Differences between Lunt BF's.

Use this section to discuss "standard" Baader/Coronado/ Lunt SolarView/ Daystar, etc… filters, cameras and scopes. No mods, just questions/ answers and reviews.
Post Reply
User avatar
GUS
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 1298
Joined: Thu May 08, 2014 12:03 pm
Location: Katoomba,NSW,Australia
Has thanked: 102 times
Been thanked: 213 times

Differences between Lunt BF's.

Post by GUS »

Does anybody know why Lunt BF's have different brightness levels, I have a B1200 and a B1800 and the image through the b1200 is a lot brighter than the b1800. I'd say that it is almost twice as bright, the other day while observing the sun, proms where very noticeable with the b1200, where as with the b1800 I could barley make them out,and surface detail was non existent. I know this has come up before and have also noticed similar threads on other forums regarding these two particular BF sizes, but curious as to what causes this difference. Is it the ITF, the diagonal mirror,the blocking filter glass or the UV blocker glass, or is it differences in coating types. Both filters look in good order, without any residue on any of the glass.
This is not a complaint regarding the filters but more of a curiosity as to the cause,as I have owned a few Coronado BF's and haven't noticed this degree of difference. These tests were visual only, I'll attempt an imaging test over the weekend.

GUS.
Last edited by GUS on Mon Jul 21, 2014 9:53 am, edited 1 time in total.


User avatar
GUS
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 1298
Joined: Thu May 08, 2014 12:03 pm
Location: Katoomba,NSW,Australia
Has thanked: 102 times
Been thanked: 213 times

Re: Differences between Lunt BF's.

Post by GUS »

Had a look tonight at both BF's,took a few shots.The first photo shows the view of a ceiling halogen light through the B1200,the second through the B1800 (both are through the eyepiece end) and the brightness difference can be clearly seen .The next two photos are of overhead lights reflecting off the BFs from both ends. Third photo is of the UV blocker/blocking filter glass(eyepiece end) and both look similar reflection wise. The Fourth photo shows the ITF (telescope side),and is a bit more telling of what could be causing the difference. In all shots the b1800 is on the left of the photo, and as can be seen in the last shot the b1800 ITF filter is reflecting a lot more light ( and IR) , and I guess letting through less. The b1200 is about 2 years old, and the b1800 was purchased last week, I know that the original ITFs had some kind of residue/rust issue and I assume my b1200 could be an early ITF type and the b1800 could be a newer ITF version. The only other component is the diagonal mirror,which I've seen when I had my b1200 cleaned recently, its not fully reflective, more like half and has a purple tint to it(maybe some sort of coating),this could also account for light transmission differences.

GUS.
Attachments
LS BFA.JPG
LS BF B.JPG
LUNT BF B.JPG
LUNT BF A.JPG


User avatar
Bill Edelen
Ohhhhhh My!
Ohhhhhh My!
Posts: 122
Joined: Mon Mar 03, 2014 6:12 pm
Location: NJ. US

Re: Differences between Lunt BF's.

Post by Bill Edelen »

Interesting and obvious. Please ask Lunt why the differences and post what they say.....


User avatar
solarchat
Site Admin
Site Admin
Posts: 4359
Joined: Sat Feb 01, 2014 3:10 pm
Location: Atlanta, GA USA
Has thanked: 83 times
Been thanked: 1315 times
Contact:

Re: Differences between Lunt BF's.

Post by solarchat »

I don't work for Lunt but I would have to go with the obvious.
I know Lunt changed blocking filter glass suppliers within the last two years due to a few issues. That may explain some of it. I also know that all of my blocking filters slightly change their characteristics over time with constant use.

Mostly, I would say because these scopes are all hand made and the etalons are all custom polished in house so there are variations. Each scope when ordered is constructed and tested by one guy and adjustments are made to the blocking filter on each scope to accommodate whatever etalon they are using. Also, it is almost impossible to get 30mm of blocking filter ITF glass to be homogenous much less a sheet of 200 of these filters. They are all slightly different. That's why they go through rigorous testing and adjustment before they ship. You are better off in my opinion with a B1200 than any other as it is big enough for all their scopes but small enough to get a mostly uniform piece of glass.

This whole idea of why they can't be interchanged between Coronado and Lunt scopes is asking a bit much, isn't it? I mean, its two different companies making two vastly different products. Why would Lunt or Coronado be interested in making their blocking filters or etalons work with a competitors product?


Stephen W. Ramsden
Atlanta, GA USA
Founder/Director Charlie Bates Solar Astronomy Project
http://www.solarastronomy.org
User avatar
swisswalter
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 17948
Joined: Thu Feb 06, 2014 4:28 am
Location: Switzerland
Been thanked: 1 time
Contact:

Re: Differences between Lunt BF's.

