How I made my own Lunt mod

[krakatoa1883]

In this same section of the forum I read a discussion about a Lunt mod that seemed very easy to implement (viewtopic.php?f=9&t=22247). I asked some questions but I got no answer so I eventually decided to proceed at my own risk.

The receiving scope is my 150 mm f/5 achromat equipped with a sturdy Tecnosky V-Power focuser that allows for 190mm backfocus, quite a lot for most purposes. I calculated that transplanting the Lunt into it required about 150 mm of focus travel so it seemed feasible without modifying the OTA, that is by simply inserting the Lunt module in the 2" eyepiece holder.

The PT red module of the Lunt can be easily separated from the white tube. These pieces are joined through a M68 standard thread, male on the module, female on the tube. I purchased from TS the following adapters:

- a Baader M68 extension tube 20mm long
- a M68 female/M48 male ring adapter

For inserting the Lunt module in the eyepiece holder of the OTA I made use of a M48 (2 inch diameter) extension usually sold as the 28 mm finetuning ring for Baader Hyperion eyepieces. All pieces were assembled this way:

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luntmod2.jpg (341.18 KiB) Viewed 859 times

luntmod3.jpg (212.62 KiB) Viewed 859 times

Now one has only to put the Lunt module into the focuser

luntmod4.jpg (475.97 KiB) Viewed 859 times

As my Lunt is already provided with a FT focuser I obtained a 2 focusers system. This is of course not necessary, those who have the basic version of the Lunt can leave in place that horrible helical focuser in the full retracted position or to unscrew it and replace with a T2 Quark adapter that has (telescope side) the same thread of the focuser, then put on the T2 male thread (BF side) a 1.25" eyepiece holder. This solution would be preferable as increases stability.

Before to point the OTA to the Sun I placed in front of the objective a 110mm Baader D-ERF. As its free aperture is of 104mm this provides a f/7.2 cone light.

Trying to use the system I immediately discovered that the original Lunt BF diagonal is unusable in this way, it is very dark and provides a strongly uneven field that can't be corrected through flats. In fact to image the whole disc of the Sun I have always been forced to put the disc on a side of the sensor area to reduce the unevenness. I am planning to replace the original BF with a Coronado one, in the meantime I made use of my Quark Chromosphere to which I removed the internal telecentric as I prefer to use an external amplifier. The overall system results to be quite long

luntmod5.jpg (400.88 KiB) Viewed 859 times

but the first results, obtained in the very poor seeing of this very hot summer, are promising

test ar luntquark gimp 2.jpg (123.37 KiB) Viewed 859 times

There is still much room for improvements, of course, and I am now curious to try the Coronado BF, so this mod will probably keep me busy for several weeks

The mod is fully reversible and one can immediately reassemble the Lunt to its original state without effort. The cost of the mod, in this case, has been only that of the two above mentioned adapters.

Thanks for looking

[MalVeauX]

Really interesting! Thanks for sharing!

Very best,

[Carbon60]

Hi Raf,

An interesting mod. Looks promising.

Stu.

[marktownley]

Looks good. I've done the same thing for my Lunt50 to go on the back of my HaT. I can get a full disk but the sweet spot is tiny.

[krakatoa1883]

I am still experimenting trying to find the best setup. For the moment I have to use my Quark chromo, still looking for an alternative to the Lunt BF.

I restored my Quark to its original configuration because this would help me to reduce the length of the train compared to the use of an external amplifier, but with its native telecentric the Quark is a poor performer so I returned to a front Barlow. Best results I have obtained so far are with a Baader VIP Barlow set to a 4x amplification.

[krakatoa1883]

Making slow progresses as seeing still refuses to cooperate.

