mm, while for the C14, it was 100mm (the poiint that more back focus causes aperture reduction).

The latests info from one of the forums is that now Celestron is saying that with the new focal reducers, the illumination is the same in both scopes.

Now this is with the focal reducer, but my guess is that without the focal reducer, the scope is less tolerant of back focus than it used to be.

But I don't know that to be true.

But the C14 lost apeture at 200mm. At 260mm of back focus (Denkmeier Supersystem in a 2" diagoanl) I could clearly see a loss in brightness as compared to less than 200mm with the Maxbright/T2 which had under 200mm of back focus.

So, until someone measures it, I can only guess, but based on the info that someone got from Celestron regarding the field illumination with a focal reducer, we know that it is more restricted than in the past.

How much though, waits to be seen.


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teskridg
Pooh-Bah


Reged: 01/15/08

Loc: PA
Re: Followup to post on aperture reduction in EdgeHD 8 new [Re: Eddgie]
      #5765466 - 03/29/13 06:16 PM

Thank you, Eddgie; my CPC-1100 is 2-1/2 yrs old, so I don't know if its baffle or other specifications are the same as present models. I have a green laser, so I guess I could bite the bullet and measure the aperture as you directed. Tim

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GlennLeDrew
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Followup to post on aperture reduction in EdgeHD 8 new [Re: teskridg]
      #5765515 - 03/29/13 06:40 PM

Derwin,
Many could lift a page from your book. Experiment!


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Eddgie
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Reged: 02/01/06

Re: Followup to post on aperture reduction in EdgeHD 8 new [Re: GlennLeDrew]
      #5765556 - 03/29/13 07:12 PM

Yes, I agree.

I am hoping more people with different kinds of scopes will take the time to measure with different configurations.

Again, I think for general observing, it is in the NBD range of worries. Global warming would be higher on my list of things to loose sleep over.

For a serious planetary observer though, I think these are more than academic issues.

But having people measure and post would be good general knowledge to help others with knowing what they can expect from a given configuration.


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