Front ERF vs Internal ERF? Minimum size?

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Front ERF vs Internal ERF? Minimum size?

Post by MalVeauX » Thu May 31, 2018 4:04 pm

Hello,

I'm trying to gather information on this subject as I have seen some really lovely truss refractor designs using internal ERFs instead of massive full aperture ERFs which are also extremely expensive. However, I'm curious what the difference is in heat blocking/rejection between a full aperture ERF and a smaller, deeper in the light cone internal smaller aperture ERF would be. For example, say you take a 6 inch or 8 inch refractor objective, F6 to F8 and put an 75~90mm aperture ERF on the inside somewhere that the light cone would fit through clearly, before the focuser. Either well before the focuser or on the nose of the focuser tube, but without cutting into the light cone. Does that work or is it inherently going to not block as much energy compared to simply being full aperture in front of the objective?

Very best,

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Re: Front ERF vs Internal ERF? Minimum size?

Post by bart1805 » Thu May 31, 2018 5:06 pm

Hi Marty,
I thought the erf should be about 2/3 of the front lens. But please wait for others to react before you act on this advice. (-;
Or try to find a second hand Erf and put it before the lens.
Bart.

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Re: Front ERF vs Internal ERF? Minimum size?

Post by Valery » Thu May 31, 2018 6:10 pm

Marty, ask any planetary and lunar imagers - do they willing to put even a very weak source of heat inside their telescopes.

Any erf can't reflect all the unwanted energy and will absorb some heat and so, produce some air turbulence.
This were especially well seen with the Quark Chromosphere worked in a 150mm telescope without front ERF
or with just UV/IR small filter. Some areas on the images were like covered by veil and look less sharp after the
post processing.
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Re: Front ERF vs Internal ERF? Minimum size?

Post by christian viladrich » Thu May 31, 2018 6:39 pm

Hi,

The classical ratio for Ha ERF is 2/3. It works pretty well with a 250 mm F/10 refractor.

For Ca K, I use a 1/3 ratio (150 mm F/7 refractor and 50 mm internal ERF). It started as a test. It looks fine until now, but I would not recommend for the moment. Definite conclusions would wait the end of the summer season.

My guess is that all ERF are not equivalent. For internal position, you have to forget about substrates absorbing energy (RG630, etc).
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Re: Front ERF vs Internal ERF? Minimum size?

Post by Merlin66 » Thu May 31, 2018 11:14 pm

Bases on numerous PST mods over the years,I always recommend the internal ERF be >50% the clear aperture andplaced as close to the objective as possible.
I also highly recommend the Baader D-ERF over the alternatives.
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Re: Front ERF vs Internal ERF? Minimum size?

Post by TheSkyBurner » Fri Jun 01, 2018 2:02 am

For h-alpha the baader planetarium d-erf is definatley by far #1 and it will always beat anything else. It seems to increase contrast noticeably on all the scopes that use them. Nothing else compares.

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Re: Front ERF vs Internal ERF? Minimum size?

Post by Valery » Sat Jun 02, 2018 11:26 am

TheSkyBurner wrote:
Fri Jun 01, 2018 2:02 am
For h-alpha the baader planetarium d-erf is definatley by far #1 and it will always beat anything else. It seems to increase contrast noticeably on all the scopes that use them. Nothing else compares.
TheSkyBurner and Merlin66,

Any factual prove? Pictures, contrast measurements on the same equipment and the same camera and the same hour?
No? Then all you said is just your speculations. As for the factual images, they loose.
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Re: Front ERF vs Internal ERF? Minimum size?

Post by Merlin66 » Sat Jun 02, 2018 1:14 pm

Valery,
I accept there are other good ERF's available, but unless you have access to quantified data on Baader D-ERF and others and prepared to share, I can only agree - "speculation".
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Re: Front ERF vs Internal ERF? Minimum size?

Post by banjo » Sat Jun 02, 2018 3:20 pm

Hello,
I just did a test with a 250mm f2800 lens.
The D-erf 75mm is about 500mm from the camera, the D-erf has a big advantage is to reject with a mirror effect all that is not Ha.
The test only a few minutes without the filter heats too much.
When the weather decides to put on a nice weather, I will leave the filter longer.
Paul

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Re: Front ERF vs Internal ERF? Minimum size?

Post by MalVeauX » Sat Jun 02, 2018 10:16 pm

Banjo...

