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Solar through fluorite?

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#1 mikeDnight

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Posted 06 February 2016 - 07:56 AM

I've recently been thinking about getting a Herschel wedge for white light solar observing. My scope is a Tak FC100D which is a fluorite Steinheil objective. Although the instruction booklet leads me to believe that the scope will be fine without the use of a full aperture filter, as it advises the use of a projection screen, I thought I'd ask for advice here first.

My question is "Will using a true fluorite scope without a full aperture filter pause a threat to the objective?"

And, does anyone here use a 4" fluorite for solar observation in this way?

 

Thanks in advance.

Mike


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#2 Rich (RLTYS)

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Posted 06 February 2016 - 08:02 AM

Your scope may work just fine for solar projection but I'm not sure I'd want to risk damaging it. I'd stick to a full aperture filter.

 

Rich (RLTYS)


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#3 waso29

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Posted 07 February 2016 - 10:55 PM

enjoy the sun with wedge

Attached Thumbnails

  • IMG_1207 fc100dl lunt wedge @adler.JPG
  • DSC_0088 fc100dc pst.JPG

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#4 mikeDnight

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Posted 10 February 2016 - 12:58 PM

enjoy the sun with wedge

You've convinced me!

Tak themselves offer no warning to the contrary, and they encourage projection, so I will buy a wedge. 

The only warning against unfiltered solar observing I've found was with regard to the FSQ, which is understandable.

 

Thanks for your encouragement.  :cool:

 

Mike



#5 Hesiod

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Posted 10 February 2016 - 01:58 PM

My FC does regularly sunbaths with the Herschel wedge. These make the FC happy, since UVs kill molds!


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#6 Mike I. Jones

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Posted 11 February 2016 - 06:59 PM

The biggest issue with all fluorocrowns is fracturing due to thermal expansion, and thermal shock in particular. The second issue is how to get over your abysmal sadness after you hear that quiet, irreversible "tik" coming from your lens. I recommended full aperture filtering to Bob Yoesle a while back. I mean, why risk it?
Mike
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#7 BYoesle

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Posted 11 February 2016 - 08:30 PM

I'd pay serious attention to Mike's advice - as professional in the optical/aerospace field, he knows what he's talking about.

 

 

I've been the principle optical designer for Lockheed Martin Aeronautics Company for nearly 36 years now.


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#8 Great Attractor

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Posted 12 February 2016 - 03:09 AM

Right, better be careful. This link shows two examples of cracked lens in triplets used for solar work.


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#9 slack

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Posted 12 February 2016 - 04:32 AM

Er, the OP inquired about a Tak FL doublet, not a triplet. And the OP states that the scope manual advises that solar projection is okay. Thus, according to Tak, this scope is fine with full aperture, unfiltered solar. Logically, that would make using a Herschel wedge okay, too.

 

If a Tak 4" FL doublet can't handle that, well... That's ridiculous. I'd think that the FL elements would likely experience more thermal "shock" in typical cold weather use than solar viewing.


Edited by slack, 12 February 2016 - 04:32 AM.

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#10 mikeDnight

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Posted 12 February 2016 - 12:16 PM

Right, better be careful. This link shows two examples of cracked lens in triplets used for solar work.

I wonder if that's an oil spaced triplet? EEK!



#11 bobhen

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Posted 12 February 2016 - 02:31 PM

If the scope is assembled correctly and the wedge used properly, then nothing will happen. The sunlight passes "through" the lens just like your windows at home. The lens will NOT get hot.

Wedges have been used for decades without issues. Lunt and others make Ha solar scopes without filtering the objective lens.

 

I have a Lunt 100 and there is no filter used before the refractor objective. I have used a wedge for many years on doublets and triplets both air and oil spaced without the slightest issue. 

If there were a problem using refractors for solar viewing when using these wedges that would have been well known a hundred years ago.

 

Here are 2 answers by Rolland Christen…

 

Would using an 1.25 inch Intes Herschel wedge in either my
oil spaced 152mm f/8 Astrophysics or my 94mm f/7 Brandon oil spaced triplet damage the oil separating the lens elements

 

No Problem
Roland Christen

 

Why would the sun heat the lens? The light from the sun is concentrated at the “focus point”,  NOT at the lens surface. You can view all day and the lens will be fine.

 

Roland Christen

Bob


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#12 mikeDnight

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Posted 12 February 2016 - 05:35 PM

If the scope is assembled correctly and the wedge used properly, then nothing will happen. The sunlight passes "through" the lens just like your windows at home. The lens will NOT get hot.

