The Charlie Bates Solar Astronomy Project is proudly sponsored by the following fine companies:
ImageImageImageImage
Image
Image
Transit of Mercury in the Spicule Layer. Phil took this great image with his Quantum 6 Scope, Solar Spectrum etalon and PGR Camera.

Welcome to Solar Chat! The best forum for Solar Astronomy
Moving forward from solar cycle 24 to 25

Took my Lunt LS50T apart to measure stuff - interesting results...

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...
Post Reply
User avatar
Roel80
The Sun?
The Sun?
Posts: 12
Joined: Sat Nov 23, 2019 8:25 am

Took my Lunt LS50T apart to measure stuff - interesting results...

Post by Roel80 » Sun Nov 24, 2019 4:39 pm

Ok, so today I took my Lunt LS50T apart to measure things up. On the internet are various claims about different diameters, lens configurations etc. so I just wanted to be sure.

First of all, my Lunt (serial number 2015662) has a doublet objective lens (instead of a singlet). Proof:
20191124_135014_resized.jpg
20191124_135014_resized.jpg (500.97 KiB) Viewed 293 times
Then I tried to measure its focal length by making a primitive optical bench. I used a crosshair eypiece and carefully focused on some trees several kilometers away. The result (by carefully measuring the distance between the center of the objective and the position of the crosshair) was the focal point being exactly 357mm from the center of the objective, making it a f/7.14.

lunttest.jpg
lunttest.jpg (402.48 KiB) Viewed 293 times
Next I made a 1:1 drawing combining several measuments on the OTA and the focal length test results. It is here that I found some slightly disturbing things...
luntdiagram.jpg
luntdiagram.jpg (583.72 KiB) Viewed 293 times
So at nr. 1 we see the 50mm free aperture f/7.14 doublet in the front.
At nr. 2 is a tilted red filter - diameter 38mm (probably an internal ERF)
At nr. 3 we enter the red etalon box. There is an aperture stop just in front of the first "collimating lens" with a free aperture of 20mm. As it is located at 188mm from the center of the objective means that it's VIGNETTING the incoming beam to about f/8.5! And this means the 50mm objective is effectively a... 42mm!!
You can imagine I was pretty suprised by this, so I measured everything again: starting with measuring the focal lenght for the second time. I got the same results. Hmm ok.
Nr. 4: Focuser-side collimating lens, 24mm in diameter
Nr. 5: the actual focal point. (I don't really know if the last collimating lens really converge the beam to f/7, so the trace lines behind the etalon unit could have a different angle in reality)

I also found the focuser-side collimating lens being positioned at exactly 133mm in front of the actual focal plane of the objective. (resulting in 127mm and 170.5mm for the front and back of the red etalon housing - convenient reference points for the mod to come.

Please tell me I did something completely wrong here, because a 50mm f/7 actually being a 42mm f/8,5 is just not funny at all. :(

User avatar
Valery
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 3350
Joined: Sun Nov 18, 2012 3:13 pm

Re: Took my Lunt LS50T apart to measure stuff - interesting results...

Post by Valery » Sun Nov 24, 2019 5:28 pm

Looks like you are correct.

Can you measure the actual diameter of the etalon itself?


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
Rusted
Ohhhhhh My!
Ohhhhhh My!
Posts: 77
Joined: Fri Jan 19, 2018 11:00 am
Contact:

Re: Took my Lunt LS50T apart to measure stuff - interesting results...

Post by Rusted » Sun Nov 24, 2019 6:46 pm

42/50 x $800 [retail] = $672. Lunt owe you a refund of $128! :shock:

User avatar
Roel80
The Sun?
The Sun?
Posts: 12
Joined: Sat Nov 23, 2019 8:25 am

Re: Took my Lunt LS50T apart to measure stuff - interesting results...

Post by Roel80 » Sun Nov 24, 2019 7:03 pm

I would have to to remove the front correction lens to meausure the etalon, and I see some glued seals on one of the retainer rings. I'd rather leave that alone. Unless the etalon is even smaller than the 20mm baffle (which appears not to be the case looking into the tube), I wouldn't be bothered by it's actual size.

I also think I'll end up using a 80mm f/7,5 donor scope considering the above test results. When it actually vignets to an f/8,5, I would still have 70mm of real aperture, and a full disk image fitting in the B600...

bart1805
Almost There...
Almost There...
Posts: 631
Joined: Mon Feb 06, 2017 9:04 pm
Location: The Netherlands

Re: Took my Lunt LS50T apart to measure stuff - interesting results...

Post by bart1805 » Sun Nov 24, 2019 7:08 pm

Remarkable, and worth the test of putting the etalon a few centimes more to the back.

User avatar
Roel80
The Sun?
The Sun?
Posts: 12
Joined: Sat Nov 23, 2019 8:25 am

Re: Took my Lunt LS50T apart to measure stuff - interesting results...

