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Aperture Reduction with EdgeHD 9.25

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#1 KJL

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Posted 20 August 2014 - 01:15 PM

There has been some excellent work by Eddgie and others regarding the aperture reduction caused by long path lengths in the EdgeHD 8. Suffice to say that the cut-off is around 160mm from the baffle nut, which is pretty short.

 

Has anyone with an EdgeHD 9.25 made similar measurements on how long the path length can be before that scope starts losing aperture?

 

Empirically it seems to be the case that my EdgeHD 11 behaves similarly to the C11 in that it doesn't drop aperture even with about 300mm of path length, achieved with a Denkmeier Power + Filter Switch Diagonal (with IVB visual back) and a pair of Maxbright binoviewers. I need to test this more rigorously but suffice to say I would not expect that from any other of the Celestron SCTs.



#2 ewave

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Posted 20 August 2014 - 05:09 PM

KJ

 

No, I haven't measured the loss of aperture with my C9.25 Edge, and any loss would be very difficult to discern at the eyepiece but I can attest to a loss in a field of view at the eyepiece.

I see you have the Delos 17.3, of which I also own.  When using this EP with my 1.25" Baader Zeiss prism diagonal in my C9.25 Edge, I can see the entire moon (~ 0.5 degrees) easily with room to spare.

However, when using this same EP in my 2" Diagonal, it is not even close with a clear increase in Focal Length and magnification, and that is just an increase in path length from 41mm to about 110mm. 

So yes, an increase in ~ 70mm path length sure caused a loss in FOV and increase in magnification.

 

Try this on your C11.

 

Clear skies,

 

Sean



#3 DesertRat

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Posted 20 August 2014 - 05:44 PM

KJ

 

I think the C11 would be more resistant to vignetting with back focus.  

 

For visual probably its ok, but for widefield imaging it is not recommended to go too far back on an Edge model.

 

I'm not clear on how people are measuring the aperture reduction however.  Perhaps I just missed that detail.  :question:

 

Glenn



#4 areyoukiddingme

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Posted 20 August 2014 - 06:44 PM

To measure fire a laser pointer down the eyepiece while the OTA is horizontal and pointed at a wall (about 1 foot away). When the laser is at the just right place, the aperture is projected on the wall. Compare a measurement of that to the true aperture, and try it with different attachments to find out if you are vignetting, and at which point.

 

I found my edge11 didn't vignette anywhere with binoviewers and a 2 inch diagonal. But from what I've read, it appears that the 11 is particularly resistant to this problem.



#5 DesertRat

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Posted 20 August 2014 - 08:02 PM

To measure fire a laser pointer down the eyepiece while the OTA is horizontal and pointed at a wall (about 1 foot away). 

 

Thanks.  I understand the concept.  However for an Edge with binoviewer and eyepiece there are so many stops and surfaces along the way out for the beam I think the measurement problematical.  In other words vignetting may be created from the test geometry alone.  Of course the backfocus distance would be a major factor.  The placement of the light source would be somewhat sensitive.  But if others have done this with success it may be ok.

 

I have an Edge 14 but no binoviewer.  I find the on-axis optical performance better at closer distances than the recommended 146mm.

 

Glenn

 

Edit: restated final sentence in better English.  :grin:


Edited by DesertRat, 21 August 2014 - 03:29 AM.


#6 KJL

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Posted 20 August 2014 - 09:23 PM

Thanks everyone for their input. It's good to have confirmation that the EdgeHD 11 is as lenient as the C11 as far as backfocus-induced aperture reduction is concerned -- namely, it seems to work with everything until you literally run out of in-focus. I still want to do some more rigorous testing though with my own OTA before I pass final judgement, but certainly nothing leaps to my eyes.

 

I have an Edge 14 but no binoviewer.  I find the optical performance better closer to the recommended 146mm back and in fact operate a little closer.

Thanks Glenn for your observations with your EdgeHD 14. I'm curious though: in what way do you find the optical performance better (I assume visually) near the recommended 146mm? I understand that technically the EdgeHD correcting optics must have an optimal position, but have not spent any time testing whether this is actually visible with my bare eyes. It would be interesting to know what to look for.

 

With a camera it has been shown that the margin of error for sensor position in an EdgeHD system is extremely small, literally down to a fraction of a millimeter for the flattest field. But I'm not certain my eyes are good enough to distinguish that sort of difference in the field.



#7 DesertRat

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Posted 21 August 2014 - 03:23 AM

Hi KJ,

I thank you for the question as it allows me to be clearer.  My earlier wording was a bit confusing now that I look at it.  :p

 

Testing the Edge14 with Roddier indicates better spherical correction closer than 146mm.  Note this is primarilly an on-axis measurement.  I found approx 100mm back provided the best correction in green light.  This has also been my experience in planetary imaging with the Edge 14.  Visually I think it would be difficult to notice the difference.  Perhaps a star test in really good seeing at different back focus positions might find the optimum distance for a given Edge scope.

 

However for wide-field applications I suspect better overall correction at the Celestron recommended back focus.  Not sure it is as critical as stated in the Edge whitepaper.  Without any contrary evidence I think it best to at least attempt to get as close as reasonably possible with any adapters on hand.