Wedges have been used for decades without issues. Lunt and others make Ha solar scopes without filtering the objective lens.

 

I have a Lunt 100 and there is no filter used before the refractor objective. I have used a wedge for many years on doublets and triplets both air and oil spaced without the slightest issue. 

If there were a problem using refractors for solar viewing when using these wedges that would have been well known a hundred years ago.

 

Here are 2 answers by Rolland Christen…

 

Would using an 1.25 inch Intes Herschel wedge in either my
oil spaced 152mm f/8 Astrophysics or my 94mm f/7 Brandon oil spaced triplet damage the oil separating the lens elements

 

No Problem
Roland Christen

 

Why would the sun heat the lens? The light from the sun is concentrated at the “focus point”,  NOT at the lens surface. You can view all day and the lens will be fine.

 

Roland Christen

Bob

Thanks for your reassuring post Bob. I felt sure Tak would have warned against solar without a full aperture filter, as they do with their FSQ, but with the new FC they are happy for the scope to be used for projection, so I assumed a wedge would be OK too. I just wanted to get the opinion of those with experience in this field and with similar scopes.

 

Many thanks

 

Mike  :)



#13 Great Attractor

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Posted 12 February 2016 - 06:42 PM

If the scope is assembled correctly and the wedge used properly, then nothing will happen. The sunlight passes "through" the lens just like your windows at home. The lens will NOT get hot.

 

Bob, you're right, of course - in general this shouldn't and won't happen.

 

The link I posted just shows unusual failures due to a possibly bad design/assembly. And it appears they were triggered simply by the prolonged, unfocused direct sunlight.



#14 Mike I. Jones

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Posted 13 February 2016 - 01:10 PM

Guys, I only recommended not exposing an objective lens with fluorite or semi-flourite glass to direct sunlight out of pure caution.  It's so easy to put a low-cost Mylar filter over the front and reduce the heat load to the lens by 99.99% or better.  Lots easier than looking with abject despair at a crack running through your forever-ruined $3-5K lens.  A cool to cold fluorite lens taken out into hot sunlight is just asking for trouble.  Maybe it will survive, maybe it won't.  Your call, your risk.  Christen can do it if he wants, I ain't doing it, ever, and I recommended the same to Bob Yoesle.  

 

Not my circus, not my monkeys.  Good luck.

Mike


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#15 slack

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Posted 13 February 2016 - 03:08 PM

It's so easy to put a low-cost Mylar filter over the front and reduce the heat load to the lens by 99.99% or better. 

 

While I respect your preference, this is not a remotely accurate statement. While Baader ASF (much better than Mylar, btw) will reduce the amount of light passing through the objective lens by 99.99%, any heating of the front objective lens by direct sunlight passing through it is negligible. The difference might be a few percent, if that. If the lens is an air spaced doublet, I would expect that the glass would be close to ambient temperature once acclimated, whether behind Baader solar film or exposed to direct sunlight in front of a Baader Herschel wedge.


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#16 George9

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Posted 13 February 2016 - 06:42 PM

I use a D-ERF on my triplet oil-spaced lens, so I am cautious, but tough to figure this one out. Seems like many have used projection or wedges over the years without a problem. Would you be willing to leave your lens cell flat on a table in the sunlight? Seems like it should be ok and yet that is probably hotter. At least one of the linked photos was a cemented lens. I wonder if the problem was not the lens in the sunlight but the black lens cell in the sunlight transferring heat to the lens. The lens itself shouldn't absorb much light.

George

#17 BYoesle

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Posted 15 February 2016 - 01:24 PM

Hi George,

 

I think the IR blocking of the DERF would be a good precaution, even for white light observation.

 

The issue is simply one of coefficient of thermal expansion, and the fluoro-glasses, including FPL 51, 52, and 53 are all significantly higher than "standard" glasses.  For the cemented element lens, this likely caused the failure.  Any mechanical impingement could possibly also result in failure.

 

Then there's the issue of changing optical figure when used for solar, which I have real-world experience with in the SolarScope filters.  These use fused silica etlaon plates, which have a far less coefficient of thermal expansion -- and they are subject to significant fluctuation with temperature gradients caused by solar, and destroy the above arguments about lack of sunlight falling on the glass affecting them... especially as they are preceded by an RG630 ERF with IR blocking (the Coronado patent for the central spacer for maintaining a uniform etalon gap was definately needed in the larger filters.)

