Jump to content


Photo

"Factory Matched" Double Stacking?

  • Please log in to reply
19 replies to this topic

#1 Gene Baraff

Gene Baraff

    sage, 1930-2010, Rest In Peace

  • -----
  • Posts: 265
  • Joined: 22 Mar 2009
  • Loc: Berkeley Heights, N.J.

Posted 27 June 2009 - 02:58 AM

Front-mounted solar etalons - such as the Coronado SM 40 or 60 and the Lunt models - are used with a blocking filter close to the eyepiece.

I understand the function of the blocking filter: it is to remove all but one of the evenly-spaced series of wavelengths transmitted.

These front mounted filters can also used be used "Double Stacked", which means that another etalon of the same (or closely the same) construction is placed just ahead of the first one. The two, used in series, transmit a narrower band of wavelengths than does the single one.

Often, when the double-stacked set up is advertised, it is advertised as "Factory Matched". This term is understood to mean that the second etalon is selected from a bunch of second etalons on hand to work especially well with the first etalon and the two are shipped out together.

Does anybody know what factors go into the choice of the "factory matched" second etalon? How would one recognize (as an observer using the system) that the double-stacked etalon had or had not been "factory matched".

Thanks in advance.

Gene Baraff

#2 brianb11213

brianb11213

    Cosmos

  • *****
  • Posts: 9047
  • Joined: 25 Feb 2009
  • Loc: 55.215N 6.554W

Posted 27 June 2009 - 05:54 AM

Here's my understanding. I may be a bit off target here so please feel free to correct me.

Double stacking works because the two etalons used are "tuned" slightly differently, the overlap between the transmissions gives the narrower bandwidth of the combination. Tuning is often done by tilting the etalons, if the etalons were identical they'd be tilted almost but not quite the same amount & you'd be liable to have an awful problem with Newton's Rings and/or double imaging from the reflections between the rear surface of the front etalon assembly and the front surface of the back one. So the etalons need to be tipped at a fair angle to each other when in normal use, this means that they need to be manufactured with slightly different centre frequencies.

Now manufacturing an etalon which works is a hard enough process, making them with specific centre frequencies is harder still, so the best way to get a usable double stack is to select two which are at opposite ends of the tolerance band in terms of centre frequency and tune them in such a way that the "short" one is being used fairly "flat" but the "long" one is tipped more than usual making it longer still ... and still have the "long" tail of the frquncy response of the "short" etalon overlapping with the "short" tail of the "long" one so that enough light gets through the stack to be useful.

#3 colinsk

colinsk

    Mercury-Atlas

  • *****
  • Posts: 2610
  • Joined: 17 Jan 2008
  • Loc: CA

Posted 27 June 2009 - 11:13 AM

I think the best image is when both etalons are tuned to the exact same center frequency. But the rest of your discussion is escentially correct. Tipping the etalons in oposite rotations helps with the interference between them as well.

#4 Wes James

Wes James

    Fly Me to the Moon

  • *****
  • Posts: 5504
  • Joined: 12 Apr 2006

Posted 27 June 2009 - 01:48 PM

The idea of double-stacking is to reduce bandwidth. Taking filters at extreme ends of the tolerance range and stacking them is going to serve to widen/flatten the overall response, not narrow it. Taking two that are at the exact same center frequency- even though it may not be the target frequency- will serve to sharpen up the bandwidth. It will make for steeper sides- but a flatter peak. Same principles apply in electronics.

#5 Jim Rosenstock

Jim Rosenstock

    Fly Me to the Moon

  • *****
  • Posts: 6590
  • Joined: 14 Jul 2005
  • Loc: MD, south of the DC Nebula

Posted 28 June 2009 - 10:35 AM

Gene,

I'm guessing the best evidence of "factory matching" would be good performance! :D

Jim

#6 Gene Baraff

Gene Baraff

    sage, 1930-2010, Rest In Peace

  • -----
  • Posts: 265
  • Joined: 22 Mar 2009
  • Loc: Berkeley Heights, N.J.

Posted 28 June 2009 - 12:37 PM

Gene,

I'm guessing the best evidence of "factory matching" would be good performance! :D

Jim


Thanks Jim, but my post was (in a high-minded sense) something of a fishing (not trolling) expedition.

I was hoping that somebody would bite by explaining the they go about selecting - whether by watching where the ghost images come in, or where the contrast enhancement is maximum, or something along those lines.

