Thermal Acclimation of My Lunt50

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Thermal Acclimation of My Lunt50

Post by marktownley » Thu Apr 26, 2018 8:15 pm

Following the 'smashing' demise of one of the etalons from my trusty Coronado double stacked 40mm scope a year or so ago I was keen to get a new scope to do those quick full frame full disk shots that give that overview of old yellow face. After some careful choosing of a scope I settled on a Lunt50 - a good one that I was able to quality control before purchase and also compare to other scopes. A relatively timely process but worth doing.

Fast forward to several months of use and i'm starting to get familiar with the idiosyncrasies of this scope.

For me the most startling is that this scope really does need some decent thermal acclimation time before it works as it should do. Let me cite examples. If I just whip the scope outside pop it on the mount and observe / image there is quite clear sweetspotting which I can predictably move around the field of view via the pressure tuner. I've found if I leave the scope outside for a while and then observe / image the problem does not exist.

It's now that time of year when I can observe the sun before work if clear in the morning, so what I do is leave the mount in the back yard setup, and then in the morning quickly get the scope / camera out and do my thing. These past couple of days have been clear first thing, with temperatures outside low single figures compared to the high teens the scope is stored inside. The scope is virtually useless with a clear 'on-band' stripe that I can move radially inwards and outwards using the pressure tuner. That time of the morning I do not have time for the scope to acclimate before work (all fast imaging!) and leaving the scope out overnight is not something I want to do (I live in the middle of deep sh!t central!). By this afternoon after work the scope was performing wonderfully again being perfectly on band and no sweetspotting.

I have given the whole thing some thought before throwing this out there, but the only conclusion I can come to is a result of thermal acclimation. I'm wondering what peoples thoughts are or if anyone has any other suggestions.

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Re: Thermal Acclimation of My Lunt50

Post by pedro » Thu Apr 26, 2018 8:32 pm

Very interesting Mark. I was able to see the same pattern in a LUNT50 I tested (belonging to a friend)

The scope is virtually useless with a clear 'on-band' stripe that I can move radially inwards and outwards using the pressure tuner



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Re: Thermal Acclimation of My Lunt50

Post by christian viladrich » Thu Apr 26, 2018 8:55 pm

Very strange. I have no clue what's going on. I hope someone there will come out with a solution.


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Re: Thermal Acclimation of My Lunt50

Post by marktownley » Thu Apr 26, 2018 9:39 pm

My next step is going to be to try some kind of blocking over the objective, probably a DERF as i've got a few, see what effect that has. I'm sure that won't be my last experiment.


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Re: Thermal Acclimation of My Lunt50

Post by Ibbo » Thu Apr 26, 2018 10:26 pm

my Lunt 60 drifts with temp, its a continual tweak for the first 30 mins or so



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Re: Thermal Acclimation of My Lunt50

Post by marktownley » Fri Apr 27, 2018 5:08 am

Thanks Steve. I was talking to Mike Garbett about this and he reminded me how Walter used to say his Lunt100 used to need pointing at the sun for 20 minutes before he could use it, and this mirrors what i'm finding above.

Thing is in the UK it isn't always clear for 20 minutes :)


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Re: Thermal Acclimation of My Lunt50

Post by GreatAttractor » Fri Apr 27, 2018 9:53 am

I've also observed the need for an acclimation with my Lunt 50 when used in my mod (with a front D-ERF). I *think* I noticed that for some short time after pointing at the Sun I wouldn't get a detailed image, or that the etalon tuning would drift (sorry, I don't remember exactly). It's possible it was in early spring, with the scope kept in a cool anteroom overnight.

Since then my routine is to first point the scope at the Sun, enable tracking, and spend the next few minutes setting up and connecting the laptop and camera. Afterwards it's good to go.


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Re: Thermal Acclimation of My Lunt50

Post by Ibbo » Fri Apr 27, 2018 3:34 pm

marktownley wrote:
Fri Apr 27, 2018 5:08 am
Thanks Steve. I was talking to Mike Garbett about this and he reminded me how Walter used to say his Lunt100 used to need pointing at the sun for 20 minutes before he could use it, and this mirrors what i'm finding above.

Thing is in the UK it isn't always clear for 20 minutes :)
Don't I know it :evil: :evil: :evil:



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Re: Thermal Acclimation of My Lunt50

Post by christian viladrich » Fri Apr 27, 2018 6:09 pm

I was just wondering whether it could make sense to gently warm the etalon. For example with a dew heater set at its lowest.
Ideally, some kind of thermoregulation might help.


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Re: Thermal Acclimation of My Lunt50

Post by Valery » Sat Apr 28, 2018 12:46 am

Before I express my opinion/advice, first of all, I need to note that I have no experience of using pressure tuning etalons. So, I can't give any advice concerning use of this etalon type.

I would advice to use another etalon type - an ordinary front mounted etalon! Early years of these etalons Lunt always underlined their very low temperature sensitivity. And this was indeed so. Two my older front mounted etalos requires from very small to no tweaking to the CWL. Etalons wich are so close to the CWL normally do not show any noticeabe banding and are perfectly in band or with minor tilt. These my etalons are SM40 and LS35T Ha.
And they work same fine from subfreezing -10C to very hot >+35C. Blocking filters become a problem much more often. I have four of them. They work differently. At lower temperatures their CWL shifts blue and this can make the image lose contrast due to leacking of light from next to blue etalon transmission peak. My Coronado BF10 works fine even at -10C, but three Lunt (two B1200 and one B500) blokers work differently at lower temp.

I also need to remind that a 20mm etalon working with 50mm aperture never will deliver as wide sweet spot as a front mounted etalon can do.

So, IMHO, the only single best way out in your specific situation is to find such a front mounted etalon (or a telescope with such an etalon) which require (at your typical atmosphere pressure) a very very little tilt to come to the CWL. It must be basically on band and require just a half of a one revolve of a tuning wheel to come to exact CWL. You probably need also a BF which works best at lower temps.

Another way to go is to have a small refractor for your perfectly uniform Quark. 50mm F/8 refractor will give you F/34 at the Quark etalon and 15mm sun disk - prefectly within 20mm Quark etalon. Once you waked up, switch on the Quark at it's best position (for F/34) and it will be already ready for observing when you will be ready to observe.

Sorry for so long list of advices you have not asked for.

Valery.
Last edited by Valery on Sun Apr 29, 2018 6:39 pm, edited 1 time in total.


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Re: Thermal Acclimation of My Lunt50

Post by MapleRidge » Sat Apr 28, 2018 1:00 am

Hi Mark...

I leave my LS80T and the DSII modded refractor out in the observatory all year round and they are usually close to ambient air temp any time I go out to observe/image. I have never had an issue as you described...I may have to adjust the pressure a bit more than simple tuning if the temperature has changed significantly since the last session but that has been the limit of issues.

If the scope if coming out of the house or other 'warm' storage it certainly will take time to acclimate and if the scope is cooling once outside the pressure in the PT cavity will slowly decrease or increase if it warms in the sun. I could see both situations maybe causing grief.

Not sure if this helps, but my thoughts on the problem.

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Re: Thermal Acclimation of My Lunt50

Post by Derek Klepp » Sat Apr 28, 2018 8:34 am

Interesting read Mark as you can imagine down here there’s not much of a problem with thermal equilibrium but I do notice with my tilt tuned Lunt a warmer day brings better results.



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Re: Thermal Acclimation of My Lunt50

Post by krakatoa1883 » Sat Apr 28, 2018 11:41 am

Same issue with my own Lunt50, the scope shows a large sweet spot that becomes detectable in FD imaging. I can make the disc almost entirely on band by rotating the handle but this shall be done some time after the scope has been pointed to the Sun. I can't say whether it needs to be "acclimated" in the same sense of an ordinary telescope or if there is some internal component that needs to reach a uniform temperature. BTW, I used my scope even at subzero (°C) temperatures with fine results.

Regarding what Valery said above, one of the most stupid things I have ever done was to sell my previous LS35T for purchasing the LS50....


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Re: Thermal Acclimation of My Lunt50

Post by MapleRidge » Sat Apr 28, 2018 12:47 pm

Hi all...