Post by Roel80 » Sun Nov 24, 2019 9:21 pm

Ok, I'm trying to wrap my mind around what that means. I got back to my 1:1 drawing to measure what would be necessary for that:

- If I want to get the f/7 beam through the first 20mm baffle completely, the etalon unit has to move 29mm to the focuser.
- It means the first baffle will be at 188mm + 29mm = 217mm behind the center of the objective.
- This is 357mm - 217mm = 140mm in front of the focal point. Hey, where dit I hear that number before?
- Then the beam gets collimated to go parallel throught the etalon, and becomes a f/7 afgain.
- From the last collimating lens to the focal point should therefore be 140mm again.

What I find very difficult to understand is what it would do with the focuser. Do I need 29mm of extra inward travel also?
I mean, the incoming beam is vignetted to f/8.5, but will this affect the outcoming beam? I think the first correction lens will parellel any beam from f/7 or slower, and the second will always make a 20mm f/7 cone out of it. In that case the focal point will always be 140mm behind the last correction lens.

Am I making the right assumption?

User avatar
marktownley
Librarian
Librarian
Posts: 26453
Joined: Tue Oct 18, 2011 5:27 pm
Location: Brierley Hills, UK
Contact:

Re: Took my Lunt LS50T apart to measure stuff - interesting results...

Post by marktownley » Sun Nov 24, 2019 10:16 pm

Roel80 wrote:
Sun Nov 24, 2019 4:39 pm
50mm f/7 actually being a 42mm f/8,5 is just not funny at all. :(
I don't doubt your measurements for one minute. However it doesn't operate as a 42mm scope, the 20mm free aperture on the front of the etalon section just functions as a field stop. That's different to an aperture stop. Lots of our professional big solar reflectors use a field stop. In a scope using a sub aperture etalon a field stop may well be a good thing to reduce the field of view that presents an 'off band' image.
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
marktownley
Librarian
Librarian
Posts: 26453
Joined: Tue Oct 18, 2011 5:27 pm
Location: Brierley Hills, UK
Contact:

Re: Took my Lunt LS50T apart to measure stuff - interesting results...

Post by marktownley » Sun Nov 24, 2019 10:21 pm

Roel80 wrote:
Sun Nov 24, 2019 9:21 pm
- This is 357mm - 217mm = 140mm in front of the focal point. Hey, where dit I hear that number before?
You heard it on Solarchat ;)
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
Roel80
The Sun?
The Sun?
Posts: 12
Joined: Sat Nov 23, 2019 8:25 am

Re: Took my Lunt LS50T apart to measure stuff - interesting results...

Post by Roel80 » Mon Nov 25, 2019 3:05 pm

Ok, I'm beginning to understand this. However, what would happen when I use an f/7,5 donor scope instead of an f/7. With a B600, an 80mm f/7,5 could be a nice candidate for a visual mod. I understand that using a shorter than f/7 scope would simply be stopped down to an f/7, but other than not be able to use the full aperture, no other problems would occur.

But what would the effect of >f/7 ratio be? Would the image be worse, same or even better than a f/7?

User avatar
marktownley
Librarian
Librarian
Posts: 26453
Joined: Tue Oct 18, 2011 5:27 pm
Location: Brierley Hills, UK
Contact:

Re: Took my Lunt LS50T apart to measure stuff - interesting results...

Post by marktownley » Mon Nov 25, 2019 7:05 pm

The 80mm f7.5 will work fine, but, has it got the necessary back focus you need? That is key, if not then some modding will be needed.
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
Roel80
The Sun?
The Sun?
Posts: 12
Joined: Sat Nov 23, 2019 8:25 am

Re: Took my Lunt LS50T apart to measure stuff - interesting results...

Post by Roel80 » Mon Nov 25, 2019 7:20 pm

If you mean cutting the tube: no problem there.

Still wondering about the exit light cone (the bundle that leaves the back of the etalon unit). Will it be f/7.5 (like the objective) or f/7 (because the rear lens is designed to make a f/7 cone out of a parallel beam)?

User avatar
marktownley
Librarian
Librarian
Posts: 26453
Joined: Tue Oct 18, 2011 5:27 pm
Location: Brierley Hills, UK
Contact:

Re: Took my Lunt LS50T apart to measure stuff - interesting results...

Post by marktownley » Mon Nov 25, 2019 8:50 pm

Roel80 wrote:
Mon Nov 25, 2019 7:20 pm
Still wondering about the exit light cone (the bundle that leaves the back of the etalon unit). Will it be f/7.5 (like the objective) or f/7 (because the rear lens is designed to make a f/7 cone out of a parallel beam)?
Wouldn't worry about it too much, it will only be a 10mm difference in focal position either way.
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
Roel80
The Sun?
The Sun?
Posts: 12
Joined: Sat Nov 23, 2019 8:25 am

Re: Took my Lunt LS50T apart to measure stuff - interesting results...

Post by Roel80 » Mon Dec 02, 2019 10:56 am

The lunt 50mm has a 40mm tilted internal ERF before the etalon. It is mounted 20mm in front of the first collimation lens. In a 80mm f7.5 mod the sun's image will be around 26mm diameter at the ERF's position, well within the diameter of the ERF, so very little risk of overheating the mounting parts.

Will this ERF be sufficient to protect the etalon?

(Although the sun's image will be larger, the amount of energy per square inch would be a little less at f/7,5 compared to the original f/7)

Post Reply