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Thread size for optolong venus u filter 2"

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#1 Daryll

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Posted 07 March 2018 - 17:29

Hi all.
Just purchased a optolong venus u filter 2" of ebay for a good price , almost as new , in box.
Just wondered what size threads are on the front and rear of this filter , I think they are 48mm , but I need to be sure.
The filter will be used on a Kaligar lense on my Full spectrum D90

Thanks in advance

Daryll

#2 Andy Perrin

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Posted 07 March 2018 - 18:02

Hey, that filter looks like it has a significant IR leak? Their LINEAR graph shows a peak near 900nm.

https://www.highpoin...e/venus-u_1.png

#3 Daryll

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Posted 07 March 2018 - 18:43

Guess I`ll have to use a IR blockfilter as well , an ideas on that

Daryll

#4 bobfriedman

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Posted 07 March 2018 - 20:25

my filter is 48mm...

#5 Daryll

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Posted 07 March 2018 - 20:27

View Postbobfriedman, on 07 March 2018 - 20:25, said:

my filter is 48mm...
Thats what I thought , Thanks

Daryll

#6 bobfriedman

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Posted 07 March 2018 - 23:54

A BG-38 filter will take care of that very very small amount of IR leakage if it matters.

#7 Cadmium

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Posted 08 March 2018 - 00:44

Although it is a different brand than the Baader U (Optolong brand), since it is a telescope filter, the 2" thread size should be 48mm.
Here is the graph of the filter from the eBay listing. It has some IR leak in the 900nm+ range, same IR leak range as the Baader U, however it looks like it is a stronger IR leak than the Baader U has.
https://i.ebayimg.co...hpI/s-l1600.jpg

Unless the BG38 is at least 3mm thick it will not block the IR leak of that U-filter. I would not use BG38.
You loose a lot of the deeper UV transmission when stacking a dichroic U-filter with an IR suppression filter, but BG40 2.5mm thick (no thinner) would be the optimal choice for this specific filter fix (black graph line).
S8612 1.5mm thick would be my second best choice for this filter fix.
Stacking will truncate the UV transmission width, peak UV transmission point, and peak UV transmission amplitude, however, depending on your lens, this may make little difference at all, if the lens curve cuts off as much or more UV transmission as the BG40 or S8612 does, then you loose nothing, and most accidentally UV friendly lenses do just that. You would need a special purpose lens to realize a difference I think (UV-Nikkor, etc.).
Attached Image: BG38_S8612_BG40_fix.jpg

Edited by Cadmium, 08 March 2018 - 01:29.


#8 Andy Perrin

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Posted 08 March 2018 - 03:42

View Postbobfriedman, on 07 March 2018 - 23:54, said:

A BG-38 filter will take care of that very very small amount of IR leakage if it matters.
If you can see the IR leak on a linear graph (rather than log or diabatic) then it seems like it's usually an issue (without additional blocking, in sunlight or under other broadband illumination).

Edited by Andy Perrin, 08 March 2018 - 03:44.


#9 Cadmium

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Posted 08 March 2018 - 07:13

Andy, I agree with you, those two graphs show a leak that appears to rise above 1E-03. Looks like about 2%. I think the Baader U has about 0.03%.
Andy, check that for me, does that all sound correct to you? You are good with that stuff.
http://www.beyondvis...BV3-filter.html

Edited by Cadmium, 08 March 2018 - 07:27.


#10 UlfW

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Posted 08 March 2018 - 08:08

Steve-
What kind of final attenuation of the stack are you aiming at when saying "BG38 at least 3mm thick" and "BG40 2.5mm thick (no thinner)"?
What is it that I am not understanding here?

The original leakage at 900nm, if we can trust the published graphs seam to be less than 2%.

To reach OD4 by adding an IR-blocking filter, that filter can block as little as 5E10-3 around 900nm.
That is fulfilled by all alternatives in your graphs above.

