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Pressure tuner behavior

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#1 drksky

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Posted 24 February 2013 - 05:04 PM

Is it normal with a pressure-tuned scope for it to fade on an off band? I was using my new LS80 today, finally, and I was seeing something like this. Like the tuning was undulating across the threshold of being on band. One minute I could see all kinds of surface detail and proms, the next all I saw was an almost featureless image.

Does temperature have any effect? While it wasn't really cold, about 30-35F, it did seem to stay on band the longer the scope was outside. But, that could have been my imagination.

I hate to think I've got a really expensive lemon, and I've got a DSII coming tomorrow. :(

#2 frolinmod

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Posted 24 February 2013 - 11:31 PM

Something like that happened to me when I received a new LS100. Right out of the box it appeared as if I was looking through floral patterned silk curtains. The field of view looked like a jigsaw puzzle of clear and blurry areas and the sweet spot was tiny and way off to one side rather than centered. The view as wonderfully fantastic through the good parts as it was horrible through the bad parts. Switching blocking filters from another scope didn't help, so it wasn't the blocking filter. I couldn't see anything on any of the internal optical surfaces. They appeared to be pristine. I returned the scope to Lunt for warranty repair. I had it back in my hands the very next week. They said they cleaned it.


#3 BYoesle

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Posted 25 February 2013 - 12:36 AM

In my experience temperature can have an effect - I left my scope off for a while once with the hot sun hitting the etalons form the side. Later when I aimed the scope back at the sun there was a definite region of the field of view where the image was on band, and the other half was off-band.

Therefore going from being inside @ 70 F to outside @ 30 F just might be the cause of the problem. I would be sure to leave the scope outside prior to use to reach ambient temperature. Then I’d have it aimed at the sun while tracking and have the temperature stabilized for at least 30 + minutes before making any definitive evaluation of performance.

#4 drksky

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Posted 25 February 2013 - 08:44 AM

Something like that happened to me when I received a new LS100. Right out of the box it appeared as if I was looking through floral patterned silk curtains. The field of view looked like a jigsaw puzzle of clear and blurry areas and the sweet spot was tiny and way off to one side rather than centered. The view as wonderfully fantastic through the good parts as it was horrible through the bad parts. Switching blocking filters from another scope didn't help, so it wasn't the blocking filter. I couldn't see anything on any of the internal optical surfaces. They appeared to be pristine. I returned the scope to Lunt for warranty repair. I had it back in my hands the very next week. They said they cleaned it.


But did it work right afterwards? :)

#5 Jim Lafferty

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Posted 25 February 2013 - 08:52 PM

Call Lunt--my guess is you may have a leak in the pressure tuner.

Jim

#6 frolinmod

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Posted 25 February 2013 - 09:07 PM

But did it work right afterwards?


Yes, perfectly. :cool:

#7 drksky

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Posted 26 February 2013 - 08:30 AM

Call Lunt--my guess is you may have a leak in the pressure tuner.

Jim


I thought about this, but wouldn't a leak manifest itself as the scope tuning on-band and then slowly going off-band as the pressure leaked out? That's not what I'm seeing. It's like the pressure is varying without my turning the tuning knob and the scope is undulating between on and off-band while I'm watching.

I just got a DSII unit yesterday. Is it interchangeable with the pressure tuner that came with the scope? That is, can I replace the stock pressure tuner with the DSII unit to see if it exhibits the same behavior? Or can the DSII only be used in concert with another etalon?

#8 frolinmod

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Posted 26 February 2013 - 01:18 PM

You can probably switch the pressure tuner knobs, but that's about it.

#9 BYoesle

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Posted 26 February 2013 - 02:13 PM

I just got a DSII unit yesterday. Is it interchangeable with the pressure tuner that came with the scope? That is, can I replace the stock pressure tuner with the DSII unit to see if it exhibits the same behavior? Or can the DSII only be used in concert with another etalon?


The DSII is designed specifically to work with the refocusing lens of the LS80. It would not work with the objective of the LS80 -- and I believe it has a smaller 35 mm etalon, verses a 50 mm etlaon of the main telescope. The air pressure tuner portion itself is most likely identical.

#10 drksky

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Posted 26 February 2013 - 03:11 PM

I've been corresponding with Rikki about missing thumbscrews that were supposed to be with my DSII (sigh), and I'm going ask about this behavior and get their opinion.

