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Blocking filters and wintertime

Posted: Sat Sep 07, 2019 4:39 am
by DeepSolar64
All,

Does temperature affect the performance of blocking filters? I know Lunt makes a heater for theirs. I read somewhere that the filters work poorly below 60 degrees. Is that true? I observe the sun daily and it would suck not to be able to use the Coronado in the wintertime. Would the Lunt heater fit the blocking filter of the Coronado? I could also use a dew strip or maybe wrap something around the blocking filter diagonal to insulate it. Or maybe use a blow dryer to keep it warm??

James

Re: Blocking filters and wintertime

Posted: Sat Sep 07, 2019 7:33 am
by marktownley
You'll clearly see whether the temperature is having a detrimental effect on you blocker, if it is use a dew strip if you have one, alternatively go for the Lunt heater, both will do the same thing.

Re: Blocking filters and wintertime

Posted: Sat Sep 07, 2019 8:58 am
by Merlin66
Interesting subject....
What documented evidence for the temperature effect on blocking filters????
Ken

Re: Blocking filters and wintertime

Posted: Sat Sep 07, 2019 1:49 pm
by MapleRidge
Hi all...

In my experience, the cold hasn't made an effect on the blockers (Ha or CaK) in the cold. I've posted images taken during the winter in airtemps down to -26C with no heat applied. This is the extreme, but -5 to -15C is typical in the winter.

I think the bigger concern is taking the cold filter into a warm storage after use and getting condensation on (inside?) the elements. Since I have my equipment in an observatory, I leave it out year round as close to the ambient temp as I can. Of course, this may not work for everyone. I will have to look at Lunt's website to see the heaters referred to.

Brian

Re: Blocking filters and wintertime

Posted: Sat Sep 07, 2019 3:27 pm
by DeepSolar64
I wonder how the Lunt heater attaches to the blocking filter, and since my scope is a Coronado will it work?

I have wondered about condensation too, since I have read reports of rust on Coronado blocking filters. Does Lunt have that issue?

And does the cold have an effect on the etalons?

James

Re: Blocking filters and wintertime

Posted: Sat Sep 07, 2019 11:03 pm
by Merlin66
Re Blocking filter failures....
It's the built in ITF ( the front element) which fails.
Both the Coronado and Lunt ITF fail....due to temperature variation affecting the bonding/ seal and moisture ingress.
It's recommended that the blocking filters be stored in a seal container with silica gel when not in use......

Re: Blocking filters and wintertime

Posted: Sun Sep 08, 2019 12:52 am
by DeepSolar64
I have a small foam lined case just big enough to fit the blocking diagonal. Now to find some silica gel.

Re: Blocking filters and wintertime

Posted: Sun Sep 08, 2019 1:25 am
by Merlin66
Hmmm
Some Lunt(?)(don't know about Coronado) users are mentioning on CN that the blocking filter seems to loose it's CWL at low temperatures...
Hence, I suppose Lunt marketing a $179 heater....
This is something I've never heard before...there will be some residual heat coming through the scope to warm the blocker (?) and what exactly causes the mentioned CWL shift?

See:https://www.cloudynights.com/topic/6747 ... ng-module/

Re: Blocking filters and wintertime

Posted: Sun Sep 08, 2019 1:34 am
by Merlin66
Hmmm
Found this reference:

""Change due to temperature
Temperature changes affect a filter's performance due to thermal expansion and contraction of the materials used to construct them. Most filters are designed and specified for operating at 23°C, and deviations from this value will produce peak wavelength shifts approximately linear with temperature. The exact shift coefficient will depend on the particular design wavelength of the filter, and, typically, ranges between 0.15Å and 0.20Å per °C at 4000Å and 8000Å respectively. Bandwidth and peak transmission changes observed are relatively minor, of the order of 0.01Å per °C and 0.013Å per °C respectively.""

See:http://www.ing.iac.es/astronomy/filters/shift.html

Re: Blocking filters and wintertime

Posted: Sun Sep 08, 2019 1:57 am
by DeepSolar64
It was, I think, on the Cloudy Nights forum where I too read about blocking filters having issues with cold temperatures. Winter is coming so I will have to address this.

What does CWL stand for?

Re: Blocking filters and wintertime

Posted: Sun Sep 08, 2019 2:00 am
by Merlin66
CWL = Central wavelength
This is important to register/ align the filter elements at the same wavelength.

Re: Blocking filters and wintertime

Posted: Sun Sep 08, 2019 2:05 am
by DeepSolar64
That was my guess. Thanks Merlin

Re: Blocking filters and wintertime

Posted: Mon Sep 09, 2019 7:08 pm
by george9
My Lunt LS80’s B1800 blocking filter begins to show effects at 45F, is noticeably worse at 35F, is not very good at 25F. A 3” dew heater solved the problem.

I have not measured whether it is a CWL shift or other induced aberration. The effect is loss of contrast and dimming.

I have a new B1800 so I am curious if it will also be affected by the cold.

George

Re: Blocking filters and wintertime

Posted: Tue Sep 10, 2019 5:20 am
by DeepSolar64
Thank you George. I need to know what to expect. And need to find a way to counter it. Fortunately winter days are not as cold as winter nights. In January where I live in the mountains of North Carolina we average a high of about 45 degrees in January. But still, below freezing days do happen. I gotta prepare for winter solar observing. In all ways!

Re: Blocking filters and wintertime

Posted: Wed Sep 11, 2019 7:58 am
by mdwmark
The temperature of the blocker is important. You need to match it up with the etalon so that it will block the next peaks.
Air spaced etalons use an blocker that is 6-8Ang at the HW. If they are at 6563Ang at 23C for the CW. Then when it is 43C the CW has moved to 6566.3Ang into the red. So Now you maybe using the edge of the bandpass. The same works if it is cold. Instead it will be on the blue side by 3.6Ang. This can drop the transmission of the overall filter system. It is the same with solid etalons. You needs to match the blocker to the operating temperature of the etalon. At Ha the blocker moves almost twice as much in wavelength than a solid etalon does with the same temperature change. These numbers are for standard blockers.
Mark W.

Re: Blocking filters and wintertime

Posted: Wed Sep 11, 2019 8:24 am
by Merlin66
Mark,
Thanks for that.
I’d be interested to know what the operating temperature of the blocker is v’s ambient temperature during a normal observing session??
Ken

Re: Blocking filters and wintertime

Posted: Tue Sep 17, 2019 9:41 am
by Montana
This is very interesting as I always have a problem getting my Solarscope on band in February when it is usually around zero degrees, all through the summer it works very well but the frustration in February maybe that it is too cold. Interesting.

Alexandra

Re: Blocking filters and wintertime

Posted: Thu Sep 19, 2019 5:33 pm
by DeepSolar64
Alexandra, All,

I am curious how my Coronado SMII 60 is gonna do at freezing temperatures. It's coming. It would be nice to find an easy way to alleviate it.

All of what I have read is the cold temperature affecting the blocking filter. Does it have any affect on the etalons themselves?

James