Omega Bob ERF

Frankenscope? Let's see it!***be advised that NOTHING in this forum has been safety tested and you are reading and using these posts at your own peril. blah, blah, blah... dont mess around with your eyesight when it comes to solar astronomy. Use appropriate filtration at all times...
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Brett
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Omega Bob ERF

Post by Brett » Tue Jan 13, 2015 9:10 am

Not sure if this is a new item or if anyone is aware of this, but here is a nice looking ERF from Omega Bob. Will have to see results first though...

http://www.ebay.com/itm/Optical-Filter- ... 2625&rt=nc

What do you think?

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swisswalter
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Re: Omega Bob ERF

Post by swisswalter » Tue Jan 13, 2015 7:29 pm

Hi Brett

the figures look good
Only stardust in the wind, some fine and some less fine scopes, filters and adapters as well. Switzerland 47 N, 9 E, in the heart of EUROPE :)

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Merlin66
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Re: Omega Bob ERF

Post by Merlin66 » Tue Jan 13, 2015 8:02 pm

I mentioned this opportunity/ need to Bob a while ago, but recommended a larger diameter...
It's pretty small, only 25mm.
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Valery
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Re: Omega Bob ERF

Post by Valery » Tue Jan 13, 2015 8:35 pm

This is a _cheap_ alternative to a full size ERF.

I have experimented with ERF coatings on internal surfaces of the solar telescope - on lenses or on small
sub-aperture ERF.

The results as this: full size ERF filter prevents heating inside the tube and no internal small scale air turbulence.
A subaperture ERF filters cause fast small scale air turbulence inside the tube. And this small scale turbulence
will be always here even when the seeing outside is good.
This is especially true for CaK line.

As the result, I make a full size 160mm and 180mm D-ERF filters for my refractors and a few of 160mm D-ERF for
free sale.


Valery.
Last edited by Valery on Thu Jan 15, 2015 4:45 am, edited 1 time in total.
"Solar H alpha activity is the most dynamic and compelling thing you can see in a telescope, so spend accordingly." (c) Bob Yoesle.

Largest full size 185 - 356mm Dielectric Energy Rejection Filters (D-ERF) by ARIES Instruments.

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Merlin66
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Re: Omega Bob ERF

Post by Merlin66 » Tue Jan 13, 2015 8:39 pm

Valery,
Re. the large aperture ERF's - what thickness glass and surface quality?
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Brett
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Re: Omega Bob ERF

Post by Brett » Tue Jan 13, 2015 9:52 pm

Have anyone used a dichroic mirror for energy reduction in CaK? This should not produce heat inside the telescope as it is transmitted through and out the back. Will work like a Herschel wedge, just the dichroic mirror in stead of a wedge prism.

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Re: Omega Bob ERF

Post by Merlin66 » Tue Jan 13, 2015 10:00 pm

Brett,
Interesting...
I used two 50/50 beam-splitters to achieve similar results in a 90mm/ SM40 mod set up.
I found the optical quality of the plates to be pretty poor (maybe aggravated by temperature effects?) - in the end it didn't work for me.
"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
https://groups.yahoo.com/neo/groups/ast ... scopy/info
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer

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swisswalter
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Re: Omega Bob ERF

Post by swisswalter » Wed Jan 14, 2015 5:24 am

Hi Brett

yes the baader B filter works perfect as an ERF. In fact that is one of the component of the secret Cak filter from Andy Murner
Only stardust in the wind, some fine and some less fine scopes, filters and adapters as well. Switzerland 47 N, 9 E, in the heart of EUROPE :)

from 7 am - 7 pm http://www.nanosys.ch

from 7.01 pm - 6.59 am http://www.wastronomiko.com some times vice versa ;)

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Re: Omega Bob ERF

Post by marktownley » Wed Jan 14, 2015 7:26 am

Yes, have seen this before.

At the end of the day it's a case of you pays your money and takes your choice. Valerys point about the heat within the tube is a very valid and important one. I'm not sure i'd be using this with say a 150mm, let alone a 200mm scope, ask Walter how he cooked a Baader K-line ;) The temperature towards the focal point is pretty high.

What we have found with ERFs mounted internally with Ha is a rule of thumb is don't use anything less than half the diameter of the scope objective. Given that this has a free aperture of 25mm it's low size really does limit how useful it actually is. 2" Baader Blue is a much better choice.

Regards the beam splitter, and I keep making this point, you have to ask the question are these designed for energy rejection with laser systems or are they designed for imaging / visual use within optical systems? I would suggest the former which is why Ken found the results he did, and also why they are not used in commercial solar systems. Baader filters on the other hand are specifically designed for visual / optical use.
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http://brierleyhillsolar.blogspot.co.uk/
Solar images, a collection of all the most up to date live solar data on the web, imaging & processing tutorials - please take a look!

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Valery
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Re: Omega Bob ERF

Post by Valery » Thu Jan 15, 2015 4:52 am

Merlin66 wrote:Valery,
Re. the large aperture ERF's - what thickness glass and surface quality?
These filters are D=165mm - 0.1mm , thickness T=13mm, surface quality 1/10 wave that mean not more 1/20 wave for wave front distorsion.
The filter's wave front quality is better than 0.02 wave P-V RMS. Means about 0.99 Strehl = no impact on telescope images.

The front side has high transmission coating in H-a and as protective coating. The back side has IR rejecting coating + protective coating.

Filter substrate Schott RG630 glass.


Valery.
"Solar H alpha activity is the most dynamic and compelling thing you can see in a telescope, so spend accordingly." (c) Bob Yoesle.

Largest full size 185 - 356mm Dielectric Energy Rejection Filters (D-ERF) by ARIES Instruments.

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