Jump to content


Photo

Optimizing the Double Stack

  • Please log in to reply
33 replies to this topic

#1 BYoesle

BYoesle

    Aurora

  • *****
  • Posts: 4961
  • Joined: 12 Jun 2004
  • Loc: Goldendale, Washington USA

Posted 14 October 2011 - 12:24 AM

Like many, I have found double stacking two H alpha etalons (in my case two front-mounted Coronado SM90's) to be a major improvement to the H alpha observing experience – resulting in a marked increase in surface detail, with a only a marginal reduction in the brightness of prominences.

However, there are a number of issues that presented themselves with double stacking, for which I have sought solutions in order to obtain optimum performance.

One of the first things that became apparent with double stacking was a considerable decrease in overall image brightness. A significant part of this could be attributed to the presence of two energy rejection filters – ERF’s – when only one is needed. Additionally, the standard ERF is made from a deep red colored glass, and has significant transmission disadvantages compared to optical glass.

Baader introduced a state-of-the-art ERF consisting of optical glass polished to 1/10 a wave with dielectric coatings (D-ERF), which has an even greater ability to block infrared wavelengths than traditional red glass ERF’s, which reduces or eliminates the heat load transmitted to the blocking filter assembly.* This is stated to prolong the life of the downstream filter components, which may also eliminate the premature “rusting” of the ITF - induced transmission filter - (I have had two previous failures of my BF30's ITF since its purchase in about 2003 from “thermal cycling.”) So I purchased the unmounted DERF 110 as being the best suited to the SM90/BF30 ( http://telescopes.ne...ic-coating.html ).

* NOTE: Lunt has also introduced a less expensive ERF with dielectric coatings on a red galss substrate with 1/4 wave flatness in order to reduce thermal loading ( http://www.lunt-sola...Hash=6251294854 ).

To properly optimize the double stack, the second etalon also needs to be first “clocked” or rotated to reduce ghost reflections. This will decrease the amount of subsequent tilting (too much of which can introduce a "sweet bar" of contrast non-uniformity) - then minimally tilted to just eliminate the ghost reflections from the field of view. See:

http://www.cloudynig...84/Main/3185080

Unfortunately, rotating the second etalon (as described in the above link) on the standard double stacking adapter loosened the filter, making the adjustment subject to excessive mechanical play, and introduced the possibility of the etalon falling off!

The solution for the dim image and thermal overloading (picture bottom) was to remove first one, then the other, original ERF’s, and substitute a Baader DERF in its own specialized cell that could be threaded on to either filter for single stack or double stacked mode. The DERF itself is tilted 0.5 degree (as is the original ERF), and additionally can be “clocked” or rotated so as to adjust its tilt orientation to ensure no spurious DERF reflections are introduced in the double stacking rotation/tilting optimization process. The DERF filter holder is essentially two nested cells that allow the inner cell (holding the DERF) to be rotated and then fixed in the correct position.

The solution for double stacking “clocking” rotation was an adapter (picture top) which is a similarly designed nested cell to allow the inner cell to rotate, and then be fixed to hold the proper etalon clocking position.

Attached Files



#2 BYoesle

BYoesle

    Aurora

  • *****
  • Posts: 4961
  • Joined: 12 Jun 2004
  • Loc: Goldendale, Washington USA

Posted 14 October 2011 - 12:25 AM

As can be seen, the transmission of the Baader DERF on the left is significantly higher than the red glass ERF on the right (double pass reflection of sunlight with filters resting on white paper):

Attached Files



#3 BYoesle

BYoesle

    Aurora

  • *****
  • Posts: 4961
  • Joined: 12 Jun 2004
  • Loc: Goldendale, Washington USA

Posted 14 October 2011 - 12:26 AM

Removing the original ERF was pretty straightforward, but exposed the front of the etalon to any atmospheric-borne dust that might enter through the T-Max tuner. To prevent this, a thin strip of black plastic was spring-fitted to the inside of the T-Max gap area:

Attached Files



#4 BYoesle

BYoesle

    Aurora

  • *****
  • Posts: 4961
  • Joined: 12 Jun 2004
  • Loc: Goldendale, Washington USA

Posted 14 October 2011 - 12:27 AM

Once the design of the DERF holder/cell and double stacking adapter was finalized, it became problematic to find a local, regional, or even a national machinist who would tackle the tilted DERF inner cell, as it was stated to require CNC machining. Large commercial operations that could do the CNC work quoted between $800.00 to $1000.00 USD for a “one-of” production, which was way beyond my budget.

