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Equipment Discussions >> Eyepieces

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Illinois
Carpal Tunnel


Reged: 12/18/06

Loc: near Dixon, Illinois USA
Re: Pushing the magnification new [Re: Sarkikos]
      #5859224 - 05/14/13 07:09 AM

Interesting posts! Field stop make sense and what about extra long dew that would block any unwanted light?

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great_bear
scholastic sledgehammer


Reged: 07/05/09

Loc: Walthamstow, London, UK
Re: Pushing the magnification new [Re: Sarkikos]
      #5859232 - 05/14/13 07:17 AM

True, but I have a Mak

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great_bear
scholastic sledgehammer


Reged: 07/05/09

Loc: Walthamstow, London, UK
Re: Pushing the magnification new [Re: Illinois]
      #5859239 - 05/14/13 07:20 AM

Quote:

what about extra long dew that would block any unwanted light?




That only gets you so far. When observing the terminator of the moon, OTA reflections in the baffle-tube can really kill the contrast at high mags on a Mak.


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Sarkikos
Postmaster
*****

Reged: 12/18/07

Loc: Scotophobia, Maryland, USA
Re: Pushing the magnification new [Re: Illinois]
      #5859291 - 05/14/13 08:06 AM

An extra long dew/light shield is important for Newts/Dobs, Cats and refractors. They prevent dewing on the secondary mirror, the corrector and the objective. They block ambient glare. And they are easy to make.

A shield about the same length as the diameter of the OTA, up to about a foot, is sufficient. I make them quite a bit longer than the diameter for small refractors, such as my 50mm and 70mm finders. If the observer is concerned about vignetting their Newt, they can plug the numbers into NewtWin or a similar program and see what would be the maximum length of a shield for their telescope. Flocking the interior with strips of ProtoStar is a good idea.

Mike


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Sarkikos
Postmaster
*****

Reged: 12/18/07

Loc: Scotophobia, Maryland, USA
Re: Pushing the magnification new [Re: great_bear]
      #5859295 - 05/14/13 08:09 AM

Quote:

Quote:

what about extra long dew that would block any unwanted light?




That only gets you so far. When observing the terminator of the moon, OTA reflections in the baffle-tube can really kill the contrast at high mags on a Mak.




Yes, I've noticed that in my Maks and SCTs. But a shield can help. They might be even more useful in Newts. But at any rate, shields will prevent - or at least slow down - dewing on the corrector for Cats. And IME, shields will prevent entirely any dewing inside a solid tube Newt.

Mike


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great_bear
scholastic sledgehammer


Reged: 07/05/09

Loc: Walthamstow, London, UK
Re: Pushing the magnification new [Re: Sarkikos]
      #5859310 - 05/14/13 08:22 AM

I agree - I made a shield from a cardboard bag from the gift-shop at a star-party once, and three-years on it's still going strong (upgraded with a thick black card lining). It not only reduces glare but does - as you say - prevent dewing very effectively - in my case, until everything else is so wet you'll have to (or at least want to) pack up for the night anyway.

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Scanning4Comets
Markus
*****

Reged: 12/26/04

Loc: Ontario, Canada
Re: Pushing the magnification new [Re: great_bear]
      #5859725 - 05/14/13 12:04 PM

I have a shield as well, fully flocked etc. I have tried using my scope without it and the difference is minimal for glare unless I am right out in my front driveway and the glare from the street lights are blazing. At the darker site I go to, I have forgotten to bring it, and it makes no difference out there when using it or not.

Never had a problem with dew on my secondary either, and my scope can get dripping wet with dew.


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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: Pushing the magnification new [Re: great_bear]
      #5859734 - 05/14/13 12:07 PM

Quote:

Hi Don,

The diagram you refer to is dual-point, mine are all single-point (kinda obvious, I thought?)

No matter... It makes no difference. Whether the diagram is single-point, dual-point or even triple-point for good measure (as shown below), it's still the case that junk light is being transmitted outside of the exit-pupil, and there's nothing that the field-stop can do about it.

You might not like this. You may even want to put your fingers in your ears and scream "no! no! no!", but the fact remains it's clearly illustrated in this diagram for all CN members to see - and anyone with a ruler can validate it.

Clear skies to you.

Fig 4:




Well OK. I had to sit down and think about it. Could light from outside the field stop of the telescope actually make it through to the eye? And, of course, you are right. I was wrong in my thinking about it.

The simplest case I could think about was a dobsonian telescope where light could get into the bottom of the eyepiece from over the lip of the other side of the scope, directly from the sky. That light is 100% outside the field stop of the telescope, yet it can come through. AND, it could be anywhere in the pupil of the eye--inside or outside the exit pupil of the eyepiece. If bright enough it could even reflect from your cornea.

Of course, inside the exit pupil it would be thought of as scattered light that would cause a reduction in contrast as glare. Most likely, scattered light could be anywhere--inside AND outside the exit pupil.

Your explanation makes good sense, and I thank you for taking the time to get it through my thick-headedness on this topic.

It goes a long way toward explaining why baffling and flocking and paying attention to reflective surfaces are so important. And it explains how we can tell if a bright object is immediately outside the field of view.


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great_bear
scholastic sledgehammer


Reged: 07/05/09

Loc: Walthamstow, London, UK
Re: Pushing the magnification new [Re: Starman1]
      #5859795 - 05/14/13 12:27 PM

Quote:

Your explanation makes good sense, and I thank you for taking the time to get it through my thick-headedness on this topic.




- and thank you for allowing me to do so :-)
(I'd never have got around to creating those diagrams otherwise!)


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Wol
super member


Reged: 03/26/07

Loc: Perth Western Australia
Re: Pushing the magnification new [Re: great_bear]
      #5861582 - 05/15/13 07:39 AM

Yes thanks to all for the questions and answers and patience - I have been following this with interest and it has been educational

regards


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Illinois
Carpal Tunnel


Reged: 12/18/06

Loc: near Dixon, Illinois USA
Re: Pushing the magnification new [Re: Wol]
      #5861787 - 05/15/13 10:03 AM

I see! Thanks! I wondered, anywhere that selling dew for my Orion 180mm Mak-Cass? Thanks!

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Hamsterdam
super member
*****

Reged: 03/23/11

Loc: smack dab in the middle of the...
Re: Pushing the magnification new [Re: Illinois]
      #5873110 - 05/20/13 05:32 AM

I wanted to do a short (me?) post on eye age/effectiveness.
While at almost 50, I have a slight amount of myopia at distance, when focusing on a small detail (and yes thru a lens), I think I have it over on many, for a specific reason. I started in the days of film scanning and wet etching, thru the 00's and today with Ps CS 5.5? (I'd have to look).

I had to be able to spot a 3-5% shift in percentage size of halftone dots, as they relate to the composition, registration and color balance of a CMYK image at 600dpi. I did this throughout my career, first in manual analog, and now, with a highly calibrated screen — matching the paper it will print on, plus the actual output color.

I since have had retirement thrust upon me, but my point is, I spent the greatest portion of my life, training my eyes to see not only obscure subtlety in color, but in detail, required in both positions I held. I continue to practice my Art, both on the computer and the canvas.

Our bodies, cell by cell, save for our neurons, are replaced every 7 years. Sadly, our repairman orders worse and worse replacement parts as we go, so I live under no illusion that our eyes do indeed fail in some respects. I also believe that both training and use, at the very least, extend that acuity, if not enhance it.

Best of views to all —davey

Edited by Hamsterdam (05/20/13 05:40 AM)


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