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Equipment Discussions >> Refractors

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greju
Carpal Tunnel
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Reged: 10/13/05

Re: 80mm Triplet with Perfect Edge Stars... possible? new [Re: Pingster]
      #5466976 - 10/12/12 03:51 PM Attachment (58 downloads)

I think this meets all of the OP's criteria.

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greju
Carpal Tunnel
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Reged: 10/13/05

Re: 80mm Triplet with Perfect Edge Stars... possible? new [Re: greju]
      #5466979 - 10/12/12 03:53 PM Attachment (34 downloads)

Oh yeh, and it's a Bushnell.

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sg6
professor emeritus


Reged: 02/14/10

Loc: Norfolk, UK.
Re: 80mm Triplet with Perfect Edge Stars... possible? [Re: greju]
      #5467069 - 10/12/12 04:45 PM

At that short a focal length you will get curvature and even a triplet has it's limits. Simply you cannot go making them shorter and shorter without something running beyond it's capabilities.

All that the new glass types do is make that limit a bit lower down they do not eliminate it.

The lens have spherical faces and they are not ideal the tighter they get the more errors you get.

Also sure I have read of that scope and a few problems were reported. Someone here got it for imaging and returned it eventually.

For visual I would always look for f/7, make life easy don't push the components, if you are imaging then you will need a flattener almost immaterial of the scope.


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Pingster
member


Reged: 06/27/11

Re: 80mm Triplet with Perfect Edge Stars... possible? new [Re: sg6]
      #5469813 - 10/14/12 01:31 PM

Hey guys thanks for the addition feedback. I have a
2in WO diaginal and so is the coorect image baader. At night both diaginals give approx the same amount of smearex edge stars. But oddly duing the day the baader with no doubt has the flatter field. Very odd and suprising.

As a few members have stated... Fast APOs all suffer field curvature. Its not avoidable.

I contacted a very popular telescope website in the UK, i asked for "a 60mm to 80mm refractor that can produce a perfectly sharp FOV using 31nagler and 17ethos, budget being 1600USD (1000GBP)".

I was told they have nothing in theor range that would be suitable, that really suprised me. They recommended another site.... Who gave me the same reply an they recommended another site. Againntheu said nothing in their range! I was very suprised. Is it truely that difficilt to make a refra tor for 1600usd that has a perfectly sharp FOV. Unbelieveable.

I find this extremely odd. As i have a pair of cannon 15x50is bins. They have a perfectly sharp fov to the edge! And no false colour. So binoculars companies can give you somthing that provides two barrels with edge shape stars and no colour for $2000usd. Yet it seems we cant get one perfect tube for 1600.

The only scopes that seem to provide a perfect fov are in the 5k region... Ie TV NP1-01


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Eddgie
Postmaster
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Reged: 02/01/06

Re: 80mm Triplet with Perfect Edge Stars... possible? new [Re: Pingster]
      #5470266 - 10/14/12 05:56 PM

Quote:

The only scopes that seem to provide a perfect fov are in the 5k region... Ie TV NP1-01




Well, if you are including all scopes in the word scopes then the good news is that you can get a Celestron EdgeHD 8" for less.

I have one, and it gives pinpoint images to the field stop when using a 31mm Nagler.

But if you are limiting yourself to refractors, depending on visual accomodation, it may be very difficult. A bigger, slower scope will make the field curvature less a factor.

But I have an EdgeHD 8", and for the same size true field, it is as sharp as my 6" APO. Of course I can get a 2 degree true field in the 6" APO, but only a 1 degree field in the EdgeHD 8". But a stunningly good 1 degree field.

So, now you have to ask yourself the hard questions... What do you value in a view.

For a decade, my message to this forum has been that telescopes can differ far more in their off axis behavior than in their center of field performance.

And the advice that I would have would be for anyone considering a major purchase would be to try to famaliarize themselves with the off axis behavior of the telescope and how it will behave with different eyepeices.

And there is another important tip.. Another message that have been repeating many years..... The effects of field curvature are reduced as the focal length of the eyepeice gets longer.

If you change a 31mm Nagler for a 35mm Panoptic, the field will only be very slightly smaller in the Panoptic, but the lower magnification will mitigate some of the field curvature.