I finally resigned myself to purchasing the un-tilted slip-fit DERF cell made by Baader, which was only available from Telescope Service of Germany ( http://www.telescope-service.com ). When I mentioned my needs in an email, Patrick Woitala at Telescope Service stated they likely could do the job. I emailed my drawings, and Patrick gave me a quote of less than 250 Euros for both parts! Patrick also noted that their engineers and machinists were amateur astronomers and would have familiarity with the SM90 components, threading, and my other requirements. After further back and forth emails detailing the specs and requirements, I gave the final OK.

It took less than three weeks to be notified my DS rotation adapter and DERF cell were ready, and took only four (4) days to receive the shipment from Germany! Total costs including shipping and import fees came to less than $400.00 USD!

As can be seen, the quality and attention to detail is truly first class, and the parts were even anodized black!

Attached Files



#5 BYoesle

BYoesle

    Aurora

  • *****
  • Posts: 4961
  • Joined: 12 Jun 2004
  • Loc: Goldendale, Washington USA

Posted 14 October 2011 - 12:29 AM

I can not wait to have a peak through the fully optimized filter system, but the weather has yet to cooperate. Regardless, I know that the optical performance and longevity will not be inhibited as before from reduced light throughput, thermal loading, and loose mechanical fittings.

And I can’t thank Patrick and the other people at Telescope Service enough for their excellent workmanship and value – the communication, willingness to help, and friendliness were beyond almost anything I have experienced.

I highly recommend “Teleskop Service” to anyone needing similar help and services. :)

Attached Files



#6 marktownley

marktownley

    Cosmos

  • -----
  • Posts: 8606
  • Joined: 19 Aug 2008
  • Loc: West Midlands, UK

Posted 14 October 2011 - 01:04 AM

Fantastic write up Bob! I'm on the same journey as you here, though to be honest weather has been so poor for me here in blighty i've actually held back in ordering my new ERF for the while. The only other thing i'm messing around with at the moment is mounting a circular polariser between the 2 etalons; this has positives in terms of contrast gains as a result of loss of reflections, however the major negative is that most circular polarisers have really poor transmission at Ha wavelengths - I have found a high transmission alternative, eventually, as a project this has gone on hold over winter and will re-emerge in spring when the sun does!

#7 BYoesle

BYoesle

    Aurora

  • *****
  • Posts: 4961
  • Joined: 12 Jun 2004
  • Loc: Goldendale, Washington USA

Posted 14 October 2011 - 02:06 AM

Thanks Mark - I'll cross my fingers for you to have an early spring!

#8 robert_arnold

robert_arnold

    Gemini

  • -----
  • Posts: 3028
  • Joined: 08 Nov 2005
  • Loc: Isle of Skye, Scotland

Posted 14 October 2011 - 04:19 AM

Excellent story, thanks for posting. I am not in the market for double stacking but it is very good to see how you got things sorted in an elegant way. I think I need a location with better seeing more than any telescope mods.... :foreheadslap:
Robert

#9 Derek Klepp

Derek Klepp

    Messenger

  • -----
  • Posts: 489
  • Joined: 27 Jun 2010
  • Loc: Yelgun NSW; Australia

Posted 14 October 2011 - 05:55 AM

Thanks Bob very informative. Robert we have areas in the Australian outback with 250+ days of sunshine a bit hot though.

#10 robert_arnold

robert_arnold

    Gemini

  • -----
  • Posts: 3028
  • Joined: 08 Nov 2005
  • Loc: Isle of Skye, Scotland

Posted 14 October 2011 - 07:58 AM

Thanks Bob very informative. Robert we have areas in the Australian outback with 250+ days of sunshine a bit hot though.

mmmm :question: sounds good!