Several responders nibbled around the edge of that area, and I do thank them. But nobody came out and said definitely: "this is how they know they have achieved a match".

My reasons for wanting this information are not at all subversive, but what they are will have to wait for a different thread.

Gene

#7 DesertRat

DesertRat

    Fly Me to the Moon

  • *****
  • Posts: 5193
  • Joined: 18 Jun 2006
  • Loc: Valley of the Sun

Posted 28 June 2009 - 01:20 PM

My reasons for wanting this information are not at all subversive, but what they are will have to wait for a different thread.



Gene, search through the archives of this forum as well. There have been some lengthy discussions on this topic (how the matching is done and installation procedures) by knowledgeable people.

Glenn

#8 colinsk

colinsk

    Mercury-Atlas

  • *****
  • Posts: 2610
  • Joined: 17 Jan 2008
  • Loc: CA

Posted 28 June 2009 - 01:29 PM

I think there is a thread in the Lunt Solar Group as well. I don't get over there ever but I am pretty sure I read a post by Andy there about it.

#9 Gene Baraff

Gene Baraff

    sage, 1930-2010, Rest In Peace

  • -----
  • Posts: 265
  • Joined: 22 Mar 2009
  • Loc: Berkeley Heights, N.J.

Posted 28 June 2009 - 01:57 PM

Gene, search through the archives of this forum as well. There have been some lengthy discussions on this topic (how the matching is done and installation procedures) by knowledgeable people.

Glenn


My unfamiliiarity asserts itself again: How does one search the archives here on CN?

Thanks in advance.

Gene

#10 Gene Baraff

Gene Baraff

    sage, 1930-2010, Rest In Peace

  • -----
  • Posts: 265
  • Joined: 22 Mar 2009
  • Loc: Berkeley Heights, N.J.

Posted 28 June 2009 - 02:00 PM

I think there is a thread in the Lunt Solar Group as well. I don't get over there ever but I am pretty sure I read a post by Andy there about it.


Thank you. I'll mosey over and take a look.

Gene

#11 DesertRat

DesertRat

    Fly Me to the Moon

  • *****
  • Posts: 5193
  • Joined: 18 Jun 2006
  • Loc: Valley of the Sun

Posted 28 June 2009 - 04:38 PM

Gene,
At the top of this page you should see a link to Search, My Home, FAQ etc. Select Search, drop down list to Solar (why it is'nt alpha sorted is puzzling), fill in keywords and date range stuff etc. I just did a search on double stacking and got too many recent hits. The discussion was maybe a year ago? We need to archive useful discussions better. I save good stuff onto my local folders, sometimes cut out and paste into a rtf file. Did not save that however :foreheadslap: Also check for tips on searching to weed things out a bit...

Glenn

#12 Gene Baraff

Gene Baraff

    sage, 1930-2010, Rest In Peace

  • -----
  • Posts: 265
  • Joined: 22 Mar 2009
  • Loc: Berkeley Heights, N.J.

Posted 28 June 2009 - 05:55 PM

Gene,
At the top of this page you should see a link to Search, .....

Glenn


Thanks. Works like gang busters.

Surprise! According to one observer of the actual process, Jim Rosenstock is closet to being correct. They just look through a batch of 'em and choose the one that gives the best image.

Who'da thunk it?

Gene

#13 Jim Rosenstock

Jim Rosenstock

    Fly Me to the Moon

  • *****
  • Posts: 6590
  • Joined: 14 Jul 2005
  • Loc: MD, south of the DC Nebula

Posted 28 June 2009 - 06:50 PM

Surprise! According to one observer of the actual process, Jim Rosenstock is closest to being correct.


I'm not at all surprised... :grin:

They just look through a batch of 'em and choose the one that gives the best image.

Who'da thunk it?

Gene


I think they sometimes twist 'em around a bit, too. :cool:

#14 BYoesle

BYoesle

    Aurora

  • *****
  • Posts: 4892
  • Joined: 12 Jun 2004
  • Loc: Goldendale, Washington USA

Posted 28 June 2009 - 11:25 PM

To the best of my knowledge, the reason for "matching" etalons lies in that the etalon plates are not mechanically fixed in their cells, but suspended in a silicon-like material that may result in slight tilt differences between etalon pairs. However, some have noted the internal tilts may change over time or possibly with varying temperatures.

