Differences between Lunt BF's.

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GUS
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Re: Differences between Lunt BF's.

Post by GUS »

Hi Mike, glad you took the leap and ordered the LS100. Imaging wise the Lunt BFs obviously work fine going by the images from this forum, and the few shots I have taken with the LS60 and Lunt BFs look good. Every Ha scope, filter or BFs I have owned I have always mixed and matched components, so if one component doesn't work for my needs,I can always try something else.
I have never taken any safety risks with mixing the components, I've always researched meticulously as many threads and articles as possible from this site, and always double checked everything before trying a new setup.

I can't emphasize enough, how much this site has helped in my enjoyment of Solar Astronomy, and the free exchange of information and helpful advise form people across the globe, make this a truly great forum.

GUS.


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Re: Differences between Lunt BF's.

Post by antonello »

Hello, I read this topic with much interest and surprise. The surprise is that I can't understand how a company like lunt can produce two articles that are supposed to do the same thing and which instead have such different results.
In these years I have built 4 solar telescopes with Lunt etalons of different diameters, with Erf Baader and with an Omega Optical BF (Bloking Filter), a small 23 mm diameter filter for just over 100 dollars. I compared this filter with the Lunt B600 filter and I didn't notice big differences, on the contrary, the Omega Optical filter was slightly better (apart from the diameter, of course).
Over the years I've always had the idea of comparing a commercial BF filter (like the Lunt B1800) with my $100 Omega Optical filter. This has never been possible. In the past few days I found an offer for the new BF Lunt B1800 at less than half the price and, despite knowing this topic, I bought it. Unfortunately here in Italy these days there is a cyclone that is causing damage and deaths and the sun hasn't been seen for a month, so the comparison of this Lunt with my BF Omega optical filter, in this moment, is not possible. However I can testify
a useful observation to explain the possible difference in brightness of the B1200 Lunt compared to the B1800 Lunt.
As soon as the Lunt B1800 arrived I immediately noticed how dark it was compared to my Omega Optical filter and compared to my B600 and I tried to understand why.
For this I did the simplest thing: I removed the mirror from both the b600 and the b1800 and I saw that the two mirrors are the same. I also saw that the two BF (5A red filters in H-alpha) are equal (in terms of brightness). These Bfs are also the same brightness as my BF Omega Optical. What changes in the B600 Vs B1800 is the front filter that blocks IR radiation. In the B600 the filter is very clear (and looks like a Schott type KG), in the B1800 the filter is very blue (and looks like a BG38).
It is probable that this different choice depends on the ERF in use in the different Lunt telescopes, which in larger telescopes requires a higher IR cut.
The difference in brightness is given by this filter and not by the BF (red filter of 5-6A) which should be the same for all Bf Lunt.
I will use the BF without diagonal Lunt and without the Lunt IR filter (therefore without the Lunt deflector mirror),
I use this sequence: ERF, ETALON, KG3-IR CUT Beloprik and BF. I don't need the Lunt diagonal.
The IR Cut Lunt filter type BG28 removes a lot of light.
Anyone can check what I wrote and continue this topic. In particular it is important to see if the difference that I discovered also exists between B1200 and B1800 Lun. Here is an image showing my two Lunt diagonals B600 (right) and B1800 (left).

20230517_150312.jpg
20230517_150312.jpg (203.44 KiB) Viewed 1109 times
Last edited by antonello on Thu May 18, 2023 4:03 pm, edited 1 time in total.


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Re: Differences between Lunt BF's.

Post by Bob Yoesle »

The BG38 filter is apparently used mostly for brightness control and some IR blocking only to about 900 nm, and additional IR blocking is needed beyond 900 mm. KG3 can provide this additional blocking out to 2500 + nm. However, the combination of the two would be quite dim at 656 nm:

BG38 v KG3 2 mm Schott plots.png
BG38 v KG3 2 mm Schott plots.png (213.77 KiB) Viewed 1069 times

Approximately 20% x 60% (0.20 x 0.60 = 0.12) > 12%

Therefore, reduced IR transmission beyond 900 nm in the Lunt filter "stew" must occur elsewhere. Note that it also incorporates a dielectric coated mirror of unknown spectral reflectivity.

If using a KG3 filter as a BG38 replacement - it should be combined with an IR blocking filter for 700 - 900 nm IR blocking, or use the BelOptik UV/IR on KG3 which incorporates this IR block coating:

UVIRcutKG3-lin.jpg
UVIRcutKG3-lin.jpg (441.77 KiB) Viewed 1069 times

As with most of the colored glass filters, the transmission through the BG38 will vary with thickness:

BG38 2 mm v 4 mm thickSchott.png
BG38 2 mm v 4 mm thickSchott.png (182.56 KiB) Viewed 1069 times

Being a soda-lime colored glass filter, there also may be slight variations from different OEMs, and within an OEM, there may be differences resulting from different "melts." The Lunt BG38 has also gone from being uncoated to now being AR (and other?) coated to prevent moisture corrosion that can occur with soda-lime filters.

Bob


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Re: Differences between Lunt BF's.

Post by antonello »

Thanks Bob, everything correct and flawless.

Naturally, the transmission of absorption filters depends also on the thickness, but, with the same thickness, no production tolerance of Schott BG38 glass can justify the difference in luminosity complained of by Gus (one Stop).

The fact is that the two IR cut filters mounted on my two Lunts (B 600 and B1800) are different. Few years ago, I had measured the front filter curve of B600 with the spectrophotometer of my university and saw that it was practically equal to that of Schott KG3, not BG38.

My contribution wanted to clarify that the difference in brightness between the B1200 and the B1800 is due to the use of two different front filters, probably KG3 in the B1200 and BG38 in the B1800 (it's a hypothesis).

This choice by Lunt is due, I imagine, to the different use of these diagonals BF. For example the B400 and B600 are given in association with small instruments, even F 11.5 (like the lunt 35), while the B1800 is recommended for much larger telescopes, with brighter focal lengths (F:8 for the Lunt 100 mm, f :7 for the Lunt 130 mm and f:6 for the Lunt 150), for which the IR protection must be higher and the brightness reduced .

I contributed to this topic to avoid that anyone reading the (old) Gus's message might think that the B1800 is a poor filter, while instead it gives different results of B1200 just because it mounts a different front filter than the B600 and1200.
As I mentioned, the true BF filter del B1800 (the 5-6A red filter) is visually the same brightness as the red BF of my B600 and my Omega Optical BF.

Anyone who wants to have, with Lunt B1800, the same brightness as with the B1200 simply has to replace the BG38 of the B1800 with a filter type KG3-IR-Cut from Beloptic.
Oliver Smie also supplies it in the 20 mm diameter, ready to be assembled.


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