Post by swisswalter »

Hi GUS

an obvious difference. We noted that too


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 ;)
User avatar
GUS
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 1298
Joined: Thu May 08, 2014 12:03 pm
Location: Katoomba,NSW,Australia
Has thanked: 102 times
Been thanked: 213 times

Re: Differences between Lunt BF's.

Post by GUS »

Thanks guys. Steven,thanks for your input, I understand these are all hand made and variations are inevitable, this post is more for a general interest value, and I know others have come across this issue, just wanted some clarity on this.

GUS.


User avatar
Valery
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 4059
Joined: Sun Nov 18, 2012 3:13 pm
Has thanked: 156 times
Been thanked: 893 times

Re: Differences between Lunt BF's.

Post by Valery »

GUS wrote:Does anybody know why Lunt BF's have different brightness levels, I have a B1200 and a B1800 and the image through the b1200 is a lot brighter than the b1800. I'd say that it is almost twice as bright, the other day while observing the sun, proms where very noticeable with the b1200, where as with the b1800 I could barley make them out,and surface detail was non existent. I know this has come up before and have also noticed similar threads on other forums regarding these two particular BF sizes, but curious as to what causes this difference. Is it the ITF, the diagonal mirror,the blocking filter glass or the UV blocker glass, or is it differences in coating types. Both filters look in good order, without any residue on any of the glass.
This is not a complaint regarding the filters but more of a curiosity as to the cause,as I have owned a few Coronado BF's and haven't noticed this degree of difference. These tests were visual only, I'll attempt an imaging test over the weekend.

GUS.
GUS, your reasoning directly reflects my experience - B1200 is 2x brighter and no such deviations in transmission in Coronado BF I have owned.
Very strange. This keep me from the B1800 purchase. I compared two my B1200 (straight and diagonal) vs B1800 diagonal owned by a friend of mine.
B1200 is the same bright as my Coronado BF10.


"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.
fjabet
Im an EXPERT!
Im an EXPERT!
Posts: 260
Joined: Sat Dec 14, 2013 5:27 pm
Been thanked: 21 times

Re: Differences between Lunt BF's.

Post by fjabet »

Some of my fellow observer switched their Lunt BF for a Coronado for much better results.


User avatar
GUS
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 1298
Joined: Thu May 08, 2014 12:03 pm
Location: Katoomba,NSW,Australia
Has thanked: 102 times
Been thanked: 213 times

Re: Differences between Lunt BF's.

Post by GUS »

Thanks for the replies Valery and fjabet, I purchased a Lunt LS 100 that came with the B1800, and I literally can not make out any surface detail, and proms are faint. The B1200 is better , as I can see more detail,on the surface and the proms stand out more. But the image is still dark and its a struggle to see fainter detail, but when using the coronado BF 10, the sun becomes dynamic and full of detail( I've tried two Bf 10s and both show the same detailed views(and one of them has the rust problem on the ITF as well).

I use Coronado BFs on both my Lunt scopes(LS60/DS60 PT and the LS100 T), mainly for visual use and use the Lunt BFs for Photography. This is not a complaint, just a personal observation, Lunt make great scopes,and my DS 60 scope is a fine example of that, the Ls 100 I've owned for a short time and only had a couple of looks through it, but all looks good with it as well. I have owned a couple of Coronado scopes as well, and understand that etalons and Bfs are complicated to manufacture and that there are differences even with the same products from each manufacturer. I'm primarily a visual observer and the Lunt scope/Coronado BF combos work great for me.(Have been using the LS60PT/BF 10 combo for close to three years now without any problem).

Just an interesting side note, both BF 10s were purchased roughly at the same time and the rusted bf10 was used in a Coronado Solarmax scope, where as the good BF was used exclusively with the Lunt scope.


User avatar
Valery
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 4059
Joined: Sun Nov 18, 2012 3:13 pm
Has thanked: 156 times
Been thanked: 893 times

Re: Differences between Lunt BF's.

Post by Valery »

GUS wrote:Thanks for the replies Valery and fjabet, I purchased a Lunt LS 100 that came with the B1800, and I literally can not make out any surface detail, and proms are faint. The B1200 is better , as I can see more detail,on the surface and the proms stand out more. But the image is still dark and its a struggle to see fainter detail, but when using the coronado BF 10, the sun becomes dynamic and full of detail( I've tried two Bf 10s and both show the same detailed views(and one of them has the rust problem on the ITF as well).