I missed explaining the conditions that the LS80T is used in at my observatory...winter has been used in as cold as -26C and as warm as 35C in summer...nice comfortable range :lol:

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Re: Thermal Acclimation of My Lunt50

Post by GUS » Sun Apr 29, 2018 9:15 am

Mark, I have the same issue with both my Lunt scopes using Lunt BF's during winter,the image lacks any detail and can't be tuned with the pressure tuner( for at least half hour or so)If I use Coronado BF's with my Lunt scope's( I have three) I have no issue, a great image straight away. I found in my case, that both mirrors in the Lunt BF's were the cause, I removed the mirror in both and used the BF as a straight thru unit(just taped together temporarily) and the image looked great. Reinstalled the mirror and the image was crap. Removing the mirror back plate initially, the mirror was wedged tight in the BF. If I remove the backplate on a warmer day, the mirror falls out easy, the BF holder seems to be pinching the mirror.



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Re: Thermal Acclimation of My Lunt50

Post by marktownley » Sun Apr 29, 2018 11:00 am

Interesting folks, thanks for your input. Seems i'm not the only person to notice this...


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Re: Thermal Acclimation of My Lunt50

Post by Ibbo » Mon Apr 30, 2018 5:51 pm

I did wonder if striaght through would be better.
I might machine something up and give it a try.
Do the filters need to be a certain distance apart or does it not matter.



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Re: Thermal Acclimation of My Lunt50

Post by marktownley » Mon Apr 30, 2018 6:52 pm

Bit of an update. Had some clear breaks on the way home from work today so decided to try and image. Got back and put the scope on top of a warm radiator while I got set up. Bingo, worked perfectly from the off. Seems to me like this scope prefers to be on the warmer side.


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Re: Thermal Acclimation of My Lunt50

Post by Carbon60 » Mon Apr 30, 2018 9:02 pm

Very interesting, Mark. Good to know.

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Re: Thermal Acclimation of My Lunt50

Post by marktownley » Tue May 01, 2018 6:39 am

And this mornings update... On the radiator for 20mins which made it nice and warm, however back to the original problem. Next step is to rotate the OTA in the clamshell and see if that makes a difference - wonder if something is sat loose in the assembly? Comparing morning to afternoon the scope would have done a meridinal flip... Just thoughts for the experiment...


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Re: Thermal Acclimation of My Lunt50

Post by GUS » Tue May 01, 2018 9:33 am

If anyone is interested, there have been a couple of threads on CN regarding the use of dew heaters wrapped around the Lunt block filter to bring them on band in cold weather.



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Re: Thermal Acclimation of My Lunt50

Post by marktownley » Tue May 01, 2018 1:23 pm

I'm using a coronado blocker in this instance GUS, which in the past has been ok, I will try specifically heating the blocker beforehand though. Something else is going on here with the 50, bit of luck trial and error will get to the bottom of it.


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Re: Thermal Acclimation of My Lunt50

Post by Bob Yoesle » Tue May 01, 2018 1:36 pm

Sounds very familiar to the issues I had with the DSF100 SolarScope filters Mark. They sometimes would present a completely off-band continuum view, and could take up to an hour or more to get to showing a half way decent on-band view. They worked well after reaching thermal equilibrium, but they were a pain in the *@# for real-world use - at least where I live.

They benefited from use of a DERF for improving thermal stability (decreasing IR load - a 50 mm UV/IR filter should do nearly the the same thing), and the ability to get them to near ambient temperatures really helps. See figure 7 and discussion here: https://sunsteinlaw.com/media/Sidorin_solar_filters.pdf


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Re: Thermal Acclimation of My Lunt50

Post by marktownley » Tue May 01, 2018 7:03 pm

You know me Bob, i'm actively exploring the options and possibilities. I do very much suspect we are experiencing pretty much the same thing though...


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Re: Thermal Acclimation of My Lunt50

Post by marktownley » Sun May 20, 2018 11:30 am

I thought I would give a bit of an update with this.

I have tried a few things, but still come to no real conclusion. The one thing that repeats itself is that the scope has an off-centred annulus sweetspot that is present when observing 'cool' - which is morning, when the scope is nice and warm in an afternoon it is brilliant with a nice wide on band area - indeed here is a 90 minute animation I took with it a couple of weeks ago, rare to have such a long spell of sunshine in the UK!

Imageha-fd-colour-ani-sm by Mark Townley, on Flickr

The scope once on band stays on band. There is no leaking of pressure either. The lens was loose in the cell - like it rattled, and the erf housing was also loose. All this served to centre the sweet spot better. I think the scope just prefers it with a dose of photons for a while before it works. I need more time with it to try and eliminate some things.

In the mean time i've decided to explore some external 50mm etalon combinations, just waiting for some adaptors to arrive...


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