A 41mm Pan will have a slighly larger true field, than the 31mm Nagler, but once again, because of the much lower magnification, the effects of field curvature from the scope will be greatly mitigated.

I can't use a 31mm Nagler in my C14 because I can't accomdate the field curvature of the scope.

But if I plug in the 41mm Pan, I can almost completly accomdate the field curvature of the scope.

For the same size true field, the longer the focal lenght of the eyepeice, the less visible the field curvature of the scope will be to the observer.

I read this in a book (Telescope Optics). And guess what.. It really works that way...


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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: 80mm Triplet with Perfect Edge Stars... possible? new [Re: Eddgie]
      #5471072 - 10/15/12 08:07 AM

Quote:

But I have an EdgeHD 8", and for the same size true field, it is as sharp as my 6" APO. Of course I can get a 2 degree true field in the 6" APO, but only a 1 degree field in the EdgeHD 8". But a stunningly good 1 degree field.




Field curvature is not much of an issue in an 80mm refractor with a 1 degree TFoV... I guess it must be there but that's about 80x with a UWA, I don't see it..

When I think of field curvature, it is most pronounced with large field stops since curvature is related to the square of the distance off-axis. A 3-4 degree TFoV... that's when it is of concern.

Jon


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Eddgie
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Reged: 02/01/06

Re: 80mm Triplet with Perfect Edge Stars... possible? new [Re: Jon Isaacs]
      #5471900 - 10/15/12 05:08 PM

Ok I agree. When you get higher powers, the field curvature is far less of an issue.

But if someone purchases an 80mm refractor because of dreams of getting very wide, pinpoint fields of view only to wind up using it at relativly high powers, perhaps they would be better off with a different scope.

For example, with the Vixen 102 ED (f/9), you can use a 31mm Nagler and get much better edge performance, but of course a smaller true field than with an 80mm scope.

This was my experience with the 80mm Celestron ED f/7.5. It was unusable for me with the 31mm Nagler. The field was just too steeply curved.

I replaced that scope with a Vixen 100EDsf, and with my 35mm Pan, got a very nice, reasonably wide true field.

And that was my point earlier. When buying a scope/eyepice combination, it helps to understand how they will behave together before purchasing.

I personally found that I was pretty happy with larger scopes (with less field curvature) being used at more modest true fields of view (2 degrees in my 6" APO), than I was with smaller, faster scopes with giant, but unsharp fields of view.

The Televue 101 was of course an excepetion, offering both a very large field and a very sharp field, but in the end was too small of an aperture to me.

Fantastic though. The only small refractor that I ever liked more than a litte. I didn't use it much, but again, it was the only small refractor that I ever enjoyed using. But only under very dark skies where I could exploit its wide field capability.


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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: 80mm Triplet with Perfect Edge Stars... possible? new [Re: Eddgie]
      #5471975 - 10/15/12 05:52 PM

Quote:

The Televue 101 was of course an excepetion, offering both a very large field and a very sharp field, but in the end was too small of an aperture to me.

Fantastic though. The only small refractor that I ever liked more than a litte. I didn't use it much, but again, it was the only small refractor that I ever enjoyed using. But only under very dark skies where I could exploit its wide field capability.




Eddgie: We are all different. I have plenty of fun with the 31mm Nagler and my 80mm F/7 apo even from my light polluted backyard. There is field curvature to be seen but there is a lot of other neat stuff too... When I purchased my NP-101, I didn't really quite understand what it was all about but I soon found out, it's a perfect fit for me and it's a great companion for scopes in the 10 inch + range.

Jon


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KJL
member
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Reged: 06/07/12

Loc: Boston, MA
Re: 80mm Triplet with Perfect Edge Stars... possible? new [Re: Pingster]
      #5593938 - 12/29/12 08:47 AM

Quote:

Hey guys thanks for the addition feedback. I have a
2in WO diaginal and so is the coorect image baader. At night both diaginals give approx the same amount of smearex edge stars. But oddly duing the day the baader with no doubt has the flatter field. Very odd and suprising.





So I've always wondered: what was the result of your investigations? Was the WO 2" dielectric diagonal simply a bad copy?


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