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Author Topic: The definative ERF risk assesment  (Read 7379 times)

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Offline Merlin66

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Re: The definative ERF risk assesment
« Reply #15 on: 05:27:45, 04 August, 2011 »
Until Colin's message, I didn't even realise the write-up had been deleted!!
Another problem!
C11, C9.25 SCT, 4" Genesis, NEQ6pro, modded 1000D,  ATik314L+, DMK41AF04, SM60DS/BF15, and Spectroscopes (many!).
http://tech.groups.yahoo.com/group/astronomical_spectroscopy/
"Astronomical Spectroscopy for Amateurs", "Grating Spectroscopes-How to Use them" -Springer

Offline Nomis Elfactem

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Re: The definative ERF risk assesment
« Reply #16 on: 09:45:59, 04 August, 2011 »
Sorry Ken - it was all part of that clean up that had to be done last christmas to stop the database crashing. 

You can either re-attach it here by editing your original posts or if you email them through to me I'll upload them onto my website and attach a link and make it a sticky... which ever suits best !

(PS we are actually as we speak actively in the process of creating more space or procurring more space so we don't have to cull quite so harshly/at all in the future.)

S.
Simon

Scopes: Astro-Tech AT-111EDT Triplet, TS65ED Quad, Orion ST80, Modded PST-90 Solar Scope, PST Cak (on loan)
Cameras: SXVF H694, Atik 16ic, Canon EOS 600d, DMK41, DMK21, QHY 5L-II (mono & colour)
Accessories: SX USB Filter Wheel, SX OAG, Baader LRGB Ha SI OII Filters, SharpSky Focuser
Mount: EQ6 (EQMOD), SW Star Adventurer, plus a lot (and I mean a lot) of other bits and pieces

Offline colinsk

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Re: The definative ERF risk assesment
« Reply #17 on: 04:23:25, 03 January, 2012 »
Any news on getting Ken's articles attached again?

Offline Nomis Elfactem

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Re: The definative ERF risk assesment
« Reply #18 on: 09:33:23, 03 January, 2012 »
Colin - I'll drop Ken a PM and ask him to either reattach or to email me the documents and I'll attach them  :thumbup:

S.
Simon

Scopes: Astro-Tech AT-111EDT Triplet, TS65ED Quad, Orion ST80, Modded PST-90 Solar Scope, PST Cak (on loan)
Cameras: SXVF H694, Atik 16ic, Canon EOS 600d, DMK41, DMK21, QHY 5L-II (mono & colour)
Accessories: SX USB Filter Wheel, SX OAG, Baader LRGB Ha SI OII Filters, SharpSky Focuser
Mount: EQ6 (EQMOD), SW Star Adventurer, plus a lot (and I mean a lot) of other bits and pieces

Offline colinsk

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Re: The definative ERF risk assesment
« Reply #19 on: 04:19:30, 04 January, 2012 »
Thank you, I have a copy but I keep needing to reference ti for other solar astronomers and it is not online. I would link the version I have but I want to make sure Ken uses the latest as I may have a draft.

Offline Nomis Elfactem

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Re: The definative ERF risk assesment
« Reply #20 on: 09:16:42, 04 January, 2012 »
I sent a PM yesterday to Ken but not heard anything back yet Colin. 

It might be worth linking it anyway and just state it may be a draft/copy of the original ?!  I'm sure Ken will respond soon if there's a problem with it !?