Just as you say, the BG40 is the optimal glass type for best UV-performance, but a 2mm thickness would also work well.
The difference in performance between 2mm and 2.5mm would be difficult to notice in the field.

If my reasoning is OK, then even a BG40 1mm thick would do the job.
However that 1mm BG40 would be useless for anything else I can imagine.

Edited by UlfW, 08 March 2018 - 08:11.

Ulf Wilhelmson
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#11 Andy Perrin

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Posted 08 March 2018 - 19:46

View PostCadmium, on 08 March 2018 - 07:13, said:

Andy, I agree with you, those two graphs show a leak that appears to rise above 1E-03. Looks like about 2%. I think the Baader U has about 0.03%.
Andy, check that for me, does that all sound correct to you? You are good with that stuff.
http://www.beyondvis...BV3-filter.html
Thanks for the compliment; here's what I get. The worst leak for the Baader 2" is at 933nm, and it's 0.027% (or between 0.025% and 0.028%):
Attached Image: baader-leak-peak.jpg

The Optolong leak is at 890nm and between 1% and 2%:
Attached Image: optalong-leak.jpg

Edited by Andy Perrin, 08 March 2018 - 19:58.


#12 Cadmium

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Posted 08 March 2018 - 22:16

Thanks Andy.

Ulf, indeed I was just eyeballing it, and didn't calculate it, so it will not actually require as much suppression as I suggested above.
Here I have made a 2% line to stack with various suppression filters.
In fact you could use BG38 2mm thick, but the transmission is a bit too close to the 1E-03 line for comfort.
Here is a BG40 2mm + 2% Optolong leak. So basically just slap some kind of BG filter on there and see how it looks. It might even look OK with nothing, defiantly try a comparison between nothing and something.
If you were to use BG40 1mm it actually crosses over and above the 1E-03 line a bit, so I would not use that.
This is as close as I can come to estimating the results, hope that helps.
Attached Image: fix_filter_calc.jpg

Attached Image: fix_filter_calc_2.jpg

Edited by Cadmium, 08 March 2018 - 22:27.


#13 Cadmium

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Posted 09 March 2018 - 02:30

Attached Image: Three_leak_fix.jpg

#14 UlfW

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Posted 09 March 2018 - 09:35

Steve,

Sorry for being picky about your statements, but they didn't add up for me.

View PostCadmium, on 08 March 2018 - 22:16, said:

If you were to use BG40 1mm it actually crosses over and above the 1E-03 line a bit, so I would not use that.


The crossing and part over the 1E-03 line is beyond 1000nm.
There the sensitivity of the sensor has decreased a bit and also is not where the Optolong filter has a leakage problem.

In this case I still think a 1mm thick BG40 might work, at least most of the times.

I agree that it is not an optimal thickness. Some extra margin is much better.
Ulf Wilhelmson
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#15 Cadmium

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Posted 09 March 2018 - 18:24

Anything would probably work. When you have 2% you will have a leak, however it will look white/gray at 900nm, and thus not as noticeable perhaps as a 700nm leak that will look warm/brown, and will alter UV false colors.
1E-03 is my personal absolute danger zone for IR suppression, I like to stay lower, in fact I like to stay under 1E-04, and some would say that 1E-05 is the true safety point.
Anything that reduces the 2% is going to be a good thing.

Also note that the graph that was posted with the filer that Daryll purchased has a considerable leak all the way up to 1100nm,
it is slightly different than what Andy posted.
Regardless of which is correct and how far you think the sensor sees IR when shooting longer exposures for UV,
the calculated stack using BG40 1mm is just too close to the 1E-03 line.
Now, if you have BG40 1mm and it works to clean up that 2% leak, then great, but I would not purchase a 1mm thick BG40 filter for this situation,
I would get something that suppresses at or below 1E-04 for that entire range.

https://i.ebayimg.co...hpI/s-l1600.jpg

Edited by Cadmium, 10 March 2018 - 05:09.