BYoesle
Pooh-Bah
Reged: 06/12/04
Posts: 1199
Loc: Goldendale, WA USA
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Quote:
I think Lee was talking about the ITF, which is at the eyepiece-end even in front-etalon configurations.
As I understand it, the off-axis rays essentially make the bandpass wider (so a 0.7 angstrom filter at f/20 might really perform as a 1.0 angstrom filter). However, the ITF is just knocking out the harmonics from the etalon, so as long as the light cone doesn't widen the ITF's bandpass to the point where it allows the next spike from the etalon to get through, it shouldn't matter.
Of course, all that depends on (1) how far apart the spikes are from the etalon (ie: its FSR), and (2) how close the width of the ITF bandpass already is to those extra spikes.
The ITF is just one component of the blocking filter. It's a wide bandpass interference filter whose main function is to eliminate infrared. The trimming filter(s) in the blocker assembly are what actually eliminate the primary etalon harmonic spikes, and being an interference filter of relatively narrow band-pass (5-10 angstroms) is more sensitive than the ITF to the incident angles striking it.
H alpha filters which place the very narrow band-pass (< 0.7 angstrom) primary etalon near the eyepiece (DayStar and Solar Spectrum) require an even tighter cone of incident light of f30 or greater to stay within design parameters, which is acheived by stopping down the aperture, or with telecentric optics.
It was David Lunt himself who advised a blocking filter works optimally at f10 (or greater), although it could still work well at f-ratios lower than f10.
So the general rule of thumb is the narrower the filter band-pass, the slower the f-ratio should be, and vice versa.
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Bob Yoesle 
You are a child of the universe,
no less than the trees and the stars...
And whether or not it is clear to you,
no doubt the universe is unfolding as it should.
Desiderata
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simonlee
sage
Reged: 04/06/07
Posts: 237
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Hi Bob,
I was thinking if we can work out an F ratio that won't "hurt" the ITF much as Mark has indicated due to thermal load but also maintaining the image quality (bandwith)as well. For the sm40 which is a 40mm to be F30, the scope will have to be 1200mm in focal lenght correct?
If so then we need at least a BF15 to see the entire image of the sun .
Anyone out there with a sm40 of F30 focal ratio experiencing rust of the ITF slower than those of the F10 scopes?
If so then thermal loading is one of the key for the rusting ITF.
lee
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BYoesle
Pooh-Bah
Reged: 06/12/04
Posts: 1199
Loc: Goldendale, WA USA
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Hi Lee,
Yes, a 1200 mm focal length would require use of a BF15 or larger.
The thought that a longer focal ratio (and perhaps therefore less thermal loading) might result in less frequent blocker failure is an interesting proposition.
Mark's comments seem to indicate a great deal of uncertainty is ineveitable with manufacturing of ITFs made with metallic coatings. Andover has clearly stated thermal loading leads to ITF failure, and we see both Solar Spectrum and Lunt taking additional steps to decrease thermal loading with IR blocking ERFs.
However, Solar Scope blocking filters seem to have been implemented in very similar designs to the Coronado BF's (i.e full-aperture f10 or faster focal ratios), with apperently far less frequent blocking filter failures to this point. Perhaps they have incorporated IR blocking in the ERF and more robust ITF requirements from the beginning.
--------------------
Bob Yoesle
You are a child of the universe,
no less than the trees and the stars...
And whether or not it is clear to you,
no doubt the universe is unfolding as it should.
Desiderata
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EddWen
sage
Reged: 04/26/08
Posts: 409
Loc: Here or There
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Jeff, curiously, the paperwork with the SM90 shows a focal length 900mm. The Coronado website indicates 800mm. So, I measured mine as best I could assuming where the focal plane is. 812mm with an unknown error band, certainly not 900mm.
Sorry Patrik, I was responding to two other posts prior to yours.
Clear skies,
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Edd Weninger
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**** Lightshield Observatory SoCal ****
Nexstar 11 GPS Questar 3.5
A-P 900 A-P Traveler
**** Blackdog Observatory Arizona ****
A-P 155EDF A-P 1200GTO Coronado SolarMax 90
Orion ED80 Baader Mk V bino-viewer
CGE STV Canon 18x50IS
Edited by EddWen (02/25/09 11:43 AM)
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colinsk
Carpal Tunnel
Reged: 01/17/08
Posts: 2170
Loc: CA
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If the etalon is internal, you can have an objective that is one f/l and a reconverging lens that is a different f/l. The PST is f/10 on both but it is certianly posible that the f# of the objective is not the f# of the converging lens in an internal etalon system.
