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Equipment Discussions >> Binoviewers

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TGModerator
Pooh-Bah


Reged: 11/02/06

Loc: Latitude 47
Binoviewers and spherical aberration in EdgeHD new
      #5852491 - 05/10/13 06:36 PM

This week, I tried my Mark V in the EdgeHD 11" for the first time ever. I tried it with and without a 2x AP barlow but in both cases I noticed asymmetrical diffraction patterns indicating that spherical aberration had been introduced. Given that the EdgeHD requires precise backfocus for optimal performance, this didn't surprise me. In comparison an AP refractor shows no degradation when the Mark V is used with it (as expected).

I'm interested in what other folks' experience is if they're using a combination of EdgeHD and any BV. Is there a solution here? An OCA to reach the precise back-focus? But how would one determine what kind of OCA is needed?

Tanveer.


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Moonglum
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Reged: 06/01/08

Re: Binoviewers and spherical aberration in EdgeHD new [Re: TG]
      #5852935 - 05/10/13 10:48 PM

I think there must be about 6 inch difference in the distance where my Barcon reaches focus and where my 2.6X Glaspath reaches focus, the Barcon being the longer. This is in my 4inch apo...point is, I think you are too long. I think a 1.2X or 1.5X Glaspath would be best with your scope.

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HowardK
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Reged: 10/20/10

Re: Binoviewers and spherical aberration in EdgeHD [Re: Moonglum]
      #5853247 - 05/11/13 04:24 AM

Look again Tanveer

I see none of what u speak


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TGModerator
Pooh-Bah


Reged: 11/02/06

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Re: Binoviewers and spherical aberration in EdgeHD new [Re: Moonglum]
      #5854037 - 05/11/13 03:45 PM

Quote:

I think there must be about 6 inch difference in the distance where my Barcon reaches focus and where my 2.6X Glaspath reaches focus, the Barcon being the longer. This is in my 4inch apo...point is, I think you are too long. I think a 1.2X or 1.5X Glaspath would be best with your scope.




I do have a 1.7x GPC that I've adapted for diagonal nosepiece use so I'll give it a try but I wasn't sure how using a Barlow or GPC would figure into the back focus equation. Of course, with a focal length of 2780mm I really wanted a 1x OCA if a good one exists.

Thanks for the suggestion.

Tanveer


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TGModerator
Pooh-Bah


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Re: Binoviewers and spherical aberration in EdgeHD new [Re: HowardK]
      #5854039 - 05/11/13 03:47 PM

Quote:

Look again Tanveer

I see none of what u speak




Are you telling me you have identical inside / outside rings when using the Mark V in a C11HD? Are you using a GPC?


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Eddgie
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Reged: 02/01/06

Re: Binoviewers and spherical aberration in EdgeHD new [Re: TG]
      #5854144 - 05/11/13 05:14 PM

I know that excessive back focus can indeed introduce spherical abberation in all SCTs.

It is very noticable in my C5, where the SA change is pretty easy to see. Maybe 1/4th wave.

In the EdgeHD 8", I can see it too, but not as much.

In the C14, I can't see any, but the C14 design has the nominal back focus set at 150mm. This is where the scope is working at f/11 and 3910mm (stated focal lenght).

All of the other Celestron SCTs used to be optimized for about 100mm of back focus, but I believe this has been extended a bit further back in the EdgeHD scopes.

I would think there would be less in the EdgeHD 11 than in the EdgeED 8 for a given amount of back focus (the focal ratio will change more the smaller the aperture).

The question though of course is wheter it is enough to do meaningful damage to the image, and my guess is that while it is enough to see in the star test, it may not be enough to really affect the image more than a tiny but.

Considering my EdgeHD 8" and my C11 both seem to have excellent correction when used at thier optimal amouts of back focus, when it changes, even a small change seems easy to see.

Star test is very sensitive for SA, so I for one am not surprised if you say that SA changed. The physics of the design indicate that this has to happen, but agian, the only question is at what point does it become severe enoug to be an issue.

I have become maniacal about back focus though. The less I can get, the better.