Really don't want to ship it back, but I supposed it's better to do so now while the weather is constantly lousy than in May or June.

#11 frolinmod

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Posted 26 February 2013 - 04:08 PM

Missing thumbscrews? Part number 92545A143, "Metric 18-8 SS Knurled-head Thumb Screw, M4 Size, 9mm Length, 10mm Head Diameter, 5mm Head Height" from McMASTER-CARR work well.

#12 drksky

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Posted 26 February 2013 - 05:03 PM

Missing thumbscrews? Part number 92545A143, "Metric 18-8 SS Knurled-head Thumb Screw, M4 Size, 9mm Length, 10mm Head Diameter, 5mm Head Height" from McMASTER-CARR work well.


Yeah, the instructions specifically mentions knurled thumbscrews that were supposed to be included with the DSII to replace the hex-head screws.

Eh, they're sending them to me. It's not like I'm going to be able to use the thing in the next week...again.

#13 bob71741

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Posted 26 February 2013 - 05:20 PM

" It's not like I'm going to be able to use the thing in the next week...again. "

Dark - The knurled thumb screws facilitate easy on/off; you can still use the DS80 w/the set screws, so no need to wait for the thumb screws.

#14 drksky

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Posted 27 February 2013 - 06:31 AM

I know. But I'm still going to wait to install the DSII until I get a chance to try it out as a single stack to see if I can reproduce the problem I originally started the thread for.

Talking with Rikki, I wasn't sure if it was "undulating" on and off band because I was chasing the tuning when it went off band. I'm going to tune it and then leave it alone and see what happens.

She said something about thermals being the cause of the detail loss, but I'm not buying that. I've never see any kind of seeing that could totally obliterate detail of an image for minutes at a time.

#15 frolinmod

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Posted 27 February 2013 - 12:36 PM

Unrelated comment: Note that the first few times you try it, you might find tuning with the DSII in place somewhat difficult. If so, please don't become disheartened by that. It's not really the n^2 problem it can seem to be at first.

#16 drksky

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Posted 27 February 2013 - 03:05 PM

Right now I'm just worried about keeping it on-band with just the single stack :)

The DSII is staying in the box until I get that sorted out.

#17 Jim Lafferty

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Posted 27 February 2013 - 04:18 PM

If its moving on and off band, something's up with the pressure being applied to the etalon.

Jim

#18 George9

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Posted 27 February 2013 - 08:10 PM

Your first description really did sound like thermals, in the sense that it is undulating. It does not sound like dirt or temperature or pressure leak because it is repeatedly going back and forth.

I have seen an entire disk go blank due to seeing, but I've never seen a significant prom actually disappear because of it. So I do understand your skepticism.

Good thought about chasing the tuning. That combined with a pressure leak could be possible.

I assume you have used it in multiple locations to eliminate some local seeing effect (like a chimney).

George

#19 drksky

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Posted 27 February 2013 - 09:29 PM

I would need to check my line of sight to see what I might have been observing over. Being as I'm in the middle of the town, it could be anything. If that were the case, it would really suck as I have two narrow windows where I can observe due to neighboring trees.

After seeing some pictures from others of the disk that day, I see that I was missing seeing significant details, so I'm still skeptical about the thermal theory.

#20 drksky

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Posted 04 March 2013 - 12:32 PM

Just a follow-up. I was finally able to get back out with the LS80 yesterday and what I was originally seeing must have been some local atmospheric disturbance as I did not see any similar issues this time out.

I also took the time to install the DSII unit which improved the view considerable. Although, there is still more "backscatter" than I'm used to than with a SMII 90DS. But, I have read that this is an issue with the etalons being so close together. It was distracting, but did not really affect the performance of the scope.

I was having some issues when I used the Flea 3 where the image seemed to fade off when on one side of the frame, but I don't know if that is the scope or the camera.

Plus, of course, the newton rings that I'm getting when using my 2.5X Powermate :(

#21 George9

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Posted 04 March 2013 - 01:39 PM

On the backscatter, there will always be some in the DSII, but make sure you minimize it by making sure the two etalons are parallel. There is some play in how you can attach them. See that the units are flat against each other. George

#22 drksky

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Posted 04 March 2013 - 04:09 PM

Thanks George. I'll make sure to take a closer look next time I have it out.






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