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#1 Mark SW

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Posted 07 April 2013 - 03:30 AM

About a year ago a member form the CSAS(Colorado Springs Astrominical Soctiey) had a filter he just placed in the upper OTA instead of treading into the eyepiece or is my eye sight as bad as my spelling.

#2 core

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Posted 07 April 2013 - 03:43 AM

Provided you're not talking about placing a filter over the objective, Borg (Japanese telescope brand) has a couple of filter holders that will fit somewhere along the OTA light path, example #1, and example #2.

#3 Mark SW

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Posted 07 April 2013 - 03:49 AM

No it was more like a round piece of plastic he put on top of the spiders.

#4 BillP

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Posted 07 April 2013 - 08:13 AM

Might have been a home made apodizing mask.

#5 GlennLeDrew

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Posted 07 April 2013 - 08:32 AM

Or a solar filter?

#6 Scanning4Comets

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Posted 07 April 2013 - 09:08 AM

No it was more like a round piece of plastic he put on top of the spiders.



Might have been a home made apodizing mask.

--------------------
Bill


This hardly sounds like an apodizing mask / screen

#7 planet earth

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Posted 07 April 2013 - 09:34 AM

Maybe it was a plastic off axis mask.
Maybe it had a larger colored filter attached to it.
Maybe............ I don't know!
Maybe Bill is right.
But then Glenn could be right.
More info needed. :)
Sam

#8 Sarkikos

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Posted 07 April 2013 - 03:14 PM

Probably a round ABS plastic off-axis mask. If a color filter, not a very cost-effective location for it. Maybe a solar filter. At any rate, doesn't sound like anything I would do. :noway:

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#9 Zamboni

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Posted 07 April 2013 - 10:32 PM

CSAS member here. I think you're thinking of either Bob Vixie or Ken, who both use apodizing masks regularly, including at various outreach events. That's probably what you saw.

#10 Scanning4Comets

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Posted 08 April 2013 - 04:09 AM

If it was indeed an apodizing mask, the description was way off. "No it was more like a round piece of plastic he put on top of the spiders."

Plastic? :lol:

#11 Sarkikos

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Posted 08 April 2013 - 07:49 AM

Yes, it didn't sound like an apodizing mask, which would look like a circle of window screen with a hole cut in the middle ... which is pretty much what an apodizing mask is. If it was an apodizing mask, the OP certainly did not get a very good look at it.

Mike

#12 Mark SW

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Posted 08 April 2013 - 08:20 AM

In fact it was Ken's telescope
Thanks

#13 t.r.

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Posted 08 April 2013 - 09:17 AM

So, now that it is brought up, what is the consensus on apodizing screens?

#14 Sarkikos

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Posted 08 April 2013 - 09:29 AM

I don't think there is a consensus.

But my own experience is that an apodizing screen does help in enhancing contrast for planet observation. At least mine does. My best - highest contrast and finest detail - views of Jupiter and Mars have been through my 10" f/4.8 Dob with an apodizing screen and a binoviewer.

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#15 Zamboni

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Posted 08 April 2013 - 10:26 AM

In fact it was Ken's telescope
Thanks


Yup. Apodizing mask. He calls it "The 60's filter." He even has an article on it on the CSAS website.

http://csastro.org/2...apodizing-mask/

Which park was the event at, by the way? Chances are I looked through that same scope that same night.

#16 Sarkikos

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Posted 08 April 2013 - 11:49 AM

Yessir. I read that article about three years ago. It was one of the sources of ideas for my own apodizing mask.

Mike

#17 Mark SW

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Posted 10 April 2013 - 03:07 AM

In fact it was Ken's telescope
Thanks


Yup. Apodizing mask. He calls it "The 60's filter." He even has an article on it on the CSAS website.

http://csastro.org/2...apodizing-mask/

Which park was the event at, by the way? Chances are I looked through that same scope that same night.

It was at Fox Run Park

#18 BillP

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Posted 10 April 2013 - 10:40 AM

So, now that it is brought up, what is the consensus on apodizing screens?


I use this.

FYI, I scaled these template masks to the proper length for my XT10's spider vanes (so scale to about 3.75" long each), then simply printed it and cut the design out of paper and secured it to the top of the spider vanes to greatly reduce the diffraction spikes. I get about a 75% reduction in the spikes and I also notice a detail improvement for planets and of course no light loss since such a small obstruction in the light path. I have not tried the conventional apodizing masks.