 

Glenn



#8 Eddgie

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Posted 21 August 2014 - 07:10 AM

Thanks everyone for their input. It's good to have confirmation that the EdgeHD 11 is as lenient as the C11 as far as backfocus-induced aperture reduction is concerned -- namely, it seems to work with everything until you literally run out of in-focus. I still want to do some more rigorous testing though with my own OTA before I pass final judgement, but certainly nothing leaps to my eyes.

 

I have an Edge 14 but no binoviewer.  I find the optical performance better closer to the recommended 146mm back and in fact operate a little closer.

Thanks Glenn for your observations with your EdgeHD 14. I'm curious though: in what way do you find the optical performance better (I assume visually) near the recommended 146mm? I understand that technically the EdgeHD correcting optics must have an optimal position, but have not spent any time testing whether this is actually visible with my bare eyes. It would be interesting to know what to look for.

 

With a camera it has been shown that the margin of error for sensor position in an EdgeHD system is extremely small, literally down to a fraction of a millimeter for the flattest field. But I'm not certain my eyes are good enough to distinguish that sort of difference in the field.

 

I am not sure that it is the same as the standard SCT.  I would measure it.

 

About a year ago, Russ (at Denk) and I had a long conversation about this.   As I recall, he told me that he did indeed have a customer that was unhappy with the apeture loss imposed by the binoviewers in low power mode.

 

Best to measure for yourself.

Disregard any concerns about the laser test.   I have used it on four different SCTs, and all showed various amounts of apeture loss (as much as an inch and a half) tjhat alwasys started at between 160mm and 200mm.

 

But I recall the conversation with Russ regarding the EdgeHD 11.

 

Again, my advice is to measure it directly, and trust the test.   You will see that if you are loosing apeture, shortenting the light path will give some back, which is just what should happen in this situation.

 

Not so sure at all that the Edge HD11 will tolerate back focus better than the other SCTs now, and until you or someone else measures it directly, the only think I have to go on is what Russ at Denk told me, though I think someone else was complaining about it on a forum a while back too.

 

Mearue to know for sure.



#9 KJL

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Posted 22 August 2014 - 03:35 PM

I am not sure that it is the same as the standard SCT.  I would measure it.

 

About a year ago, Russ (at Denk) and I had a long conversation about this.   As I recall, he told me that he did indeed have a customer that was unhappy with the apeture loss imposed by the binoviewers in low power mode.

 

Best to measure for yourself.

Disregard any concerns about the laser test.   I have used it on four different SCTs, and all showed various amounts of apeture loss (as much as an inch and a half) tjhat alwasys started at between 160mm and 200mm.

 

But I recall the conversation with Russ regarding the EdgeHD 11.

 

Again, my advice is to measure it directly, and trust the test.   You will see that if you are loosing apeture, shortenting the light path will give some back, which is just what should happen in this situation.

 

Not so sure at all that the Edge HD11 will tolerate back focus better than the other SCTs now, and until you or someone else measures it directly, the only think I have to go on is what Russ at Denk told me, though I think someone else was complaining about it on a forum a while back too.

 

Mearue to know for sure.

 

Thanks for the advice Ed. I will certainly find the time to more precisely measure the aperture reduction characteristics of my EdgeHD 11, especially if I end up returning it to downgrade to an EdgeHD 9.25.

 

What I'm trying to do here is lay the foundation for such a switch. If the EdgeHD 9.25 starts losing aperture even with mono EPs in my Power/Filter Switch diagonal, I'd have to take that into account.

 

BTW, my Power + Filter Switch diagonal's train is so long that I can't even achieve focus with the reduction "A" lens in when using the Maxbrights. My casual measurement when using the Maxbright with both arms out resulted in 11" of clear aperture. This would be consistent with your vignetting document which indicates the aperture reduction on the C11 kicks in beyond 300mm, which even my train doesn't hit.

 

The fact that no EdgeHD 9.25 owner has tested for aperture reduction tells me that people really don't seem to care about it! I care a great deal because when I use my IIE the focal ratio is very important, and if I also lose aperture ....



#10 Eddgie

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Posted 22 August 2014 - 04:42 PM

I think that for normal visual use, it is not at all anything to be concerned about.    It becomes far more important with binoviewers or when using a 2" diagonal and focal reducers.

 

Each user has to decide for themselves whether a little aperture loss or spherical aberration is enough to cancel the benefit of the wider field.    

 

My true belief though is that if a wide field is really the goal, the SCT is not at all the right scope design to provide it.    But that is just an opinion of mine.   I don't use any reducer on the EdgeHD 8" and love binoviewing it, even though I am limited to about 63x in low power when using the Mark Vs.   If I need a wider field, I use the SV110ED, and the vast majority of my observing is done with a 12" dob, so my personal viewpoint is that it is better to match the scope to the task, and recognize that sometimes it is better to just use a different telescope.

 

But I have a lot of scopes.  I can pick the one I want to use for a given target.  One size does not have to fit all for me.








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