S.
Simon

Scopes: Astro-Tech AT-111EDT Triplet, TS65ED Quad, Orion ST80, Modded PST-90 Solar Scope, PST Cak (on loan)
Cameras: SXVF H694, Atik 16ic, Canon EOS 600d, DMK41, DMK21, QHY 5L-II (mono & colour)
Accessories: SX USB Filter Wheel, SX OAG, Baader LRGB Ha SI OII Filters, SharpSky Focuser
Mount: EQ6 (EQMOD), SW Star Adventurer, plus a lot (and I mean a lot) of other bits and pieces

Offline Merlin66

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Re: The definative ERF risk assesment
« Reply #21 on: 13:41:53, 04 January, 2012 »
Just returned from a Xmas trip to Oz...
Simon, I'll contact you regards getting it re-posted.
Ken
C11, C9.25 SCT, 4" Genesis, NEQ6pro, modded 1000D,  ATik314L+, DMK41AF04, SM60DS/BF15, and Spectroscopes (many!).
http://tech.groups.yahoo.com/group/astronomical_spectroscopy/
"Astronomical Spectroscopy for Amateurs", "Grating Spectroscopes-How to Use them" -Springer

Offline markthais

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Re: The definative ERF risk assesment
« Reply #22 on: 04:03:17, 01 February, 2012 »
WHAT is the ERF really for?
There seems to be a misunderstanding that it is for the safety of your eyes.
The blocking filters are for that. This is where you get your safety. Standard bandpass filters are only blocked to 1000nm. This is fine for PM tubes and film. Most CCD camera need to be blocked to at least 1200nm. Now for your eye. YOU ARE LOOKING AT THE SUN. There is still a lot of energy getting through at 1200nm. To be safe with limited and not continues  use. Your eye can handle the bandpass starting to leak at 1500nm and became transparent after that. Most of the safety rules you read about are for lasers, UV lamps, Arc lamps, industry and laboratory uses. While looking at the sun, it has been left up to the company's. In the early days the filters where not block as well as they are today. It is safe er to go with the farther you block the side bands the better. The spec will read from X-ray to Far IR density 4     
The ITF (Induced transmission filter) is what is used to block the IR. Red glass like RG630 will block everything worth blocking on the short side of the spectrum. If you are using a ITF, you might as well add the extra layers and have it blocked to the far IR. This is 2500nm and farther red. This energy may not hurt your eye but if you can block it you might as well.
The other way to block the IR is to use a edge filters. You can stack them so that one  over lapping the other on the same substrate  and move out into the red. The problem is that you can only do that so far before the thickness is to thick and the coating will fail. So you can get out to about 1600nm if you are lucky. This filter is far more complicated and cost much more then a ITF. 
The only problem is that you will need to replace the ITF some day.       
The original idea for the ERF was to keep as much heat off the etalon as you could.  The etalon's bandpass moves with temperature. This way the filter would stay on band longer .
In the days of 4" F/15's we would just stop them down to 2" and not even use any ERF. But even at 2" with an filter that ran cool it would drift red after a while. Then you would take it off the sun for a while to cool back to the temperature where the etalon was on band.
But if you wanted more aperture then you had more heat to deal with. This is when we started using filter glass to cut the energy down.
The other reason you want to use a ERF is the more heat you keep out of the scope the better the seeing will be. It is the same problem night scope have in reverse. The seeing starts out fine and as the scope heats the seeing goes down.
For high magnification this can be a problem. You will fine that most solar scopes are used at very low powers.
So the ERF is more for performance then eye protection.
Mark     

Offline Nomis Elfactem

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Re: The definative ERF risk assesment
« Reply #23 on: 13:37:23, 01 February, 2012 »
Hey Mark - firstly welcome to UKAI !

Very interesting reading....

I think the key point to make here is that those of us that are using ERF's are doing so because we are not using commercially made solar scope but making our own by using large(ish) scopes (mine is 90mm) to obtain higher resolution images than are possible otherwise.

Therefore, to answer your question regarding what is the purpose of an ERF; well, as you rightly say, from my point of view I'm using one as otherwise the thermal stress that would be placed on the PST etalon when using a 90mm refractor would not be tolerated and would almost certainly cause the barlow lens at the front of the PST etalon that I'm using to crack/fail or even worse cause the etalon glass sandwich itself to break. I'm not using an ERF specifically to "save" either my eyes or my CCD or in fact to reduce convection currents in the scope tube (although clearly, agian as you say, all of these are fringe benefits to using one) but to save the delicate etalon optics from thermal stress and failure when subjected to a larger amount of energy than it was originally designed for !!