This is not to state you have to have matching done at the factory, and I and a few others have had very good success with randomly selected off-the-shelf etalons for double stacking.

Here's the basic mechanical matching process from a previous thread as related to the poster by Andy Lunt:

(1) Test each etalon by itself for contrast on surface detail by using the Tmax tuner. Note the amount of tilt each etalon requires to achieve maximum contrast. There may be a "sweet spot"; that's OK, just go for maximum contrast using tilt.

(2) Select the etalon that requires the *lesser* amount of tilt as noted in (1) and dial in this amount of tilt. This Tmax tuner will now remain "permanently" fixed in this position and all adjustments while viewing will be done with the second etalon's Tmax tuner.

(3) Place this etalon closest to the telescope OTA. Doing this results in the least amount of "pre-tilt" for the second etalon. Pre-tilt is undesirable because it tends to throw the second etalon off-band or at least limits its adjustment range.

(4) Mount the second etalon to the first etalon and adjust its Tmax tuner to the optimum contrast position noted in step (1).

(5) Mounting the second etalon guarantees the creation of a "solar ghost" by reflection from the additional optical surfaces. Since the ghost (apparently) cannot be eliminated, the goal is to move the ghost out of view by rotating this etalon. If the etalons are nearly parallel, rotating the second etalon causes the visible ghost image to spin in place or move in a tight circle but not leave the field of view that includes the true Solar image. However, since there is (hopefully) some tilt between the etalons, rotating the second etalon relative to the first etalon now causes this ghost image to move around a center point that is a fair distance away from the true Solar image. If all goes well, half or more of the "circle" that the ghost image now traverses will be outside the scope's field of view. If the ghost remains visible, add some tilt to the second etalon using its Tmax dial and repeat this step.

(6) Stop rotating the second etalon when the ghost image is acceptably out of the FOV. The two etalons are now positioned with their best match.

#15 colinsk

colinsk

    Mercury-Atlas

  • *****
  • Posts: 2610
  • Joined: 17 Jan 2008
  • Loc: CA

Posted 29 June 2009 - 12:17 AM

We need a wiki!

#16 Oldfield

Oldfield

    Fly Me to the Moon

  • *****
  • Moderators
  • Posts: 6431
  • Joined: 20 Mar 2002
  • Loc: Hong Kong

Posted 29 June 2009 - 02:11 AM

To the best of my knowledge, the reason for "matching" etalons lies in that the etalon plates are not mechanically fixed in their cells, but suspended in a silicon-like material that may result in slight tilt differences between etalon pairs. However, some have noted the internal tilts may change over time or possibly with varying temperatures.

This is not to state you have to have matching done at the factory, and I and a few others have had very good success with randomly selected off-the-shelf etalons for double stacking.

Here's the basic mechanical matching process from a previous thread as related to the poster by Andy Lunt:

(1) Test each etalon by itself for contrast on surface detail by using the Tmax tuner. Note the amount of tilt each etalon requires to achieve maximum contrast. There may be a "sweet spot"; that's OK, just go for maximum contrast using tilt.

(2) Select the etalon that requires the *lesser* amount of tilt as noted in (1) and dial in this amount of tilt. This Tmax tuner will now remain "permanently" fixed in this position and all adjustments while viewing will be done with the second etalon's Tmax tuner.

(3) Place this etalon closest to the telescope OTA. Doing this results in the least amount of "pre-tilt" for the second etalon. Pre-tilt is undesirable because it tends to throw the second etalon off-band or at least limits its adjustment range.

(4) Mount the second etalon to the first etalon and adjust its Tmax tuner to the optimum contrast position noted in step (1).

(5) Mounting the second etalon guarantees the creation of a "solar ghost" by reflection from the additional optical surfaces. Since the ghost (apparently) cannot be eliminated, the goal is to move the ghost out of view by rotating this etalon. If the etalons are nearly parallel, rotating the second etalon causes the visible ghost image to spin in place or move in a tight circle but not leave the field of view that includes the true Solar image. However, since there is (hopefully) some tilt between the etalons, rotating the second etalon relative to the first etalon now causes this ghost image to move around a center point that is a fair distance away from the true Solar image. If all goes well, half or more of the "circle" that the ghost image now traverses will be outside the scope's field of view. If the ghost remains visible, add some tilt to the second etalon using its Tmax dial and repeat this step.