#19 Sarkikos

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Posted 10 April 2013 - 10:57 AM

Bill,

Very interesting article. Should be easy to set up with stiff paper for a field test.

But isn't this mostly to reduce the diffraction spikes from the spider vanes? The purpose of an apodizing screen is to manipulate the diffraction pattern in order to improve contrast. AFAIK, most of the important diffraction effects produced by the secondary assembly in a Newt are due to the central obstruction, not the spider vanes. SCTs do not have spider vanes, but they most certainly do have a loss in contrast due to the central obstruction. And there does seem to be a tendency for Newts to be better planet performers than comparable SCTs.

An apodizing screen certainly does not reduce the diffraction spikes. Actually, I believe it makes them more prominent. Personally, though, the spikes never bothered me. They can get in the way sometimes for doubles, but I'm not that into doubles to worry about it.

An apodizing screen is analogous to a contrast filter. But the apodizing screen works by directly changing the diffraction pattern rather than by reducing transmission of selected wavelengths.

You've used the template mask for planets and have seen an increase in observed detail? Now that is interesting! :grin: I wonder what results a combination of the template mask and apodizing screen would show for planet observation?

So far, I've used the apodizing mask for bright planets with good results. I've never tried it for doubles, though I've heard some observers use it for them.

I've also found the apodizing mask can improve contrast for lunar areas away from the terminator. But this is anathema. Many folks say the apodizing mask is not meant for lunar, and they will give theoretical reasons why it should not be used for lunar. Pfft!... :shrug: I know what I see and what I like.

:grin:
Mike

#20 Sarkikos

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Posted 10 April 2013 - 11:45 AM

I have to admit, though, that the apparent improvement produced by the template mask in the diffraction pattern and the intensity graph are impressive.

But I'm put off by the comments in the last paragraph of the article. AFAIK, a curved spider does not reduce diffraction effects. Instead, it merely spreads the diffraction effects over the image. Yet again, a main impetus of the attitude here is that somehow the spider vanes are responsible for most of the diffraction problems in a Newt. Not true. The main culprit is the CO. Do the professional scientific writers of these articles not understand this? Or do we lowly amateur astronomers have it all wrong? I don't think so. All you have to do is think about contrast loss in the SCTs. No spider vanes, no spikes, but worse contrast than comparable Newts.

A four-vane spider with thin vanes is actually superior to a curved spider, both in terms of slightly better - or at least, no worse - diffraction effects and a more sturdy structure. Again, this is AFAIK, according to what I've read on the subject. Of course, there are some who think it's worthwhile to replace a four-vane spider with a curved one. I remain very skeptical about that. Yes, the spider spikes will be gone, if that's what you want. But the diffraction from the curved vanes will still be there. Only now it is spread out over the image, rather than localized in the spikes. Exactly how is that better for observing planets? :shrug:

Mike

#21 Zamboni

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Posted 10 April 2013 - 01:54 PM

In fact it was Ken's telescope
Thanks


Yup. Apodizing mask. He calls it "The 60's filter." He even has an article on it on the CSAS website.

http://csastro.org/2...apodizing-mask/

Which park was the event at, by the way? Chances are I looked through that same scope that same night.

It was at Fox Run Park


Ah, the infamous "night of the sprinklers" when Jason won the first awarded CSAS medal of valor for diving on a live sprinklerhead in order to save everybody's equipment.

Good times.

#22 Sarkikos

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Posted 10 April 2013 - 02:09 PM

The amateur astronomer's equivalent of diving on a hand grenade.

:grin:
Mike

#23 Scanning4Comets

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Posted 10 April 2013 - 04:37 PM

Good points made on diffraction, etc Mike! :waytogo:

Cheers,

#24 buddyjesus

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Posted 10 April 2013 - 07:14 PM

Ah, the infamous "night of the sprinklers" when Jason won the first awarded CSAS medal of valor for diving on a live sprinklerhead in order to save everybody's equipment.

Good times.


My hero!

#25 Sarkikos

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Posted 11 April 2013 - 05:48 AM

Good points made on diffraction, etc Mike! :waytogo:

Cheers,


Thank you, Markus. It seems to me there is still a lot of work to be done in figuring out the optimal method of apodization for Newtonians. It's certainly not all about the spider spikes!

Mike






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