I use Coronado BFs on both my Lunt scopes(LS60/DS60 PT and the LS100 T), mainly for visual use and use the Lunt BFs for Photography. This is not a complaint, just a personal observation, Lunt make great scopes,and my DS 60 scope is a fine example of that, the Ls 100 I've owned for a short time and only had a couple of looks through it, but all looks good with it as well. I have owned a couple of Coronado scopes as well, and understand that etalons and Bfs are complicated to manufacture and that there are differences even with the same products from each manufacturer. I'm primarily a visual observer and the Lunt scope/Coronado BF combos work great for me.(Have been using the LS60PT/BF 10 combo for close to three years now without any problem).

Just an interesting side note, both BF 10s were purchased roughly at the same time and the rusted bf10 was used in a Coronado Solarmax scope, where as the good BF was used exclusively with the Lunt scope.
I don't remember who said this (from the Lunt Engineering team) but it was said: Coronado blocking filters will work with Lunt etalons just fine, but the Lunt blocking filters are not warranted to work with Coronado etalons.
However many PST mods work with Lunt blocking filters and we all know this.

I hope that Lunt's new blocking filters (after they changed the BF supplier) are fine and there is no difference in transmission between new B1200 and new B1800.
But this need to be proved.


"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.
User avatar
GUS
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 1298
Joined: Thu May 08, 2014 12:03 pm
Location: Katoomba,NSW,Australia
Has thanked: 102 times
Been thanked: 213 times

Re: Differences between Lunt BF's.

Post by GUS »

My B1800, as far as I know is a relatively recent BF,(scope was purchased two weeks ago) and might contain the newer ITF. I tried using my Lunt B1200 on 3 seperate SM60 and two SM40 etalons, and the B1200 would not work with any of them, lacking contrast and very limited Ha detail,very washed out image. I've also read that they work with PST mods, read somewhere that it might have something to do with the Baader ERFs, which most PST modders seem to use( not sure if this is correct).
Another thing is that while the Lunt BFs didn't work in single stack mode with the Coronado Etalons they did show reasonable views when the Coronados were double stacked to my Lunt scope.
GUS.


User avatar
swisswalter
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 17948
Joined: Thu Feb 06, 2014 4:28 am
Location: Switzerland
Been thanked: 1 time
Contact:

Re: Differences between Lunt BF's.

Post by swisswalter »

hi all

another thing is puzzling me. You all speak of an ITF , me I used too, but Rikki corrected me, she wrote: the blue glass ahead of the blocking filter it is not an ITF, it is a blue filter. Now what is the blue glass ? What is the definition of an ITF?


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 ;)
User avatar
marktownley
Librarian
Librarian
Posts: 42272
Joined: Tue Oct 18, 2011 5:27 pm
Location: Brierley Hills, UK
Has thanked: 20435 times
Been thanked: 10245 times
Contact:

Re: Differences between Lunt BF's.

Post by marktownley »

GUS wrote:My B1800, as far as I know is a relatively recent BF,(scope was purchased two weeks ago) and might contain the newer ITF. I tried using my Lunt B1200 on 3 seperate SM60 and two SM40 etalons, and the B1200 would not work with any of them, lacking contrast and very limited Ha detail,very washed out image. I've also read that they work with PST mods, read somewhere that it might have something to do with the Baader ERFs, which most PST modders seem to use( not sure if this is correct).
Another thing is that while the Lunt BFs didn't work in single stack mode with the Coronado Etalons they did show reasonable views when the Coronados were double stacked to my Lunt scope.
GUS.
Coronado ERFs are just a piece of Schott RG630 glass, what this means is wavelengths shorter than 630nm are blocked, but those longer (and into the infra red) are not. PST mods as you say often use a Baader ERF which is blocked into the IR, as a result the Lunt blockers work. Basically the ITF (the filter that rusts in the coronado blockers) in coronados blocks IR, Lunt blockers do not block IR which is why they give a contrast lacking washed out image with a coronado scope - in this configuration there would be an IR leak which causes what you observe.


Image
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!
User avatar
GUS
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 1298
Joined: Thu May 08, 2014 12:03 pm
Location: Katoomba,NSW,Australia
Has thanked: 102 times
Been thanked: 213 times

Re: Differences between Lunt BF's.

Post by GUS »

Thanks Mark,that's the information I needed. So the Lunt ERF blocks !R(in the lunt cutaway diagram it shows an IR blocker in the BF).So the Coronado BF should be safe to use with the Lunt scopes.What I found with the Coronado BFs is that the one I have been using with the Lunt is fine, no rust, but the one with the SM 40 scope is rusted, the ring of rust on the ITF coincides almost exactly with the light cone at that point(both were purchased at roughly the same time). So on the rusted BF, is it loosing its IR protection.

Thanks again

GUS.


User avatar
marktownley
Librarian
Librarian
Posts: 42272
Joined: Tue Oct 18, 2011 5:27 pm
Location: Brierley Hills, UK
Has thanked: 20435 times
Been thanked: 10245 times
Contact:

Re: Differences between Lunt BF's.

Post by marktownley »

GUS wrote: So on the rusted BF, is it loosing its IR protection..
It won't lose any of its ability to block IR, just lose some contrast and transmission.


Image
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!
User avatar
GUS
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 1298
Joined: Thu May 08, 2014 12:03 pm
Location: Katoomba,NSW,Australia
Has thanked: 102 times
Been thanked: 213 times

Re: Differences between Lunt BF's.

Post by GUS »

Thanks for that mark.

GUS


User avatar
Valery
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 4059
Joined: Sun Nov 18, 2012 3:13 pm
Has thanked: 156 times
Been thanked: 893 times

Re: Differences between Lunt BF's.

Post by Valery »

marktownley wrote:
GUS wrote: So on the rusted BF, is it loosing its IR protection..
It won't lose any of its ability to block IR, just lose some contrast and transmission.
My BF10 Coronado has rusted. However, no contrast loss and a few % transmission which (transmission change) I can't really detect by my eyes.


"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.
User avatar
GUS
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 1298
Joined: Thu May 08, 2014 12:03 pm
Location: Katoomba,NSW,Australia
Has thanked: 102 times
Been thanked: 213 times

Re: Differences between Lunt BF's.

Post by GUS »

Hi Valery, I have noticed dimmer images and a loss of contrast on the Solarmax 40, but with the LS 60 can't notice any difference visually(figure because of the brighter image)this is compared to my good BF10.
.


User avatar
marktownley
Librarian
Librarian
Posts: 42272
Joined: Tue Oct 18, 2011 5:27 pm
Location: Brierley Hills, UK
Has thanked: 20435 times
Been thanked: 10245 times
Contact:

Re: Differences between Lunt BF's.

Post by marktownley »

My BF10 lost contrast and also transmission, you must have been lucky valery!


Image
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!
User avatar
Valery
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 4059
Joined: Sun Nov 18, 2012 3:13 pm
Has thanked: 156 times
Been thanked: 893 times

Re: Differences between Lunt BF's.

Post by Valery »

marktownley wrote:My BF10 lost contrast and also transmission, you must have been lucky valery!
May be I am lucky. But I still want the best performance for my BF10. A few questions, if I may.

1. How did you unscrew the front element of the BF10?

2. What the best replacement for the front element?

Thanks,

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.
User avatar
marktownley
Librarian
Librarian
Posts: 42272
Joined: Tue Oct 18, 2011 5:27 pm
Location: Brierley Hills, UK
Has thanked: 20435 times
Been thanked: 10245 times
Contact:

Re: Differences between Lunt BF's.

Post by marktownley »

Hi Valery.

To remove the front element of the BF10 there is just a retaining ring that unscrews, a lens spanner will do the job easily.

Maier Photonics do a replacement ITF for the PST here http://maierphotonics.com/656bandpassfilter.aspx which is 100% safe for visual. If you want to use this as a replacement in the BFxx you will need to use a Baader Ha also. Used with a PST mod (with Baader D-ERF) it will work as the D-ERF has excellent out of band blocking on the long and short sides of 656nm.

Alternatively if imaging is your main thing you can use a baader Ha filter (7nm or 35nm), if you wanted to do this visually too a piece of AR coated KG3 used in conjunction with the Baader will block out to >2500nm which is visual safe too.

Mark


Image
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!
User avatar
pedro
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 12256
Joined: Sun May 01, 2016 8:26 pm
Location: Portugal
Has thanked: 14 times
Been thanked: 6577 times
Contact:

Re: Differences between Lunt BF's.

Post by pedro »

I replaced my rusted BF10 with a Baader 35nm + a IV/IR filter.

It works fine as far as imaging is concerned.

I have several BF and they are all different: LUNT BF3400 BF1200; Coronado BF30 BF10 (2)


User avatar
Valery
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 4059
Joined: Sun Nov 18, 2012 3:13 pm
Has thanked: 156 times
Been thanked: 893 times

Re: Differences between Lunt BF's.

Post by Valery »

marktownley wrote:Hi Valery.

To remove the front element of the BF10 there is just a retaining ring that unscrews, a lens spanner will do the job easily.