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Mahalo,
Colin Kaminski
Coulter 10.1" Dobsonian
TV-76/Baader Film White Light
LS60T/DS50/FT/BF1200
LDX-75
AT Voyager
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Jeff Young
Post Laureate
Reged: 08/04/05
Posts: 4330
Loc: Ireland
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All very interesting. (Thanks Bob for the clarification on ITF vs. trimmer.)
So I currently have my Solarscope ITF/trimmer in an f/6.8 light cone, but with a Barlow behind it. I'm now curious if f/6.8 is fast enough that I'm letting some light in from the harmonics on either side of h-alpha.
Next time I have clear skies I could swap the blocking filter housing and Barlow and see if anything changes (although it might be hard to tell with the resulting higher mag from the Barlow). Perhaps it would be easier just to call Ken....
-- Jeff.
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Nikon 18x70s / UA Millennium Colorado:
Solarscope SF70 / TV Pronto / AP400QMD Coronado SolarMax40 DS / Bogen 055+3130
APM MC1610 / Tak FC-125 / AP1200GTO Tak Mewlon 250 / AP600EGTO
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colinsk
Carpal Tunnel
Reged: 01/17/08
Posts: 2170
Loc: CA
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There are no true zeros in an etalon (which is why I am a fan of Lyot filters). It just may help you to narrow the bandpass of your trim filter. You would see it as slightly enhanced contrast.
PS. Perhaps I should expand... The lowest transmission of an etalon is exactly 1/2 way from peak to peak. It never reaches zero. If your trim filter is wider than the FSR of the etalon you will be harming contrast. However, being narrower than the FSR does not help until you are getting close to the bandpass of the etalon. (Which will never happen.)
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Mahalo,
Colin Kaminski
Coulter 10.1" Dobsonian
TV-76/Baader Film White Light
LS60T/DS50/FT/BF1200
LDX-75
AT Voyager
Edited by colinsk (02/25/09 04:58 PM)
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markthais
member
Reged: 06/26/04
Posts: 89
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Colin is right, no real zero, If your mirrors on the etalon are increased to 97% you get better rejection at the bottom of the curve. OR you can add another cavity(etalon) and the contrast increases with the same bandwidth. Its signal to noise ratios. You need to make sure that your blocker does not leek the next order on the etalon. The front mounted etalons are about 10-15ang apart. This would need a narrow single cavity or better a 5ang two cavity blocker.
The rear mounted are about 25-35 apart so they would use some thing around 10Ang 2 cavity to make sure they are clear. If you have any of the other orders coming through the contrast will be less.
The ITF are not the part that takes out the other orders. They are for the IR blocking. The can be 200Ang wide where the bandpass are never wider that 20Ang's.
Mark
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ragebot
Carpal Tunnel
Reged: 08/26/05
Posts: 1721
Loc: Tallahassee, FL, USA
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Quote:
"From what I understand the Coronado OTAs are nothing really special;..."
Coronado advertised that the optics, Cemax eyepieces and Barlow were specifically designed and optimized for Ha use by Richard Buchroeder. What that really means, I don't know but Buchroeder is a well known and respected designer. I do know the objective of the SM90 has two elements but I can't tell if they are airspaced, oiled or cemented.
"I think the SM90 is also around f/7"
My SM90/BF15 is f/10, 900mm focal length.
What I do know is that even the most superficial reading of past threads will confirm that Cemax EPs are almost universally viewed as inferior to almost any other EP.
Not to mention that there are lots of threads about using the Coronado stuff on other ETAs for better views.
There is no doubt in my mind that I get much better views with my DS SM40 stuff on my ED80 than on the stock Coronado OTA that came with it. I know that is not really fair, but it is not easy to find a 40mm OTA.
I still stand by my comment that Coronado OTAs are nothing special, ad hype aside. 
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Meade ETX 90, Meade AR5, Orion ED80, Atlas GT, 8 in Newt, Coronado DS SM40, Garrett 10.5X70, Sigma SD10, SD14, Canon 1D2, Xti, Nikon CP4500, C-14
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colinsk
Carpal Tunnel
Reged: 01/17/08
Posts: 2170
Loc: CA
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Quote:
...no real zero...
Thank you Mark. That is the more correct term. When solving for zeros in equations there are no "real zeros" in the etalon equation. 
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Mahalo,
Colin Kaminski
Coulter 10.1" Dobsonian
TV-76/Baader Film White Light
LS60T/DS50/FT/BF1200
LDX-75
AT Voyager
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simonlee
sage
Reged: 04/06/07
Posts: 237
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Quote:
Hi Lee,
However, Solar Scope blocking filters seem to have been implemented in very similar designs to the Coronado BF's (i.e full-aperture f10 or faster focal ratios), with apperently far less frequent blocking filter failures to this point. Perhaps they have incorporated IR blocking in the ERF and more robust ITF requirements from the beginning.
Yes Bob,
Mark did say that ITF can be soft coated and hard coated. Maybe Solar Scope's ITF is hard coated. But mark did say that the hard coated ITF may last 2-3 times longer than those soft coated ones, but they are also 2 to 3 times much costlier than the soft coated ones.
So perhaps because of this, it does not matter if they manufacture a hard or soft coated ITF as both will eventually rusted out with the same amount of expenditure by the consumer.
lee
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colinsk
Carpal Tunnel
Reged: 01/17/08
Posts: 2170
Loc: CA
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Mark also made the very good point that once you add silver all filters are "soft coated" regardless of whether the DE layers are "hard" or not.
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Mahalo,
Colin Kaminski
Coulter 10.1" Dobsonian
TV-76/Baader Film White Light
LS60T/DS50/FT/BF1200
LDX-75
AT Voyager
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markthais
member
Reged: 06/26/04
Posts: 89
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Hi Group,
The point I wanted to make was that an ITF can last the same if it is hard coated or soft coated. Most last many years but sometimes they don't. I am sure some day the blockers on a Solarview scope will need to be changed.
Colin pointed out that if you scan an etalon at the bottom of the curve you may not be at zero. but the higher the reflectivity of the mirrors the lower % of light gets through at the bottom of the curve.
If we look at band passes filters the simplest is the single cavity
If the half width(HW) is 1 ang then you can expect to measure 10X that(10ang)at the 1% and 20X at the .01% and below that it is not really defined. This would be similar to an 1ang etalon.
At two cavity it improves. At the .1% it is only 6X and you can measure down to .001% before it become undefined.
The three cavity filter is the one I always though would be great for a solar filter. A .5Ang three cavity filter would beat a Lyot filter. But to made such a thing is still a wish. The three cavity filters has only 5X HW at .01% and drops below .001% with out any other blocking.
So Colin is right when measuring an etalon or a band pass it's low but not zero % Transmission at the bottom of the curve.
Mark
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NickH
Post Laureate
Reged: 05/09/06
Posts: 3717
Loc: Wiltshire, UK (near StoneHenge...
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I am sure you are right, but that's not what the optical experts at CVI told me about the SV filters (these guys having just put some of the optical system on the Phoenix Lander on Mars...I guess they know what they are talking about), and not what Ken also seems to think, both knowing their product and coating systems very well, I tend to trust their judgement. PErhaps it will degrade, but having had 3 Coronado PST's fail on me (rust on two, astigmatism on the third), one SM40 and two blocking filters (rust) in less than 3 years, and not a hint of any problems with the SV60, SV50 or SF70 I have used over the same period.. I think I'll stick with a product I trust.
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EddWen
sage
Reged: 04/26/08
Posts: 409
Loc: Here or There
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Quote:
What I do know is that even the most superficial reading of past threads will confirm that Cemax EPs are almost universally viewed as inferior to almost any other EP.
Not to mention that there are lots of threads about using the Coronado stuff on other ETAs for better views.
There is no doubt in my mind that I get much better views with my DS SM40 stuff on my ED80 than on the stock Coronado OTA that came with it. I know that is not really fair, but it is not easy to find a 40mm OTA.
I still stand by my comment that Coronado OTAs are nothing special, ad hype aside.
Yes, I have read multiple threads regarding the Cemaxs compared to others, and while I concur with your synopsis, my comparisons with Radians and Brandons do not produce the same result for me. I have a Tele-vue zoom but have never tried it. I know lots of people here like them, but I would think it unlikely one could avoid some loss of contrast by putting more glass in the optical train.
Regarding using the SM90 filters on other OTAs, I do plan to mount them on my Traveler and give it a try. It will take an adapter I haven't had time to make yet. I'll report what I find. Meanwhile, I'm very content with the SM90 system as it is. Double stacking is somewhere in its future.
I'm not sure I understand why you think putting the SM40 etalon on the ED80 provides a big advantage. You end up with a longer focal length, but doesn't the etalon still restrict resolution to that of a 40mm objective? The light cone going through the BF will be smaller on entry, perhaps avoiding the edges? I have an ED80 and am planning a Lunt 75mm etalon/blocker for it, once they have been produced in sufficient quantity to get through the growing pains.
edit: boy, did we ever hijack this thread, sorry to the OP.