#11 stephenramsden

stephenramsden

    Soyuz

  • *****
  • Posts: 3519
  • Joined: 17 Aug 2007

Posted 14 October 2011 - 08:11 AM

man, I would certainly have dropped my $4500 etalon somewhere in that proccess... :)
  • AlphaGJohn likes this

#12 BYoesle

BYoesle

    Aurora

  • *****
  • Posts: 4961
  • Joined: 12 Jun 2004
  • Loc: Goldendale, Washington USA

Posted 14 October 2011 - 09:25 AM

Thanks Robert and Derek, and I know exactly what you mean by needing better 'seeing' conditions!

Stephen - I can't think of a reason why the rotating DS adapter and ERF mod wouldn't be almost as beneficial for external/internal DS scopes, such as your "Luntanoado." ;)

#13 marktownley

marktownley

    Cosmos

  • -----
  • Posts: 8606
  • Joined: 19 Aug 2008
  • Loc: West Midlands, UK

Posted 14 October 2011 - 11:57 AM

I am not in the market for double stacking


You could pick up a second hand sm40 etalon for £300ish - you'd have some fun with it for full disks ;)

#14 BYoesle

BYoesle

    Aurora

  • *****
  • Posts: 4961
  • Joined: 12 Jun 2004
  • Loc: Goldendale, Washington USA

Posted 14 October 2011 - 02:59 PM

First light:

Despite high thin clouds, some blue sky appeared and has allowed a preliminary evaluation.

A much brighter disk is evident with the new DERF replacing the original ERF - previous removal of one of the original ERF’s resulted in a similar brightness increase. So overall the Baader DERF provides a very significant improvement in image brightness.

Initial secondary ghost reflections were much less numerous and more offset from the primary image. Clocking of the second etalon removed any overlap and left only one ghost image. However, the final position seemed to result in the same hinge placement as before - 180 degrees from each other... Additional tilting to remove the single ghost image resulted in little noticeable change compared to the non-clocked adapter used previously. Again the caveat might be the poor transparency, and additional observations under better conditions will have to be made.

When the clouds were thinnest, the sun was set in an inky-black background, despite the much brighter image.

A faint blue secondary reflection from the DERF was initially observed, but easily removed by clocking the DERF so the low side was lined up with the secondary etalon hinge.

Attached Files



#15 Andy Devey

Andy Devey

    Mercury-Atlas

  • -----
  • Posts: 2794
  • Joined: 06 May 2011

Posted 14 October 2011 - 03:57 PM

Hi Bob, great write up and methodical approach you have, a very brave and successful attempt. I hope it delivers the performance you require - you certainly deserve the rewards.

Thanks for sharing

Regards

Andy

#16 ASTROTRUCK

ASTROTRUCK

    Ranger 4

  • -----
  • Posts: 386
  • Joined: 02 Jan 2010

Posted 14 October 2011 - 04:26 PM

KUDOS TO YOU BYOESLE - but not for us who dont have it in our fingers -

now since both meade & lunt sell etelons also intended for double stacking why should they not make them special order modified as you have ??
would be easier if they sold etelons intended for double stacking modified by them - at least lunt 's work would be trusted.

#17 bill1234

bill1234

    Viking 1

  • -----
  • Posts: 804
  • Joined: 21 Apr 2011
  • Loc: NJ, USA

Posted 14 October 2011 - 04:31 PM

Bob. Thanks for that info.Well written....

#18 colinsk

colinsk

    Mercury-Atlas

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

Posted 14 October 2011 - 09:54 PM

Can you also clock your blocking filters?

#19 marktownley

marktownley

    Cosmos

  • -----
  • Posts: 8606
  • Joined: 19 Aug 2008
  • Loc: West Midlands, UK

Posted 15 October 2011 - 01:42 AM

Brilliant to hear Bob! :D

#20 BYoesle

BYoesle

    Aurora

  • *****
  • Posts: 4961
  • Joined: 12 Jun 2004
  • Loc: Goldendale, Washington USA

Posted 15 October 2011 - 12:11 PM

Thanks guys. Quite a journey to try and seek out the very best performance... sometimes two steps forward and one back. But it keeps it fun and interesting! I am a big believer that small incremental tweeks can add up to significant improvements. For example, I have found proper collimating of the objective and focuser are critical to H alpha performance.