What is that thing? Looks like a wooden box? Home brew refractor?

Very best,

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Re: Front ERF vs Internal ERF? Minimum size?

Post by christian viladrich » Sun Jun 03, 2018 7:20 pm

Maybe a word of clarification is needed there ...

An ERF has no impact at all on contrast, except if it is of bad optical quality or bad uniformity.
The contrast of Ha images depends on the Ha filter (= blocking photospheric contribution), the optical quality of the telescope and optical set-up(including level of diffusion) and the quality of seeing.
I have both the Baader D-ERF (160 mm) and the AiryLab ERF ( 210 mm). Both are of excellent quality. This is proven by a lot of Ha images I took with the TOA 150 and the C8 EDgeHD, and also mesurements on optical bench.
My 160 D-ERF is L/9.6 P-V :
Image
My 210 mm AiryLab is L/10 P-V.

This beeing said, I won't pretend there is no other good / excellent ERFs, since ... I have tested no others. ;-)
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Re: Front ERF vs Internal ERF? Minimum size?

Post by TheSkyBurner » Sun Jun 03, 2018 11:27 pm

Well from what I have seen, anyone that owns the baader planetarium D-erf filter, or Airylab Erf filter, or the Aries erf filter is producing images that look WAAAY better than anything coming out of a ls152 without the erf filter.....

The best comparison would be with a daystar erf filter.


With that being said, I have mounted my 2" baader planetarium filter to use as ERF ONE and a secondary bandpass filter to use as ERF TWO. I have the Ir/UV cut filter before ERF two. So this is technically a three part erf.
skybender filter cell 2a.jpg
skybender filter cell 2a.jpg (173.86 KiB) Viewed 1410 times
skybender filter cell 2b.jpg
skybender filter cell 2b.jpg (143.95 KiB) Viewed 1410 times
skybender filter cell 4.jpg
skybender filter cell 4.jpg (205.27 KiB) Viewed 1410 times
skybender 393.4bp1 1.25 filter cell B.jpg
skybender 393.4bp1 1.25 filter cell B.jpg (173.18 KiB) Viewed 1410 times
skybender 393.4bp1 1.25 filter cell C.jpg
skybender 393.4bp1 1.25 filter cell C.jpg (191.55 KiB) Viewed 1410 times

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Re: Front ERF vs Internal ERF? Minimum size?

Post by TheSkyBurner » Mon Jun 04, 2018 11:54 pm

Bad news about the Baader planetarium B-ccd filter. It creates a reflection that burns. Not sure what this is, but my scope is made of solid magnesium, not one speck of plastic anywhere inside of it.

Perhaps this is the powder coating on the inside? This filter is 100mm away from the focal point, and the gold reflection is creating an internal focus somewhere in the middle of the scope, vaporizing the air.

Going to try moving it to the 200mm zone.
baader planetarium B-ccd filter interna tube reflection burning.GIF
baader planetarium B-ccd filter interna tube reflection burning.GIF (5.45 MiB) Viewed 1381 times

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Re: Front ERF vs Internal ERF? Minimum size?

Post by TheSkyBurner » Mon Jun 04, 2018 11:57 pm

here is a better shot of the focus point being created, I think it is just literally condensating the air inside the scope? The beam isnt touching anything.
focal reflection.jpg
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Re: Front ERF vs Internal ERF? Minimum size?

Post by Merlin66 » Tue Jun 05, 2018 12:06 am

If the filter is 100mm from the focus, it will be re-focusing the solar image 100mm in front - towards the objective.
Interesting.....
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Re: Front ERF vs Internal ERF? Minimum size?

Post by TheSkyBurner » Tue Jun 05, 2018 12:28 am

Merlin66 wrote:
Tue Jun 05, 2018 12:06 am
If the filter is 100mm from the focus, it will be re-focusing the solar image 100mm in front - towards the objective.
Interesting.....
Interesting indeed considering none of the other filters do this :) . I think the gold coating is reflecting a spectrum that excites water molecules?

I am going to move it another 100mm, and recheck. I wonder if this is happening inside of christians 150mm scope with his blue filter too and he just has not realized it yet.

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Re: Front ERF vs Internal ERF? Minimum size?

Post by Merlin66 » Tue Jun 05, 2018 2:56 am

Well, if nothing else, it shows that the Baader CCD filter reflects the majority of the incoming light and energy.
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Re: Front ERF vs Internal ERF? Minimum size?