(6) Stop rotating the second etalon when the ghost image is acceptably out of the FOV. The two etalons are now positioned with their best match.


fantastic! :bow: :bow: :bow: :bow:

#17 Gene Baraff

Gene Baraff

    sage, 1930-2010, Rest In Peace

  • -----
  • Posts: 265
  • Joined: 22 Mar 2009
  • Loc: Berkeley Heights, N.J.

Posted 29 June 2009 - 09:48 AM

To the best of my knowledge .....

Here's the basic mechanical matching process from a previous thread as related to the poster by Andy Lunt:

(1) Test each etalon by itself ........


BINGO!

The content of your reply is exactly what I needed. What you posted suggests a step in tuning my double-stacked filters that I had overlooked - believing that the double stacker was factory chosen by a different method.

I'm enthusiastically waiting for a two hour solar stretch in July, similar to the one we had in June, where I can put the new procedure to the test.

Gene Baraff

#18 Gene Baraff

Gene Baraff

    sage, 1930-2010, Rest In Peace

  • -----
  • Posts: 265
  • Joined: 22 Mar 2009
  • Loc: Berkeley Heights, N.J.

Posted 29 June 2009 - 10:01 AM

Doing this results in the least amount of "pre-tilt" for the second etalon. Pre-tilt is undesirable because it tends to throw the second etalon off-band or at least limits its adjustment range.


Adding only a grain of sand to the mountain of information you have put before us (and again, thanks for that), another reason for aiming at minimum pre-tilt is that excessive tilt can also cause a sweet bar to develop. By sweet bar, I mean an area of best contrast bounded above and below (but not bounded left to right) - ie, a bar shaped sweet spot.

Gene

#19 DesertRat

DesertRat

    Fly Me to the Moon

  • *****
  • Posts: 5193
  • Joined: 18 Jun 2006
  • Loc: Valley of the Sun

Posted 29 June 2009 - 09:09 PM

Gene,

I found the following threads with pertinent stacking information:

Double stacking Coronado filters 03/18/09
http://www.cloudynig...&Number=2991054

Lunt Triple Pass Filtering 05/09/08
http://www.cloudynig...&Number=2382639

The good info Bob related was found in this last thread (the one I was recalling) provided by a user named Gary whose handle is "half meter".

Glenn

#20 Gene Baraff

Gene Baraff

    sage, 1930-2010, Rest In Peace

  • -----
  • Posts: 265
  • Joined: 22 Mar 2009
  • Loc: Berkeley Heights, N.J.

Posted 30 June 2009 - 10:41 AM

Gene,

I found the following threads with pertinent stacking information:

Double stacking Coronado filters 03/18/09
http://www.cloudynig...&Number=2991054

Lunt Triple Pass Filtering 05/09/08
http://www.cloudynig...&Number=2382639

Glenn


I wanted to share this with all you who have already provided so much helpful information. Everything below follows from my experience this morning with "Factory Matched" SM40's.

One of your links or quotes was that recommended orientation of the frontmost of the double-stacks is so that its tuning knob is 180 degress from the tuning knob of the etalon closest to the objective.

I went out to test this idea, and found that when the second one is screwed into the first as far as it will go, the two tuning knobs ARE 180 degrees apart.

One of you mentioned that the two etalons of a factory matched pair are chosen so that the inner one has less tilt to bring it on line than does the outer one.

I went out to test this idea, hoping to attach one and the other separately to the objective and tune each separately for maximum contrast, noting how far I had moved each tuning knob away from the fully closed position.

No way to test that idea: the back threads on the second etalon are different from the back threads on the first so there is only one order in which the etalons - singly or double stacked - can be mounted.

One of you said that the first etalon is chosen so that it has very little tilt. In setting up this morning, I paid special attention to getting the adapter EXACTLY square to the scope. That meant using the Cheshire Colliimator and being really fussy to make sure that the cross hairs and their reflection really lined up.

(For those who don't understand what I mean here, I had talked about this in a thread on the C.N. A.T.M. - D.I.Y. and Optics Forum.)

http://www.cloudynig...5/o/all/fpart/1

With that amount of fussiness, the best position of the first etalon was very very close to fully closed. That is, my "factory matched" etalon had been chosen to have really small tilt.

There is another observation I'd like to ask you about, but I'll start a new thread for it later - here in "Solar Observing" - in the interest of keeping this forum well organized.

Thanks again for all the help.

Gene Baraff






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics