Help! Lunt: trying to get a diameter upgrade in full disc

[davidem27]

Hello y'all!
In these days I'm trying to figure out how to improve sampling, resolution and diameter of my h-alpha full disc, using a Borg 76ED (76/500 f/6.5) instead of the native Lunt LS50THa (50/350 f/7).

Recently I got a Thousand Oaks HAU, 0.9 Angstom bandpass filter that can be a blocking filter too of 2 cm diameter.
So I solved the optical path issue of Lunt LS 50T that is too short due of the diagonal blocking filter.

I would like to use only one OTA for the three bands, so I played with my favourite toy like a LEGO.
Borg is known for the capability of dismantle the tube in so many part, so I can use it with Herschel Wedge, with Calcium K module and with an internal etalon like my LS50 one.

Borg76_WL_178M.jpg (624.6 KiB) Viewed 2770 times

Borg76_CaK_178M.jpg (618.62 KiB) Viewed 2770 times

Borg76_HaDS_178M.jpg (611.39 KiB) Viewed 2770 times

So I draw, on graph paper, the optical path of the Borg and I measured the distance from the lens where the cone makes a 2 cm diameter of light, just to cover the full etalon diameter.

The result does not satisfy me and you can check it out here:

2023-04-21-0952_C.jpg (1.74 MiB) Viewed 2770 times

I had a double times fine tuning of the etalons and this is the best I could achieve.
You can see a very even field on LS50T Ha tube and a uncomplete centered field in the Borg configuration.
I also placed the etalon before of what calculations suggest me, but the result is really the same: disappointing.

I cannot figure it out and there is something I am missing...

The only thing my thoughts go to is the incident angle of the optical path near the etalon: Borg f/6.5 versus the original 7.

What do you think, masters of 656.3 nm light?

[marktownley]

Hi David.

I think you have discovered the saying 'there is no such thing as a free lunch'.

The size of the etalon in a lunt 50 is matched to make the size of the sweetspot (area in tune and on band) the same size as a 'full disk' If you put this etalon in a larger scope (your borg76) then the sweet spot is smaller than the full disk, which is what you can see in your pictures. This type of mod is only really suitable for closeups not full disks.

If you are using the scope for CaK you need to stop it down, I would sugest 50mm, to get the best out of it.

Mark

[davidem27]

It seems that Ivan put the Lunt 50 etalon on his 90/500 and it runs quite good, isn't it?

For CaK I stop the aperture to diameter of 50 (f/10) and 40 mm (f/12,5) and the optic performs so good.

[marktownley]

It seems that Ivan put the Lunt 50 etalon on his 90/500 and it runs quite good, isn't it?

For CaK I stop the aperture to diameter of 50 (f/10) and 40 mm (f/12,5) and the optic performs so good.

I think Ivan makes mosaics for his full disks (I might be wrong?)

Good work stopping down for CaK, it is the way to go.

[marktownley]

Borg76_HaDS_178M.jpg (611.39 KiB) Viewed 2580 times

Looking at this picture David.

The Lunt module goes in the back of the Borg with a 2" fitting?

You should look to screw the lunt module direct to the back of the Borg.

The uneveness on your full disk could well be impacted by sag in the optical chain.

[davidem27]

I tried with Baader Clic lock, more sturdy than simple screws.
The issue had been still present.

The definitive try costs me 92 euros:
https://www.teleskop-express.de/shop/pr ... hread.html

[christian viladrich]

Hi David,
Just to be sure I understand properly the setup : you are using the TO 0.9 A filter as an additional filter (say "double-stack") after the Lunt filter ?

If so, try to rotate the position of the 0.9 A TO (while keeping the Lunt and camera at the same position) and see if the non uniformities rotate or not with the TO.

[davidem27]

You got it, Christian!
Thanks for the suggestion, I'll try as soon as I could!

[marktownley]

Let us know how you get on. Before and after pictures please

[davidem27]

As you demand, monsieur

I thought that I placed the long side of the sensor perpendicular to the direction of the tilting, just to reduce this effect.
I haven't tried to rotate the camera yet although.

Stay tilted... oops, tuned

[Ivan]

Hi, Davide,
Mark is right, I use a mosaic for my FD's. If I put a camera with a large chip, remove the Barlow lens, then I will get a picture even worse than yours. I also noticed that the "sweet spot" may decrease slightly if the camera sensor is not perpendicular to the optical axis, or the optical train is sagging.

Ivan

[mdwmark]

Hi David
you have the .9Ang filter in the collimated beam and not after it. If its before the diagonal( F/7 beam ) it will not be an .9Ang Hw filter anymore but much wider and shifted towards shorter wavelength. This type of filter will have an low index spacer and needed to be in the collimated beam.
Mark W.

.7Ang low index different focal ratios.pdf

(705.77 KiB) Downloaded 81 times

[Oak]

Hi David,
Just to be sure I understand properly the setup : you are using the TO 0.9 A filter as an additional filter (say "double-stack") after the Lunt filter ?

If so, try to rotate the position of the 0.9 A TO (while keeping the Lunt and camera at the same position) and see if the non uniformities rotate or not with the TO.

Looks like the 0.9A TO is being used as a blocking filter, not a double stack here. This should work and still be safe for visual correct? Not sure it does much for the bandpass though at that f ratio.

[christian viladrich]

As said by Mark W, because the TO 0.9 A is placed in a f/7 focus, its effective FWHM becomes huge :

astrosurf.com/viladrich/astro/instrumen ... spaced.jpg

If it was a filter with a 1.56 index placed in a f/10 light beam, its FWHM would become larger than 3.4 A. Here, the situation is worse since the f-ratio is about f/7.

It is probably worth trying without the TO 0.9A to check whether there is a difference in uniformity or not.

The distance between the collimating lens of the Lunt and the final focus is probably also to be checked.