Lunt LS50+Visionking 80mm/F7.5 MOD And Questions...

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soulvagabond
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Lunt LS50+Visionking 80mm/F7.5 MOD And Questions...

Post by soulvagabond » Tue Jun 06, 2017 7:56 am

Got this Chineese 80mm refractor from Aliexpress, which apparently has good optics, but the rest is so so..
Almost a perfect candidate , for the Lunt 50 ethalon, so I did that.

Image
Image
Image
Image
Image

The result is, both positive and negative, let me explain:

Pros:
Obviously, more details visible
Higher contrast, more 3d perception, compared to stock Lunt LS50

Cons:
Little more reflections
Keyhole effect, and lower magnifications, much harder to position the eye now, for comfortable viewing.
With the full disk in view, the prominences at the sides are less visible, than with the stock Lunt.


So, my questions are rather concerning the last 2 cons.
Guess, the reduced eye relief is due to the focal distance of the eyepieces, or, due to the BF600 filter, being at the max limit, of the 600mm focal distance of the Visionking refractor.
Which 2 of the above is more important?
Which eyepieces could give less keyhole effect, at lower zoom?
What could affect the sides of the sun not being so much on-band as vs the stock Lunt? If the sweetspot is reduced, why it doesn't seem to affect the upper and lower borders of the sun?

Thanks!

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GreatAttractor
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Re: Lunt LS50+Visionking 80mm/F7.5 MOD And Questions...

Post by GreatAttractor » Sun Jun 11, 2017 2:07 pm

What could affect the sides of the sun not being so much on-band as vs the stock Lunt? If the sweetspot is reduced, why it doesn't seem to affect the upper and lower borders of the sun?
Mods like this are a trade-off: better resolution for a smaller sweet-spot relative to Sun disk size (etalon incidence angles are magnified due to longer donor's focal length; you can find detailed discussion elsewhere). The non-symmetric shape of sweet-spot could be caused by some misalignment in optical train.

As for the keyhole effect, I'm afraid nothing can be done. Using a larger blocking filter won't help (it won't improve the smaller sweet-spot). Simply, use a sufficiently short eyepiece that its whole FOV is on-band. For instance, I use a 9 mm Antares Ortho with my 660 mm f.l. donor (73x).
My software:
Stackistry — an open-source cross-platform image stacker
ImPPG — stack post-processing and animation alignment
My images

SW Mak-Cass 127, ATM Hα scope (D=90 mm, LS50 etalon), Lunt LS50THa, ATM SSM, ATM Newt 300/1500 mm, PGR Chameleon 3 mono (ICX445)

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Re: Lunt LS50+Visionking 80mm/F7.5 MOD And Questions...

Post by marktownley » Sun Jun 11, 2017 3:34 pm

Missed this one - looks a nice mod!
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!

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Re: Lunt LS50+Visionking 80mm/F7.5 MOD And Questions...

Post by Merlin66 » Sun Jun 11, 2017 5:25 pm

Questions:
How did you calculate the "majic" spacing for the Lunt etalon???
"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
https://groups.yahoo.com/neo/groups/ast ... scopy/info
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer

soulvagabond
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Re: Lunt LS50+Visionking 80mm/F7.5 MOD And Questions...

Post by soulvagabond » Sun Jun 11, 2017 8:20 pm

Set my lunt onto the mount, focused onto the Sun with 20mm eyepiece.
Unscrewed the ethalon with the diagonal and eyepiece from the scope.
Put the refractor, with the focuser removed, onto the mount instead.
Held the ethalon in the hands and tried to get the Sun in focus.
Thought I nailed it pretty accurate.
But, the adapter in the photos above, was off by 6mm. But, having it done, allowed me to do precise measurement, since, I didn't have to sweat anymore, trying to get the image in focus and aligned.
I have already ordered N.2 adapter, which is 6mm longer. Should be here Tomorrow.

Perhaps, an easier alternative, would be a 2-piece adapter, with draw tube design and the grub screws. Once you have the distance figured, you can cut off the excess.

Bear in mind, I'm not really sure, if this method really figures out the magical distance, but that's what my intuition lead me to.

Regards!

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