Maier Photonics do a replacement ITF for the PST here http://maierphotonics.com/656bandpassfilter.aspx which is 100% safe for visual. If you want to use this as a replacement in the BFxx you will need to use a Baader Ha also. Used with a PST mod (with Baader D-ERF) it will work as the D-ERF has excellent out of band blocking on the long and short sides of 656nm.

Alternatively if imaging is your main thing you can use a baader Ha filter (7nm or 35nm), if you wanted to do this visually too a piece of AR coated KG3 used in conjunction with the Baader will block out to >2500nm which is visual safe too.

Mark
Thanks, Mark, for your detailed input.


"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.
User avatar
GUS
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 1298
Joined: Thu May 08, 2014 12:03 pm
Location: Katoomba,NSW,Australia
Has thanked: 102 times
Been thanked: 213 times

Re: Differences between Lunt BF's.

Post by GUS »

The point of this thread originally was that I was having trouble getting any usable view through the Lunt BFs as opposed to the Coronado Bfs. After spending a couple of days recently comparing the views through the different BFs, I found that if I move my eye away from the eyepiece, just over an inch, I can actually see as much detail in the Lunts as in the Coronados, the images are still darker but the detail is there and for some reason is not seen when I place my eye up to the eyepiece.

Over the years I have seen mentioned the blackness of the view around the sun using the Lunt BFs, but I have never seen the sky black around the sun using any of my BFs, its always been orange-red, and that's using any eyepiece, from simple to multi lens, and I've seen mentioned that this could be caused by light reflected off my eye saturating the view. So in my case this might account for the fact that when I move my eye away from the Lunt BFs, the reflected light is not as strong and I start to see detail, and the image being dim to start with doesn't help.

In the case of the Coronado BFs, the image is so bright initially that reflected light off my eyes doesn't saturate the view as much, hence I can see detail straight away, but have noticed here as well that moving my eye an inch or so away, the detail improves as well. I read that different coloured eyes reflect, absorb light differently, I wonder if this could have something to do with it as well. In the end I think the problem is more to do with my eyes than any real problem with the BFs. Sorry for waffling on about this subject so much.

GUS


User avatar
grimble_cornet
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 3537
Joined: Tue Jul 10, 2012 5:10 pm
Location: UK West Midlands
Been thanked: 47 times

Re: Differences between Lunt BF's.

Post by grimble_cornet »

Don't apologize GUS; I for one am fascinated by your observations. :bow2

I know that my eyesight is deteriorating with age which is one reason I do imaging - I have probably only looked into a solar scope eyepiece a couple of times in the last year :oops:

When I had a Lunt 60 on loan a couple of weeks ago, I did have a quick look before starting to image and actually thought it might be broken as it was so dim compared to my SolarMax40. This was single stacked with a B1200. When it was double stacked I could barely make out anything visually despite it working very well once a camera was attached :o

I didn't give it too much thought at the time as a) I didn't bother to 'work at' seeing detail or use a cloth over my head for shade, b) I didn't have much to compare it with and c) I'm not really interested in visual work and wanted to get on with imaging which worked very well :lol:

At the time, I didn't even consider trying my SolarMax BF as I assumed it wouldn't work with a Lunt scope. However, when my Lunt 100 B1800 arrives (hopefully in October) I will certainly try switching blocking filters. Your experiences initially put me off the idea of getting the B1800 but it seems that the hand built nature of the beast makes any kind of generalization rather suspect and I decided to go with recommendations from Stephen and Walter who have probably forgotten more than I will ever know about the subject :lol:


.

.



.


Mike Garbett

Selection of Solar and Deep Sky images on Flickr: https://www.flickr.com/photos/mikegarbett/
User avatar
GUS
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 1298
Joined: Thu May 08, 2014 12:03 pm
Location: Katoomba,NSW,Australia
Has thanked: 102 times
Been thanked: 213 times

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.


antonello
Ohhhhhh My!
Ohhhhhh My!
Posts: 86
Joined: Tue Mar 24, 2015 12:34 pm
Been thanked: 17 times

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 1103 times
Last edited by antonello on Thu May 18, 2023 4:03 pm, edited 1 time in total.


User avatar
Bob Yoesle
Almost There...
Almost There...
Posts: 994
Joined: Thu Mar 08, 2012 7:24 pm
Has thanked: 540 times
Been thanked: 807 times

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 1063 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 1063 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 1063 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


Diagonally parked in a parallel universe.

Curiosity is the father of knowledge; uncertainty is the mother of wisdom.

Dark-Sky Defenders
Goldendale Observatory
antonello
Ohhhhhh My!
Ohhhhhh My!
Posts: 86
Joined: Tue Mar 24, 2015 12:34 pm
Been thanked: 17 times

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.


Post Reply