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binoviewer and f/3.1 mirror

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#1 Apollo20

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Posted 07 March 2013 - 02:44 PM

Dear all,

I'm completing my 71 cm f/3.1 telescope
(http://www.facebook....166723403460447).
I'll use it both as monocular and with a binoviewer.
With a f/3.1 cone the front aperture of the binoviewer vignettes, of course.
I'm testing a 1.7x glasspath/coma corrector that does a good job, but anyway it sees about 67 cm, not the whole mirror.
Any idea on how to see the whole mirror?
Has someone with very fast mirror (below f/3.3 I mean) already solved this problem?

Clear and dark skies.
Fabio Falchi
www.cielobuio.org
www.istil.it

#2 Mark9473

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Posted 07 March 2013 - 03:20 PM

Denkmeier now has a 45mm clear aperture OCS. Otherwise I guess Harry Siebert could have the parts you need.

#3 Apollo20

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Posted 07 March 2013 - 04:00 PM

Thank you, Mark.
But has anyone tested this Denkmeier 45 mm OCS on an f/3 telescope and found that there is no vignetting at the center of the field? Even my 1.7x glasspath/coma corrector has about 45 mm clear aperture in a 2" mount, but it sees about 66-67cm out of the 71cm of my primary mirror (I photographed the exit pupil of the eyepiece mounted on the binoviewer).

Fabio

#4 johnnyha

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Posted 07 March 2013 - 04:03 PM

You'll probably have to go up to 2X.

#5 GlennLeDrew

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Posted 07 March 2013 - 06:14 PM

From my own not-so-thorough checking, binoviewers cannot accommodate a light cone faster than *about* f/5 at its own front aperture. Whatever that limit is for your unit, you'll need to introduce a Barlowing action which will get the light cone to around this value.

#6 Astrojensen

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Posted 08 March 2013 - 03:41 AM

Hi Fabio

I haven't tried it, but a combination which MIGHT work is this:

Baader Mark III coma corrector + M48 extension rings + TeleVue 2x Powermate + T2 adapter + Baader binoviewer.

I am *not* sure the Baader Mark III coma corrector can get the ideal working distance to the Powermate. But using just the Powermate + T2 adapter + binoviewer WILL work. Coma will not be eliminated, but I think it will be reduced to that of an f/6 telescope. Maybe.


Clear skies!
Thomas, Denmark

#7 Eddgie

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Posted 08 March 2013 - 09:50 AM

binoviewers cannot accommodate a light cone faster than *about* f/5 at its own front aperture.



I would have thought that it would be a little faster than f/5, but I agree that there is no way it will work at f/3.1 without an OCS.

Of course I could be wrong about this, but don't you just divide the opening of the binoviewer in millimeters to the light path lenghth of the binoviewers to get the limiting light cone?

For example, the Mark V has a front opening of 28mm (as measured at the quick coupler face). The light path lenght is about 120mm I believe (though I could be wrong about this but it seems like I read that somewhere).

If that is the way it works, this would suggest that for 100% illumination at the center of the field, the Mark V would be (120 / 28) about f/4.3?

Again, forgive my ignorance, but I tought you just needed to divide the entry aperture of the binoviewer light path lenght to get the limiting focal ratio.

To your point though, I think that f/3.1 is impossible without a barlow or OCS.

#8 Apollo20

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Posted 08 March 2013 - 12:45 PM

Eddgie,
your reasoning is correct.
But the problem for me is that even with a big OCS the center of the field receive only 66 cm out of 71 of the main mirror. Is it possible that none out there has ever tried to get a fully illuminated center of field with f/3 scopes?

Fabio

#9 Eddgie

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Posted 08 March 2013 - 01:26 PM

I have my doubts that it can be done, but my advice would be to ask someone to ray trace the system.

Also most spherical optical elements will no longer be diffraction limited with this fast a light cone. Not that it matters with this much aperture anyway, but my guess is that the OCSs themselves are no longer diffraction limited at f/3.1.

I honestly don't know the answers though. I just think that from a simple geometry standpoint, it will be a challenge.

That is why I think a ray trace for the entire system would be in order.

#10 GlennLeDrew

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Posted 08 March 2013 - 03:15 PM

Fabio,
How are you determining how much of the aperture you're using?

#11 Apollo20

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Posted 09 March 2013 - 12:56 AM

I put the binoviewer with the 1.7x GPC in in observing configuration, i.e. with eyepieces (well, one will suffice. At 3 cm over the mirror surface level I put a rule meter.
Then I photographed the circular exit pupil during the day (see the photo enclosed). Measuring how much of the meter is visible I got the result of about 67 cm.
Fabio

Attached Files



#12 GlennLeDrew

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Posted 09 March 2013 - 08:37 AM

That both the exit pupil and the image within are both out of focus is *slightly* suspect. And a more finely divided ruler would inspire greater confidence.

#13 Apollo20

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Posted 09 March 2013 - 09:39 AM

Yes, I've to repeat it with more light, in fully daylight and not in twilight as I did. So I'll get a sharper picture. But it will not change the substance.

#14 faackanders2

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Posted 09 March 2013 - 12:33 PM

I used it wit f4.1 wit great success.






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