 

Bottom line, I will not use my irreplaceable AP130 EDT objective for unprotected solar observation -- its not worth it from a structural failure point of view, and would not provide as stable optical performance as an "ordinary" objective in any event.  It is confined to H alpha use with a DERF, or white light use using Baader film, and not a Herschel wedge.



#18 slack

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Posted 15 February 2016 - 02:18 PM

Bottom line, I will not use my irreplaceable AP130 EDT objective for unprotected solar observation -- its not worth it from a structural failure point of view, and would not provide as stable optical performance as an "ordinary" objective in any event.  It is confined to H alpha use with a DERF, or white light use using Baader film, and not a Herschel wedge.

 

I wouldn't either. Because it's a triplet.



#19 R Botero

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Posted 16 February 2016 - 03:27 AM

I'm sorry to disagree here on the generalisation for triplets (I cannot comment on flourites as I don't own one) but even Roland says there should be no issues whatsoever using his triplets for solar observing. See here for example:

https://groups.yahoo.../messages/40293

He states the front glass should not suffer as light does not focus on it. It's just like a car's windshield on a hot summer day. Problems arise if there are any plastic parts inside the OTA; then you'll see fumes!
My 6" AP does white light happily during the long days of summer in SE England.

Roberto
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#20 George9

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Posted 19 February 2016 - 01:35 AM

In the other ERF thread, a DayStar quote alludes to the UV light of the sun being bad for the oil, which would not need heat.

George

#21 R Botero

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Posted 19 February 2016 - 03:16 AM

George
To each their own. I completely understand people not wanting to risk their expensive scope as the sun is a dangerous object when handled incorrectly but if you think about it there should be no issues with the oil in between the lenses. Unless there is a mechanical problem or a leak, the oil expansion should be minimal. In fact oil's expansion coefficient is low, that's why it is used in engines (in addition to being a lubricant).
My 6" triplet is oil spaced and now 24 years old (1992 vintage); no issues here.
Roberto

#22 George9

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Posted 19 February 2016 - 11:22 PM

I agree with you on the heating part. I just hadn't thought of the UV light directly breaking down oil. I don't know if it is an issue or not. Certainly it does break down plastics and some oils. I have not heard of sun-induced oil problems, and in fact I don't remember many oil failures of any kind (I guess leaks).

 

I remember George of AP cautioning about internal ERFs, but I don't remember the reason. Obviously Roland is not worried.

George



#23 AllanDystrup

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Posted 21 February 2016 - 03:39 AM

I have been doing Herschel Wedge solar obs. with my 3" and 4" Vixen fluorite Steinheil doublets for a couple of years now.

I never had any issue with the objectives. On the contrary: splendid images, better than with the Baader AstroSolar!

 

 

Here's a picture of one my solar observation setups with the 3" FL-80S. The sleeve around the objective cell is for a more gentle temperature equalization (and also dew prevention) in he winter season, where I often have a diff. of 20-30 dg.C when carrying the rig out / in. The sleeve may also counteract a steep rise in temperature from direct solar radiation on the lens cell when bringing out the OTA on a hot summer day.)

 

2015-07-18%2014.35.37.jpg


Edited by AllanDystrup, 21 February 2016 - 04:10 AM.

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#24 Sky Muse

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Posted 22 February 2016 - 02:29 PM

With this...

 

fluorite doublet2b.jpg

 

...I use this...

 

TO RG solar filter3.jpg

 

...and for H-Alpha, I'd use this... http://www.thousando...com/halpha.html ...or something very, very similar.

 

But most of the time, her lovely visage is exposed only to the gentle, nocturne photons of the deep, dark night.

 

:grin:



#25 slack

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Posted 22 February 2016 - 03:10 PM

With this...

 

attachicon.giffluorite doublet2b.jpg

 

...I use this...

 

attachicon.gifTO RG solar filter3.jpg

 

...and for H-Alpha, I'd use this... http://www.thousando...com/halpha.html ...or something very, very similar.

 

But most of the time, her lovely visage is exposed only to the gentle, nocturne photons of the deep, dark night.

 

:grin:

 

Well, your nice scope becomes somewhat irrelevant behind that white light filter. Baader AstroSolar Safety Film far outperforms what you're using (and a Baader CCS Herschel wedge is better still). And as for Ha, I believe that you'd also find TO to have few users compared to Coronado, Lunt, and others (though I like TO and use some of their non-solar filters).




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