S.
Simon

Scopes: Astro-Tech AT-111EDT Triplet, TS65ED Quad, Orion ST80, Modded PST-90 Solar Scope, PST Cak (on loan)
Cameras: SXVF H694, Atik 16ic, Canon EOS 600d, DMK41, DMK21, QHY 5L-II (mono & colour)
Accessories: SX USB Filter Wheel, SX OAG, Baader LRGB Ha SI OII Filters, SharpSky Focuser
Mount: EQ6 (EQMOD), SW Star Adventurer, plus a lot (and I mean a lot) of other bits and pieces

Offline markthais

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Re: The definative ERF risk assesment
« Reply #24 on: 17:25:45, 01 February, 2012 »
Hi Nomis,
That's right, you need to keep the heat low for that air spaced etalon. The OLD DAYS amateur solar filters that I referred to was  the Carson Instruments(Spectro Lab was the really the first but Don Carson who work there took over that division and started his own company)and they used a mica etalon.   The mica etalon can take a lot more heat with out any damage. The idea for the F/30 was that it doesn't broaden out to bad but also the heat across the etalon is more even. I have seen where they have used an 4" F/15 with out any ERF and the blockers and polaroids  where ruin but the etalon was OK.
When picking an ERF you need to consider first is it going to hold its wave form. For an 90mm ERF the thickness should be about 8mm thick. The ones that are 2 and 3mm thick, won't hold the 1/4wave when used as a solar ERF.
And how much heat is it going to keep out. 
As you probably know, The Baader D-ERF's will block the most heat, and then you drop from there. The cost does the same thing.
If you want to drop even more heat in your system I think Baader has a 70mm Ha bandpass which is hard coated you could put in front of the barlow  lens.
Suggestion: If you are making your own solar scope and just using the etalon from a PST. I would design a telecentric instead of a barlow. The etalon will preform better.  And the longer the focal length the narrower the etalon will be. If you are using an F/8 system you can do it with 3 singlets, or use an TZ4 from Baader.
Mark
 
 

Offline Merlin66

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Re: The definative ERF risk assesment
« Reply #25 on: 09:18:00, 02 February, 2012 »
Mark,
Welcome on board.
I'm glad someone has taken an interest in the subject.
The whole object of an ERF is to reduce the amount of energy entering the optical system. It will, as you say, provide protection for both the optics, etalon BF's and the observer.
Many different filter combo have been recommended and tried, but again, as you mentioned, many of the larger "photographic" filters are too thin and unstable.
That's why I like the thicker, optical surface quality of the Baader D-ERF - it does the job it was designed for.
Re fratios- trials in Germany with a PST etalon with BOTH the front barlow and rear imaging lens removed, indicate that above f25 (target f30) the performance was very good. The small physical size of the PST etalon becomes an issue. I'm interested in the etalon from the Lunt 60 for a future mod...
There's a review of solar observing "accidents" being conducted at the moment to determine what the reality is of eye damage while using telescopes (not the solar eclipse naked eye reported blindness - BTW they couldn't find any of those either!).
So far, including reports from Opthalmic doctors, there are no cases of eyeballs being cooked, popping or significant long term damage. Having said that, it's always better to be safe than sorry. ( A lot of newbies read these forums and their enthusiasm exceeds their knowledge - a dangerous mix)
C11, C9.25 SCT, 4" Genesis, NEQ6pro, modded 1000D,  ATik314L+, DMK41AF04, SM60DS/BF15, and Spectroscopes (many!).
http://tech.groups.yahoo.com/group/astronomical_spectroscopy/
"Astronomical Spectroscopy for Amateurs", "Grating Spectroscopes-How to Use them" -Springer

 

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