Can you also clock your blocking filters?


Hi Colin - yes, and in a way I have. The BF30 ITF and trim filters are both tilted in their cells, as I discovered after disassembling the filter. This was a very difficult task, as some sort of thread lock material resembling a hard silicon is used to secure them in their threaded housing (probably so guys like me wouldn’t disturb the clocking angles). Once I began to loosen the trim filter to see if it consisted of more than one element, this material fell off within the filter assembly, and then required a complete disassembly to clean up – what a mess – and quite a nerve racking procedure! A closer inspection of each filter in its cell showed they were tilted just like the ERF ahead of the etalon.

I assumed, and it was later confirmed by a former Coronado staff person, that these filters needed to be oriented so that they were clocked to have the maximum divergence from each other... that is if you viewed them from the side of the filter, hypothetically you would see they formed a “V” with respect to each other - which makes sense in order to prevent internal reflections from becoming visible. I have since noted that on all the technical drawings and cutaway views of the Lunt scopes that seemingly every filter component except the etalon has some tilt applied to it... but the only instance I have heard about actually needing to clock the filters is the double stacked etalon itself as described in the above link.

#21 colinsk

colinsk

    Mercury-Atlas

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

Posted 15 October 2011 - 02:03 PM

I have clocked the BFs in my Lunt and found the factory rotation was the best location. Remember you have to tilt every flat surface or you get a reflection (unless you use a polarizer like the solid etalons). The optimum tilt is the minimum tilt for moving the reflection outside the FOV. The smaller BF diameters make imaging possible but also make it so you have to tilt the ERF and etalons more. Tilting the etalons causes its own issues.

#22 Jim Lafferty

Jim Lafferty

    Mercury-Atlas

  • *****
  • Posts: 2640
  • Joined: 30 Jun 2007
  • Loc: Southern California

Posted 15 October 2011 - 11:17 PM

Wow! Nice Bob!!

Jim

#23 colinsk

colinsk

    Mercury-Atlas

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

Posted 16 October 2011 - 11:04 AM

I have clocked the BFs in my Lunt and found the factory rotation was the best location. Remember you have to tilt every flat surface or you get a reflection (unless you use a polarizer like the solid etalons). The optimum tilt is the minimum tilt for moving the reflection outside the FOV. The smaller BF diameters make imaging possible but also make it so you have to tilt the ERF and etalons more. Tilting the etalons causes its own issues.


Woops, Bob pointed out a mis-write of a sentence here. It should read:

The LARGER BF diameters make imaging possible but also make it so you have to tilt the ERF and etalons more. Tilting the etalons causes its own issues.


:foreheadslap:

#24 Keith Howlett

Keith Howlett

    Apollo

  • *****
  • Posts: 1447
  • Joined: 06 Mar 2007
  • Loc: Northumberland, UK

Posted 16 October 2011 - 07:21 PM

Hi Bob,

I love the clocking adapters and the use of the D-ERF, there is plenty to think about there.

Thanks,

Keith

#25 BYoesle

BYoesle

    Aurora

  • *****
  • Posts: 4961
  • Joined: 12 Jun 2004
  • Loc: Goldendale, Washington USA

Posted 17 October 2011 - 04:43 PM

Thanks for the feedback Jim, Colin, and Keith!

Sun today, so a new "clocking" of the secondary etalon again revealed no overlapping ghost images, and about 2/3rd's the tilt required previously to remove the remaining ghost image from the BF30 field of view... I don't know if it's the increased brightness, less tilt, or both, but the image detail also seems subtlety improved -- but it's a subjective assessment.

However, with regard to the image brightness, this is more easily assessed...

I did a run of imaging today, some of which I will load later in a separate post. Most all of the H alpha images I have posted in the past three or more months were taken with a an Orion ED 100/900, with the DS's Coronado 90's, and a single original red glass ERF. Close ups at the same image scale as below are obtained with a TeleVue 2 x telenegative, and previously required exposures of 15-20+ ms.

Using the new DERF, the 1800 mm (f-20) EFL limb surge prominence image below was captured with only a 5 ms exposure:

Attached Files








Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics