Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Question re. LS80THa thermal stability

  • Please log in to reply
29 replies to this topic

#1 Seldom

Seldom

    Surveyor 1

  • -----
  • topic starter
  • Posts: 1555
  • Joined: 05 Aug 2012
  • Loc: N of Cedar City Light Dome

Posted 03 June 2015 - 02:38 PM

I take my LS80THa out of the house (70F) and set it up in the sun (80F).  I pressure tune for most detail by turning the tuning cylinder all the way in, and backing off one or two turns.  I take some AVIs and take the laptop back to the house to transfer my movies.  When I come back outside (an hour and a half after setup) all the detail is gone.   The edge of the disk is crisply in focus, but it looks like a red version of a white light image.  I can then restore tuning by backing the tuning cylinder ALL the way out until I hear it vent and tighten it back to the point where I see detail. It's a minor nuisance, nothing more, but the scope manual says I should only need to reset the tuner rarely, and this is happening every day.  Should my tuner behave this way, and is it an indicator of something serious? 


Edited by Seldom, 03 June 2015 - 04:18 PM.


#2 bobhen

bobhen

    Mercury-Atlas

  • *****
  • Posts: 2635
  • Joined: 25 Jun 2005

Posted 03 June 2015 - 04:56 PM

I don’t see the need to turn the tuner “all the way in” and then backing out?  Why not just turn the tuner in until you see the most detail and leave it there?

 

Here is what I do with my Lunt 100…

 

Before I take the scope out, I remove the knob and reset the pressure. That way I’m sure the pressure will be “fresh” and ready to be tuned.

 

Next I take the scope out and start turning the pressure tuner knob inward until the best detail is visible. That’s it.

 

If the scope is out in the sun for an hour or more, I might have to adjust the tuner by half a turn or so until the best detail is again visible. I presume the retuning by a turn or so is because the sun’s heat is changing the pressure and this causes the need for a slight adjustment.

 

I’ve had my 100 for a year with no problems so far.

 

You could try tuning until you get the best detail then quickly take the scope inside but leave the tuner alone. Then set up 4 or 5 hours later or the next day. If the detail is the same then you don’t have a problem. If the detail is gone then you might have a pressure leak and that is causing the complete loss of detail over time – in that case you will need to call Lunt. But try changing your set-up procedure first.

 

Bob



#3 Seldom

Seldom

    Surveyor 1

  • -----
  • topic starter
  • Posts: 1555
  • Joined: 05 Aug 2012
  • Loc: N of Cedar City Light Dome

Posted 03 June 2015 - 05:46 PM

Thanks for the reply, Bob.

Best detail is "almost all the way in."  If I stop tuning before overshooting I'll never know how far short of perfect I am stopping. You say you're resetting the tuner daily at the start of every viewing.  The manual says you should only need to do this rarely.  Also, when you make your half turn retune, do you still have detail, or have you lost most or all of it?



#4 bill1234

bill1234

    Apollo

  • *****
  • Posts: 1033
  • Joined: 21 Apr 2011
  • Loc: NJ, USA

Posted 03 June 2015 - 07:49 PM

Here is another option....What I have always done is leave pressure tuner alone after each use.The next time I use it it is right on (or very close). I adjust as necessary and only re pressurize when I see my adjusting range move in... ( seldom) ...For me at sea level, right on is with about 4 threads showing. Lunt  told me keeping  pressurized  will do no harm, so why let in moisture and air-born contaminates regularly?



#5 Seldom

Seldom

    Surveyor 1

  • -----
  • topic starter
  • Posts: 1555
  • Joined: 05 Aug 2012
  • Loc: N of Cedar City Light Dome

Posted 03 June 2015 - 11:10 PM

For me at sea level, right on is with about 4 threads showing.

Good point, Bill.  I'm at 5700 feet.



#6 nickatnight

nickatnight

    Skylab

  • *****
  • Posts: 4374
  • Joined: 13 Apr 2008
  • Loc: Santa Clarita, CA (LA Suburbs)

Posted 04 June 2015 - 01:02 AM

Interesting. I never heard of the "screwing it all the way in" method until now. I have heard people mention their positioning of the tuner leaves threads exposed, when mine, even after burping, requires screwing in past the thread a good bit. I also found when removing the tuners plunger to burp, that there does not appear to be much grease on mine, and more like a graphite looking residue. I was thinking of cleaning and re-greasing, but I'd have to research grease and all first, and as there isn't any issue with my scopes performance (as far as i know as a newbie), I'm holding off on that for now.


Edited by nickatnight, 04 June 2015 - 01:02 AM.


#7 bobhen

bobhen

    Mercury-Atlas

  • *****
  • Posts: 2635
  • Joined: 25 Jun 2005

Posted 04 June 2015 - 07:19 AM

Thanks for the reply, Bob.

Best detail is "almost all the way in."  If I stop tuning before overshooting I'll never know how far short of perfect I am stopping. You say you're resetting the tuner daily at the start of every viewing.  The manual says you should only need to do this rarely.  Also, when you make your half turn retune, do you still have detail, or have you lost most or all of it?

 

Yes I still have detail - just not perfect.

I'm in PA (near sea level) and sometimes between sessions can be more than a few days - like this week.

My best detail seems to be with 3-5 turns left.

I would experiment and keep notes and if you still have a problem call Lunt. 

 

Bob



#8 George9

George9

    Apollo

  • *****
  • Posts: 1427
  • Joined: 11 Dec 2004

Posted 04 June 2015 - 09:00 PM

Seldom, your first description just sounds like your tuner is leaking a little. You set up and an hour and a half later you need to vent to get it to work again. Do you find that after 30 minutes, you need to tighten it further to stay at the same detail? That would confirm it.

 

Also the fact that if you vent at the beginning of the session and then re-vent it 1.5 hours later, you should not be hearing any air moving because it has been too short a time for a pressure differential. Again it points to a leak.

 

Could be as simple as the rubber seal or could be something else. I am sure that Lunt would take care of it if there is a leak.

 

I can leave mine a month without venting.

 

George



#9 Seldom

Seldom

    Surveyor 1

  • -----
  • topic starter
  • Posts: 1555
  • Joined: 05 Aug 2012
  • Loc: N of Cedar City Light Dome

Posted 04 June 2015 - 09:54 PM

George, that was my thinking too.  I spoke to Faye at Lunt today and the first thing she asked me to do was fully remove the tuner cover, not just burp it.  If that doesn't work, she said Lunt will come up with another fix.



#10 Seldom

Seldom

    Surveyor 1

  • -----
  • topic starter
  • Posts: 1555
  • Joined: 05 Aug 2012
  • Loc: N of Cedar City Light Dome

Posted 04 June 2015 - 09:59 PM

Interesting. I never heard of the "screwing it all the way in" method until now. I have heard people mention their positioning of the tuner leaves threads exposed, when mine, even after burping, requires screwing in past the thread a good bit. I also found when removing the tuners plunger to burp, that there does not appear to be much grease on mine, and more like a graphite looking residue. I was thinking of cleaning and re-greasing, but I'd have to research grease and all first, and as there isn't any issue with my scopes performance (as far as i know as a newbie), I'm holding off on that for now.

I think the screwing it in all the way business may be a requirement of altitude.  Where I am air pressure's 70% of sea level. That could mean that I need to squeeze out another 30% more volume with my tuner cap.  I'll ask Faye if they have an upper altitude limit next time I speak with her.



#11 George9

George9

    Apollo

  • *****
  • Posts: 1427
  • Joined: 11 Dec 2004

Posted 04 June 2015 - 11:03 PM

George, that was my thinking too.  I spoke to Faye at Lunt today and the first thing she asked me to do was fully remove the tuner cover, not just burp it.  If that doesn't work, she said Lunt will come up with another fix.

 

Yes, if there is a small piece of something stuck on the seal then that could cause the leak. Completely removing the cover could dislodge it. George



#12 BYoesle

BYoesle

    Fly Me to the Moon

  • *****
  • Posts: 6343
  • Joined: 12 Jun 2004
  • Loc: Washington USA

Posted 05 June 2015 - 01:05 AM

 

I think the screwing it in all the way business may be a requirement of altitude.  Where I am air pressure's 70% of sea level. That could mean that I need to squeeze out another 30% more volume with my tuner cap.

 

That is indeed the case.

 

The etalon math for transmission wavelength is λp = 2 n t cos θ

 

etalon wavelength formula.jpg

 

Where:

n = refractive index of gap
t = gap thickness
θ = angle of incidence through the etalon

 

For an air pressure-tuned etalon, increasing the gap pressure (air density) increases the refractive index n.  Increasing n therefore shifts the peak wavelengths to the red.  As you go up in altitude, air pressure will decrease, and the gap refractive index n will decrease and the peak wavelengths will shorten toward the blue.

 

A pressure-tuned etalon is therefore designed to be on-band at a high air pressure (low altitude), and this pressure must be maintained (thanks George ;-) to keep the etalon on-band.  I believe the Lunt pressure-tuned etalons are actually designed to be on band at below sea level.  The gap-pressure needs to be increased at sea level, and the greater the altitude, the greater volume compression needs to be accomplished to keep the pressure of the gap stable and the filter on-band at the proper wavelength.

 

Also note that as the angle of incidence θ through the gap increases, the cosine will decrease, and consequently so will the transmission wavelengths (e.g. they get “bluer”), demonstrating why tilt-tuning can only shift the etalon blue-ward.  This therefore shows why a tilt-tuned etlaon is designed to be on-band at a high altitude.  As one decreases altitude, the gap pressure increases, n increases and the filter transmission peak(s) will shift to the red, and tilting the etalon shifts the peaks blue-ward so it can remain on-band.

 

Incident angle θ also explains the origin of the "sweet spot" and "banding" -- as the field angles and instrument angles through the etalon increase, the rays passing through the filter begin to shift blue-ward and can go off-band...


  • abberation likes this

#13 Seldom

Seldom

    Surveyor 1

  • -----
  • topic starter
  • Posts: 1555
  • Joined: 05 Aug 2012
  • Loc: N of Cedar City Light Dome

Posted 05 June 2015 - 08:56 AM

So, Bob, does that mean I would be better off with tilt tuned etalons?



#14 BYoesle

BYoesle

    Fly Me to the Moon

  • *****
  • Posts: 6343
  • Joined: 12 Jun 2004
  • Loc: Washington USA

Posted 05 June 2015 - 09:12 AM

No -- tilting introduces the "banding" issue for internal etalons, and they have a much more restricted range of altitudes (unless you know you will only use it at your current altitude).

 

As George states, it sounds like you might have a leak issue.

 

To get more tuning range (if that is the issue) Lunt should be able to address this for you.  If you have a bit of Macgyver in you  :smash: ,  you can find solutions for increasing the available pressure -- say with a three-way stopcock valve between the etalon chamber and the pressure adjuster to supply a higher intial pressure, and use the OEM PT to add a bit more and fine-tune the final pressure.  Or you might also consider this:  http://www.cloudynig...5040-lunt-pc-1/  -- check with Lunt on its pressure range.



#15 Seldom

Seldom

    Surveyor 1

  • -----
  • topic starter
  • Posts: 1555
  • Joined: 05 Aug 2012
  • Loc: N of Cedar City Light Dome

Posted 05 June 2015 - 09:32 AM

Thanks again, Bob.  If it clears up here I'll be able to see if the "back the pressure chamber all the way out" worked.  I should note that I got my double stack filter yesterday, and it's pressure chamber takes much more effort to turn than the original chamber furnished with the OTA.



#16 nickatnight

nickatnight

    Skylab

  • *****
  • Posts: 4374
  • Joined: 13 Apr 2008
  • Loc: Santa Clarita, CA (LA Suburbs)

Posted 05 June 2015 - 04:32 PM

I think you're supposed to add an exclamation point after you announce the arrival of a double stack filter. Take me for example: today I ordered a DS filter for my LS50! Now tell us about the DS views with that LS80!
  • saguaro likes this

#17 Seldom

Seldom

    Surveyor 1

  • -----
  • topic starter
  • Posts: 1555
  • Joined: 05 Aug 2012
  • Loc: N of Cedar City Light Dome

Posted 05 June 2015 - 05:34 PM

Well, nick.  Still sliding down the learning curve. Like the man with two watches who never knew what time it was, I'm not sure how to manage each pressure cover to be sure I've really got it optimized.

 

This is what I got today double stacked on AR2356, with 5x Powermate.  Weather wasn't ideal, puffy clouds. And I don't think it's as good as the single stacked hot blonde AR2356 which was also a 5x Powermate  found here.

 

YellowSunAR2356_6_5_15_MosaicReg1_RCurves-Wavelets copy.jpg  

 

Double stack darken's the disk nicely though.  This is with the 2.5x Powermate.

YellowSun_2015_06_05_1810_4_L_24_g6_ap882_conv_R.jpg


  • BYoesle likes this

#18 nickatnight

nickatnight

    Skylab

  • *****
  • Posts: 4374
  • Joined: 13 Apr 2008
  • Loc: Santa Clarita, CA (LA Suburbs)

Posted 05 June 2015 - 06:07 PM

I hadn't heard the man with two watches tale, but I like and fully understand the analogy. I had wondered about the dual tuning issue too as I'm never fully confident it's spot on. Images look very good though, and I'm sure it will get easier tuning both with practice. Thanks for the DS update.

#19 Paul Hyndman

Paul Hyndman

    Apollo

  • -----
  • Posts: 1063
  • Joined: 13 Jul 2004
  • Loc: Connecticut Shoreline USA

Posted 05 June 2015 - 08:46 PM

I think you're supposed to add an exclamation point after you announce the arrival of a double stack filter. Take me for example: today I ordered a DS filter for my LS50! Now tell us about the DS views with that LS80!

 

The past two years, George (George9) presented some of the best views at the NEAF SSP with his bino-viewed DS LS80... of course George has a master touch when it comes to tuning (and he has an uncanny knack for instantly sizing up viewers interpupillary distance... it's almost spooky!)

 

You're gonna' love your Lunt even more with the DS module! :bigshock:

 

Paul



#20 Seldom

Seldom

    Surveyor 1

  • -----
  • topic starter
  • Posts: 1555
  • Joined: 05 Aug 2012
  • Loc: N of Cedar City Light Dome

Posted 05 June 2015 - 10:27 PM

Thanks, Paul.  I'm feeling better already.  Today's image of AR2356 posted above was processed from a convoluted AutoStakkert output.  The one below is processed from an unconvoluted output.  Not perfect but a lot better.

 

YellowSunAR2356_6_5_15_MosaicNoconvReg1_RCurves.jpg

 

I should add that based on the recommendations in your solar tutorial, both were exposed by centering the red channel of my ASI120MC's histogram in FireCapture, and leaving the green and blue histograms off to the left.



#21 BYoesle

BYoesle

    Fly Me to the Moon

  • *****
  • Posts: 6343
  • Joined: 12 Jun 2004
  • Loc: Washington USA

Posted 06 June 2015 - 08:15 AM

Looking good!  The limb image looks almost perfect with only the slightest hint of the "double limb."



#22 bill1234

bill1234

    Apollo

  • *****
  • Posts: 1033
  • Joined: 21 Apr 2011
  • Loc: NJ, USA

Posted 06 June 2015 - 11:08 AM

"The past two years, George (George9) presented some of the best views at the NEAF SSP with his bino-viewed DS LS80... of course George has a master touch when it comes to tuning (and he has an uncanny knack for instantly sizing up viewers interpupillary distance... it's almost spooky!)"

 

 

I agree and wish I had a video of his tuning  method. He turns both tuners at the same time,changing directions,turning together and in opposing directions and manages to get both tuned just right, and it only takes about 10 seconds . 



#23 George9

George9

    Apollo

  • *****
  • Posts: 1427
  • Joined: 11 Dec 2004

Posted 06 June 2015 - 09:13 PM

Thanks, Paul and Bill (I learned a lot from Paul, whose 90DS is amazing, and have been co-learning with Bill each year at NEAF, who donates his time the Lunt scopes).

 

Seldom, it is mostly just a matter of practice. I've been at it a while and I still keep learning new things. One hint is that there are more degrees of freedom to adjust than you realize. I am only visual, so my list is a little different from an imager, but here goes: tuning each of the tuners (obviously), whether the tuners are on the same side or opposite side or elsewhere, the exact angle between the DSII unit and the telescope body (there is some play possible as you tighten the thumb screws), use of the new anti-reflection filter, which eyepiece (for visual), whether you use a binoviewer, and whether you break from Lunt advice and put the single-stack spacer between the DSII and the telescope body.