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tomcody
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Re: Binoviewers and spherical aberration in EdgeHD new [Re: Eddgie]
      #5869036 - 05/18/13 09:37 AM

Tanveer or Eddgie,
As Celestron lists the design backfocus distance of the 8" edge at 133mm and the 11" edge as 146mm could one of you try an experiment for me? Attach the MarkV's (without any GPC ) directly to the back of the scope without using any diagonal, in the case of the 8" a 2" to T-thread adapter should provide about the perfect distance ( i.e. MarkV = 123mm + T adapter = 10mm? = total of 133 distance), in the case of the 11" (MarkV = 123mm + 2" nosepiece = 20mm? total = about 145mm ). I surmise that the corrector lens in the Edge should work the same as the GPC or recommended barlow to help narrow the image and the resultant views should be about as good as possible with the widest views possible. Either of you care to try? ( I am considering getting an Edge 8" for use with MarkV's and this information would help me make a decision). Thanks
Rex

Edited by tomcody (05/18/13 09:39 AM)


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Eddgie
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Re: Binoviewers and spherical aberration in EdgeHD new [Re: tomcody]
      #5869598 - 05/18/13 03:40 PM

Well my sky conditions have been a bit cloudy, but next time I am out with the EdgeHD I might try it.

It may be a while though.

But the EdgeHD 8" is going to loose a little aperture when used in a normal configuration, but the amount of spherical aberration should be quite miminmal, and honestly, I would not worry about it at all..

For general binovieweing, if you keep the light path short (Baader prism and direct connection with the 10mm SCT/T2 and Baader Lock ring) that is about as good as you will get, and frankly, for general observing, more than good enough.

And if you want to view planets, stick in a high power GPC and this will get you full aperture and minimal SA because when used with a very short light path, you will be at about 176mm of back focus, and with a 2.6 (2.7?) GPC, this will put you down to below 150.

That should mean full aperture and quite minimal SA. Maybe less than 1/15th wave. In other words, not even enough to see in the star test, and not enough to matter.

Most people can't see the damage done by 1/6th wave so even without the GPC, you will likely not notice any degradation in the view for planets. Scope will only be at 7.8" of aperture, but that is the best you are going to do with a Mark V and EdgeHD 8" I think.

I do love using the binoviewers with the EdgeHD, but I know the scope is not working at optimal. So it goes.


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TGModerator
Pooh-Bah


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Re: Binoviewers and spherical aberration in EdgeHD new [Re: tomcody]
      #5877844 - 05/22/13 11:44 AM

Quote:

Tanveer or Eddgie,
As Celestron lists the design backfocus distance of the 8" edge at 133mm and the 11" edge as 146mm could one of you try an experiment for me? Attach the MarkV's (without any GPC ) directly to the back of the scope without using any diagonal, in the case of the 8" a 2" to T-thread adapter should provide about the perfect distance ( i.e. MarkV = 123mm + T adapter = 10mm? = total of 133 distance), in the case of the 11" (MarkV = 123mm + 2" nosepiece = 20mm? total = about 145mm ). I surmise that the corrector lens in the Edge should work the same as the GPC or recommended barlow to help narrow the image and the resultant views should be about as good as possible with the widest views possible. Either of you care to try? ( I am considering getting an Edge 8" for use with MarkV's and this information would help me make a decision). Thanks
Rex




Good idea but we're currently rained out with a system coming in from Alaska. Perhaps I'll get a chance by this weekend. I have a pier extension tall enough that I could comfortably view without a diagonal as long as it's not near the zenith.

I'm planning to buy the T2 prism diagonal so I can get some of the backfocus back. However, I'm not sure whether the non-Zeiss one is just as good.

Tanveer.


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Eddgie
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Re: Binoviewers and spherical aberration in EdgeHD new [Re: TG]
      #5878007 - 05/22/13 01:10 PM

I have of course not used the Ziess one, but the Baader appears to perform about as good as I think a diagonal can perform.

When used in my 6" APO, I could fine no difference between it and a Telvue Dielectric. Views were identical.

It is not at all difficult to make a diagonal to prism to an excellent standard of quality, and the coatings on the Baader look quite excellent to me.