I have been coping with internal reflections due to the non-perfect alignment of some parts: using a Quark and an external Barlow adds some joints that make the (long) optical train less rigid. The sweet spot obtained with my setup - at least until I get a new BF - is not very large and if all components are not perfectly aligned the following appears on the screen:

luntmod8.jpg (120.22 KiB) Viewed 712 times

The problem is particularly severe when using of a 0.7x or 0.5x reducer but it can easily be cured. Quark and Barlow have been joined through more stable connection and I also replaced the 2" M48 threaded extension (Hyperion finetuning ring) that goes into the OTA focuser with a 2"/T2 adapter that has a well defined stop surface which prevents tilting of the system

luntmod7.jpg (142.81 KiB) Viewed 712 times

This eliminates the unwanted reflections but I still got Newton's rings that required a TS tilting adapter between Quark and camera

luntmod6.jpg (137.89 KiB) Viewed 712 times

The 1.25" eyepiece holder of the Quark is not suitable for this kind of work and should be replaced with something better as an auto-centering one, via the Quark-T2 adapter. eventually I removed also the FT focuser of the Lunt as the travel of the V-Power focuser installed on the 150mm is more than enough.

Unfortuntely the images I obtained so far are still unsatisfactory, I hope to do better in the fall season as the seeing generally improves in September.

luntmod9.jpg (127.97 KiB) Viewed 707 times

[krakatoa1883]

Seeing slightly improved today, and images too. Not my target, yet, but somewhat closer...

[TheSkyBurner]

you need to get an extension on that focuser so you are not using it. the problem with these mods is that the negative element needs to be fixed in a very specific spot with less than 1mm of error. Focusing is only done at the eyepiece.

You should put the lunt objective back on, and measure the exact location where the sun is focused BEFORE you put the blocking filter on.

This focal point is not going to change because it is the focal point on the refocus lens.

The second step is as simple as getting an image back at that same focal point from your new objective! You cant miss.

Your last image looks great , but it seems a little soft on the focus. (this may be an incompatibility issue with your objective because I believe the LS50 is a singlet lens, and this does have an affect when you choose a doublet to replace it.)

[krakatoa1883]

Thanks for your suggestion, you're certainly right. I will try this way and see if images improve.

[bart1805]

Nice mod Raf! I use a Lunt B1200 in my pst mod and it works really nice. What gain and gamma settings did you use and what was the resulting shutter time of your last images?

[krakatoa1883]

Sorry, Bart, I did not save the log of the video with gain and gamma settings, however the image is the stacking of 898 frames out of 1374 at 30 fps with Chameleon mono CCD.

[krakatoa1883]

This focal point is not going to change because it is the focal point on the refocus lens.
The second step is as simple as getting an image back at that same focal point from your new objective! You cant miss.

bingo. I surely overlooked this aspect of the mod. The Sun focuses at 150mm from the rear end of the red module in original Lunt50, keeping this distance when the module is inserted in the focuser of my achromat made a significant improvement indeed, thanks for reminding me of this point. I checked the setup only visually due to bad seeing but the contrast is really impressive. Hope to be able to image in the next two days.

Of course I can't just scale up the Lunt50, I have to accept some compromises, for example the f/ratio of my system is 7.2 instead of 7, I can't change it because the free aperture of the front ERF is 104mm. Moreover, as you noted, my achromat is certainly not optimized for H alpha.

A further improvement arrived when I put my Quark on the tail of the scope

luntmod13.jpg (198.74 KiB) Viewed 648 times

today I couldn't take any decent image, however this is a rough shot of a filament with the Lunt module only

luntmodonly.jpg (42.92 KiB) Viewed 648 times

and this with Luntmod + Quark

luntmodquark1.jpg (51.06 KiB) Viewed 648 times

just look at the contrast, the difference is noticeable.

[TheSkyBurner]

Looking forward to your progress, you may not need the extra erf since the ls50 has the ir cut rg650 on its nose for the time being. But, this is not for long duration imaging. The tube currents and temprature typically change the center wavelength on the pressure tuners

[krakatoa1883]

A beautiful filament appeared in the N hemisphere on days 27th and 28th. I taken an image with the mod @1780mm EFL, nothing special, I tried to highlight spicules around

[marktownley]

The mod is coming along nicely!

[krakatoa1883]

I am slowly progressing in trying to understand how to get good results from my mod. The following is an interesting region of the chromosphere close to center of the disc although there were no spots. Seeing wasn't good with strong wind but sky very transparent.

[TheSkyBurner]

Dude you hit it dead on, dont change it! Congrats

[marktownley]

Epic shots! Totally agree with Apollo!

[krakatoa1883]

Thanks Mark and Apollo, I have to say that this mod saved me lot of money for replacing my poor Quark with something better