Post by marktownley » Tue Jun 05, 2018 5:29 am

Interesting chaps.
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Re: Front ERF vs Internal ERF? Minimum size?

Post by TheSkyBurner » Tue Jun 05, 2018 7:24 am

I have been doing some thinking about the problem with reflection energy, and I believe that placing a B&W HTC KSM filter in front of the B-ccd filter will further prevent any damaging rays from returning to the ota.

The first initial pass of light will be polarized and the intensity will be reduced by 1.5 f-stops; then the reverse transmitted reflection will make a second pass further reducing it's energy to 3 f-stops, or potentially dimmed to extinction .

This particular polarizer will have a minimal affect on the output transmission, and each filter can then be clocked properly.

When dealing with a filter that is 95% transmission, and the reflection is 99%. It seems reasonable a small sacrifice can be made with a potential benefit.

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Re: Front ERF vs Internal ERF? Minimum size?

Post by banjo » Tue Jun 05, 2018 8:51 am

MalVeauX wrote:
Sat Jun 02, 2018 10:16 pm
Banjo...

What is that thing? Looks like a wooden box? Home brew refractor?

Very best,
hello,
my telescope in white light version, there is obviously no filter in front of the lens for the Ha. :lol:

Paul
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Re: Front ERF vs Internal ERF? Minimum size?

Post by marktownley » Tue Jun 05, 2018 3:20 pm

Nice scope!
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Re: Front ERF vs Internal ERF? Minimum size?

Post by TheSkyBurner » Tue Jun 05, 2018 4:09 pm

Yes Paul that is a very nice home brew telescope indeed!

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Re: Front ERF vs Internal ERF? Minimum size?

Post by christian viladrich » Tue Jun 05, 2018 7:30 pm

BTW, air cant' be "vaporized". The humidity in the air is already in the form of vapor. O2 and N2 molecules are already in the form of gaz ;-)

Humidity can condensate, if the temperature decreases below the "dew point", but obviously this can't be the case here.
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Re: Front ERF vs Internal ERF? Minimum size?

Post by christian viladrich » Tue Jun 05, 2018 7:35 pm

Just some ideas about the "Baader blue filter that burns the air" :

- First of all, these filters are interference filters. They don't absorb light, they reflects light (absoption is typically lower than 0.1%).
- Accordingly, a blue filter reflects yellow light (= green + red), a green filter reflects magenta (= blue + red) , a red filter reflects cyan light (= blue + green).
- So, it is not suprising that the reflected light is yellow :-)
- I can see that the reflected beam meets partly with the tube on the refractor. This is indeed very bad. It means that the reflected beam heats the tube (remember the tube is black), which in turns heats the air, which finally builds up convection. This might be an explanation of what you are experiencing. I am not sure at 100%, because I am not there at your side to check what is going on ;-)
- So, the first thing to do is to tilt the blue filter so that the reflected beam remains inside the aperture of the objective of your refractor. It should not be center on the objective (otherwise you might have reflection at the focus), but off-center.

Hope this helps.
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Re: Front ERF vs Internal ERF? Minimum size?

Post by TheSkyBurner » Tue Jun 05, 2018 9:17 pm

Christian thanks for the tips, i can adjust the tilt with the skybender device from 0-18 degrees. I do believe you are right in that there is some kind of glare affecting the image, so adjustments will be made to get rid of this.

Today I tested a full aperture linear polarizer filter)105mm), and It has completely stopped the smoke/vapor from the b-ccd reflection.
105mm polarizer.jpg
105mm polarizer.jpg (132.56 KiB) Viewed 1298 times
full aperture polarizer.jpg
full aperture polarizer.jpg (178.27 KiB) Viewed 1303 times
However whatever was burning did deposit a nasty film over the b-ccd filter so now i have to clean it because it is noticeable on the solar image. .
film.jpg
film.jpg (193.64 KiB) Viewed 1303 times
I did not yet move the b-ccd filter another 100mm, i left it where it was to test the polarizer's affect on the reflection. So moving it will have to wait another day. This is a full aperture polarizer.xc

There is intermittent clouding today so my testing ability is diminished. However I am very pleased with the performance of the full aperture polarizer, So i will get one to place in front of the b-ccd filter for the double reduction of the reflection. I cannot wait to get some images out of this scope!
IMG_4766.GIF
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