I have no reservations at all about giving a 100$ recommendation to the standard T2. I personally believe that any difference between it and the Ziess prism will be to small to be seen at the eyepecie in any way, shape or form.

That is of course only an opinion, but the unit appears to be flawless as compared to very fine, high reflectivilty mirrors.


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TGModerator
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Re: Binoviewers and spherical aberration in EdgeHD new [Re: Eddgie]
      #5878170 - 05/22/13 02:23 PM

Quote:

I have of course not used the Ziess one, but the Baader appears to perform about as good as I think a diagonal can perform.

When used in my 6" APO, I could fine no difference between it and a Telvue Dielectric. Views were identical.

It is not at all difficult to make a diagonal to prism to an excellent standard of quality, and the coatings on the Baader look quite excellent to me.

I have no reservations at all about giving a 100$ recommendation to the standard T2. I personally believe that any difference between it and the Ziess prism will be to small to be seen at the eyepecie in any way, shape or form.

That is of course only an opinion, but the unit appears to be flawless as compared to very fine, high reflectivilty mirrors.




Thanks Eddgie, you helped me make up my mind. The Zeiss on is out of stock anyway. Now what needs to leave the stable...

Tanveer.


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Eddgie
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Re: Binoviewers and spherical aberration in EdgeHD new [Re: TG]
      #5878340 - 05/22/13 03:20 PM

I think you will be pleased with the Maxbright.

I am considering getting the 10mm to T2 adapter and the SCT Lock Ring.

This is only a bit shorter than the 15mm SCT to T2. The 15mm to T2 allows rotation of the diagonal by slipping the collar. The problem is that it is very difficult to feel confident that the unit will not break loose when on the left.

The Locking Ring would allow I think for a more secure connection.

This was my main problem with the 15mm Connector. If you tighten it enough to make sure nothing moves, it can be a bear to get loose again when you want to rotate the diagonal.

Still, now you have the issue of having to take it on and off when you want to go back and forth to a standard 2" nose.

For the C14, it is meaningless, but for the EdgeHD 8", I think that every little bit helps..

It is actually listed for the Maxbright mirror diagonal, but I can't see any reason why it would not work equally as well with the 10mm connector. It would just thread out and lock up against the end of the 10mm Connector I would think. A long as the 10mm connector has three or four revolutions of thread engagment, I don't know why that would not work.


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TGModerator
Pooh-Bah


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Re: Binoviewers and spherical aberration in EdgeHD new [Re: TG]
      #5878662 - 05/22/13 05:41 PM

[quote The Zeiss on is out of stock anyway. Now what needs to leave the stable...

Tanveer.






Turns out the Zeiss is in stock and the Maxbright is out of stock everywhere (except perhaps direct from Baader in Germany). I'm tempted to buy the more expensive one but I'm not sure the difference will be worth $130 more.

Tanveer.


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TGModerator
Pooh-Bah


Reged: 11/02/06

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Re: Binoviewers and spherical aberration in EdgeHD new [Re: TG]
      #5950118 - 07/01/13 06:14 PM

UPDATE:

Due to the generosity of a fellow CN'er, I'm now in possession of the Baader Maxbright prism diagonal, sent all the way from Germany. With this diagnonal and the 1.25x corrector, the diffraction patterns returned to normal, or as close to normal as I can remember. Even though this isn't the more expensive Zeiss prism, the coatings on the prism are excellent and there is no induced aberration that I can see.

I'm a happy trooper . Now only if the seeing would cooperate.

Tanveer.


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Eddgie
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Re: Binoviewers and spherical aberration in EdgeHD new [Re: TG]
      #5950133 - 07/01/13 06:28 PM

Happy to hear this.

I feel high confidence when recommending the Baader 1.25" standard prism diagonal. The quality appears quite excellent.

And not surprised about the change in spherical aberration. As I mentioned, I can see it change in the C5 quite a bit when the binoviewers are in place with a long light path.

For the EdgeHD 8", I think keeping the light path short is a good idea too.

Again, glad to hear that the T2 Prism seems to be working well for you. I have high regard for it and don't see the value in purchasing the far more expensive Zeiss unit.


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