Thx,
--------------------
Edd Weninger
-----------------
**** Lightshield Observatory SoCal ****
Nexstar 11 GPS Questar 3.5
A-P 900 A-P Traveler
**** Blackdog Observatory Arizona ****
A-P 155EDF A-P 1200GTO Coronado SolarMax 90
Orion ED80 Baader Mk V bino-viewer
CGE STV Canon 18x50IS
Edited by EddWen (02/26/09 03:24 PM)
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Keith Howlett
Pooh-Bah
Reged: 03/06/07
Posts: 1029
Loc: Northumberland, UK
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Edd, I have a spare SM90 / AP Traveler adapter.
Cheers,
Keith
Edited by Keith Howlett (02/26/09 03:44 PM)
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ragebot
Carpal Tunnel
Reged: 08/26/05
Posts: 1721
Loc: Tallahassee, FL, USA
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Quote:
SNIP
SNIP
I'm not sure I understand why you think putting the SM40 etalon on the ED80 provides a big advantage. You end up with a longer focal length, but doesn't the etalon still restrict resolution to that of a 40mm objective? The light cone going through the BF will be smaller on entry, perhaps avoiding the edges? I have an ED80 and am planning a Lunt 75mm etalon/blocker for it, once they have been produced in sufficient quantity to get through the growing pains.
edit: boy, did we ever hijack this thread, sorry to the OP.
Thx,
My understanding is that resolution is a function of focal length; however stopping down the ED80 with the SM40 might lower the brightness or contrast, but that would be something you would notice with your eyes and memory, or perhaps with the exposure if you were trying to take a pix.
By putting the SM40 on the ED80 you wind up with a virtual ED40 with a 600mm focal length and a focal ration of around f15. But because the sun is bright the f15 may not be that much of a problem, while the extra 200mm focal length will result in about a 50% increase in resolution.
Course I could be wrong about this. 
--------------------
Meade ETX 90, Meade AR5, Orion ED80, Atlas GT, 8 in Newt, Coronado DS SM40, Garrett 10.5X70, Sigma SD10, SD14, Canon 1D2, Xti, Nikon CP4500, C-14
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brianb11213
Post Laureate
Reged: 02/25/09
Posts: 3309
Loc: 55.215N 6.554W
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Quote:
My understanding is that resolution is a function of focal length
I don't think that's right.
If the etalon is positioned on the sunny side of the objective the focal ratio of the scope doesn't matter. If it is in the converging cone then it won't work properly unless the focal ratio is large enough.
Resolution is, as always, a function of clear aperture only. Given that the optics are good. As the etalon needs to be constructed to a tolerance of 1/100 wave, or better, in order to work, that isn't going to affect the resolution, unless it restricts the aperture. As a 40mm etalon on an 80mm scope most certainly will. OTOH a 40mm etalon is better than nothing, and is maybe affordable when a larger one might not be.
Visually the working focal ratio matters not all. For imaging, a "faster" system allows shorter exposures, but to make images showing as much detail as the scope is capable of showing, it's necessary to use a barlow lens to increase the image scale - with the DMK41 camera (monochrome, 4.65um pixel size) I find f/24 is enough, but other people are known to use projection to focal ratios up to f/64.
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EddWen
sage
Reged: 04/26/08
Posts: 409
Loc: Here or There
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Quote:
Edd, I have a spare SM90 / AP Traveler adapter.
Cheers,
Keith
Keith,
If you'd like to sell it, contact me @:
e d d w e n a t a o l d o t c o m
Thx,
--------------------
Edd Weninger
-----------------
**** Lightshield Observatory SoCal ****
Nexstar 11 GPS Questar 3.5
A-P 900 A-P Traveler
**** Blackdog Observatory Arizona ****
A-P 155EDF A-P 1200GTO Coronado SolarMax 90
Orion ED80 Baader Mk V bino-viewer
CGE STV Canon 18x50IS
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EddWen
sage
Reged: 04/26/08
Posts: 409
Loc: Here or There
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I concur with Brian. Resolution of a telescope is a direct function of the primary objective diameter, even in obstructed systems.
In a monochromatic system, if your Maxscope doesn't seem to work well, abberations would be limited to astigmatism, spherical or possibly pinched optics. My SM90 does not seem to show any of this.
Good luck,
--------------------
Edd Weninger
-----------------
**** Lightshield Observatory SoCal ****
Nexstar 11 GPS Questar 3.5
A-P 900 A-P Traveler
**** Blackdog Observatory Arizona ****
A-P 155EDF A-P 1200GTO Coronado SolarMax 90
Orion ED80 Baader Mk V bino-viewer
CGE STV Canon 18x50IS
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