40mm Lunt front mount mod

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40mm Lunt front mount mod

Post by DavidP »

Hi all. I am considering a mod to be used with my 150 mm F/10 with a DERF and or a 9.25” Tri-band. I am considering using a Lunt 40 mm front mount Rafcamera adapter. I have never done a mod, but have a lot of experience building telescopes. I would appreciate any advice and direction or experience.
Last edited by DavidP on Thu Mar 16, 2023 2:31 am, edited 1 time in total.


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Re: PST vs Lunt front mount mod

Post by marktownley »

Assuming you are going to use it rear mounted, you will need the etalon, adapter, TZ3 and blocking filter.


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Re: PST vs Lunt front mount mod

Post by Sebastien Lebouc »

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C9 with ERF coating on schmidt plate.
Beloptik TZ3, lunt50 front body, BF30 then lunt35.
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lunt 35 in homemade aluminium body
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it performs also with AP130 refractor!

some key points:
telecentric beam at least FD 30, more is better, etalon contrast increases drastically.
in double stack, the more distance between etalons, the less ghost images. (I put the BF30 between)
for viewing, the more distance after the last etalon, the less ghost images in eyepiece (long tube and diagonal, or best, binoviewer)


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Re: PST vs Lunt front mount mod

Post by DavidP »

Thank you Mark. Yes it will be rear mounted. Since I have a Baader front mounted DERF, I assumed I would not need a Lunt blocking filter, perhaps just an additional 2” uv/ir filter downstream somewhere?
Sebastian. Thank you. That helps a lot. Are you saying that I want the telecentric before the etalon? I have a TZ3 (and 4).
I use a Solarspectrum 18. I don’t plan to use it together in this set up, but would my TZ-3 be used in the same way in regards to distances in relation to focus?
I am completely confused as far as what happens to the light cone diameter-wise as it passes the telecentric through the Lunt 40.


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Re: 40mm Lunt front mount mod

Post by DavidP »

I am also confused on why the 40 mm front double stack is so much less expensive than the 100/80mm double stack ( $650 vs $2000.) the DSII SFPT can’t have a significantly larger clear aperture. I realized that the ladder is pressure tuned. Is that it? What am I missing?


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Re: 40mm Lunt front mount mod

Post by marktownley »

You will definitely need to use a blocking filter with the Lunt idea.

Telecentric beam most definitely before the etalon. (Lunt or SS)

If you have a SS i'm not sure why you would want to go the Lunt route as you will get significantly better results with the SS.


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Re: 40mm Lunt front mount mod

Post by Sebastien Lebouc »

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David, you put the lunt40 exactly at the same place as your SS 18mm.
You can even put them in double stack, you get more contrast, it's still bright enough, and you don't need to buy a large BF (it is already inside the solarspectrum!!!)

yes, these lunt40 etalons are crazy cheap, clearly more than PST etalon, and they are full 40mm in diameter.
You can use them as they are.
For internal double stack module (with pressure system and pressure room between the 2 lenses of collimated beam), you can't screw off the lenses, and need to put the entire system inside a beam the same FD


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Re: 40mm Lunt front mount mod

Post by Sebastien Lebouc »

marktownley wrote: Thu Mar 16, 2023 6:36 am
If you have a SS i'm not sure why you would want to go the Lunt route as you will get significantly better results with the SS.
Mark, I am not so sure,for many reasons:
-David has a 18mm SS, so the field is in proportion. If he succeed to illuminate the entire etalon, he will get a drastically 4x larger field.
-etalons in telecentric beam just need tilt, no electricity to run the solarspectrum in its thermoregulated oven.
Solarspectrum filters are pure jewels, but very rare on market, need internal blockers regeneration every 5 to 10 years.

Of course David can use the two systems, they are very complementary.
Air etalon is brighter than solid one.
On my pictures, double stack of air etalon is still brighter than one solid etalon of my solarspectrum (and mine is bright...!!!)


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Re: 40mm Lunt front mount mod

Post by Bob Yoesle »

For tuning the etalon as proposed in a rear double stack arrangement it would be interesting to see if the DIY mechanical pressure tuning method would be superior to the built-in tilt tuning of the etalon...


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Re: 40mm Lunt front mount mod

Post by Oak »

If you use the Raf adapter, it will fit the etalon off the Lunt 40 dedicated scope with the white housing to possibly use with Bobs method. The etalon-only double stack that Lunt now sells will be too long to fit due to the coupler at the end. Thread size in the double stack etalon is probably the same but I haven't actually seen one in person and the installation method does not strictly speaking require it.


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Re: 40mm Lunt front mount mod

Post by DavidP »

Mark. I’m interested in this for a greater FOV. My RG18 is great but has a very small field.
Oak. When you say that the double stack etalon is too long, are you referring to Bobs pressure tune mod?


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Re: 40mm Lunt front mount mod

Post by Oak »

Just that the Raf holder is internally sized to fit the etalon off the dedicated 40mm scopewith the ERF/white housing in place and leave 2mm to spare at maximum tilt . If you want to use the slightly cheaper 40mm double stack-only unit from Lunt, the white housing and thread adapter at the end would be too long to fit.

It's no problem to remove the white housing and you may want to do that anyway to get rid of the extra ERF. However, Bob's way of compression tuning uses the etalon housing. If you want to potentially use this method, it would be easier just to have the shorter etalon. Does that make sense?

IMO, better to just pay the extra $100 for the full scope anyway and get a nice little grab and go. You will also need a big blocking filter.


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Re: 40mm Lunt front mount mod

Post by DavidP »

I didn’t realize that the RAF adapter was sized to fit the small scope’s primary etalon. I was under the impression it was for the double stack unit. I’m interested in Bob’s modification. Does anyone know if these have generally been successful?
It’s a shame Lunt requires you to order one of their smaller blocking filters with the scope. I already have a large blocking filter.


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