 

I do tune them both at the same time. I keep my tuners on opposite sides. I first open and close them both (which means moving my hands in opposite directions because the tuners are on opposite sides), sweeping up- and down-band to find the highest contrast. Then I move my hands in the same direction (opening one while closing the other), looking for maximum brightness, and repeat once or twice. But any way you get there is fine. Part of it is just knowing what it is supposed to look like.

 

Here is my current approach:

 

If I am using a single eyepiece, I do put the antireflection filter in. It's a little dimmer but the sky is black and the scope is relatively insensitive to the other tweaks. It just works. The LS100 DSII at NEAF this year used the anti-reflection filter, and it was great, especially because the LS100 is a bright scope.

 

If I am using a binoviewer, then I skip the antireflection filter to maximize brightness. Then it becomes important to get everything else perfect. Mainly adjusting the exact angle of the DSII unit against the telescope body to get the best view.

 

If I am going for high power (140x), especially on proms, then I skip the antireflection filter (to maximize brightness), but I put the single-stack spacer between the telescope and the DSII unit. This makes the reflections much worse, so I use a few layers of paper as a shim to force a tilt between the spacer and the telescope body. This pushes the reflections to one side or another of the sun. The side without reflections is sharp, high contrast, and with a black sky. (Remember this is high power, so the reflections are out of the field of view.) The view is also relatively bright given the power. Partly because in the normal configuration, the DSII vignettes the light cone a little, and with the spacer it doesn't. The question is whether those outer couple of millimeters add more brightness or add aberrations. In my experience, high power ends up better. On proms it is worth switching it around for that added brightness. For disk details, it is probably not worth the effort and is not clearly better.

 

Good luck, and have fun.

 

George


Edited by George9, 06 June 2015 - 09:19 PM.

  • saguaro likes this

#24 Seldom

Seldom

    Surveyor 1

  • -----
  • topic starter
  • Posts: 1555
  • Joined: 05 Aug 2012
  • Loc: N of Cedar City Light Dome

Posted 06 June 2015 - 09:46 PM

 and whether you break from Lunt advice and put the single-stack spacer between the DSII and the telescope body.

Thanks, George.

 

I noticed that with the DII in place the focuser travel is short by about the depth of the spacer.  Haven't tried re-inserting it yet.  Are there any downsides besides having to re-cut all the foam in the carrying case?

 

And thanks for the suggestion about the paper shims.  Would that also work for Newton's rings?  I've noticed them when I'm focusing but they seem to be gone by the time I get into focus.


Edited by Seldom, 06 June 2015 - 09:46 PM.


#25 George9

George9

    Apollo

  • *****
  • Posts: 1427
  • Joined: 11 Dec 2004

Posted 07 June 2015 - 09:58 AM

Yes, in certain configurations, my Feathertouch is blocked from closing all the way. I use a Denkmeier OCS on the end of my blocking filter, which acts as a Barlow and lets me come to focus with the binoviewer. I leave it there even for single eyepieces, so I am generally needing to rack the focus out rather than in, and the interference with the Feathertouch movement does not affect me.

 

I expect shimming could affect Newton's rings. But if the in-focus image lacks them, then I might not worry. I had Newton's rings visually with single eyepieces (in focus) but not with the binoviewer. Lunt had me send them my blocking filter and they replaced it, and I never saw the rings again. My first blocking filter was fine, I am sure. It is just that when you put all the parts together, they can produce various artifacts. I hadn't tried changing the tilt for that. I didn't even notice them for the first year because I almost always used the binoviewer.

 

By the way, just looking at my previous post, one might ask, why can't this be easy? My answer is that it is easy if you are not picky. If you are picky, then you need to remember that we are talking about a 0.5 Angstrom filter. People pay $200 for a 10nm (100 Ang) Lumincon nighttime filter that is just has a piece of glass, deposited film, and a metal ring. An H-alpha filter is 1/200th as wide, is still supposed to transmit a near-perfect wavefront, comes with an objective and focuser, and costs $1000-$10,000. That is ridiculously cheap. It should cost $50,000-$100,000 based on the cost of laboratory instruments. The tradeoff is that they are a little finicky if you want a near-perfect view.

 

George




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics