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12-04-2012, 09:32 AM
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Bust Duster
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Join Date: Nov 2008
Location: Brisbane, Australia
Posts: 4,846
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flattener spacing/backfocus - theoretical vs real
Trying to get a completely flat field with the TMB92SS. Did some more testing last night with the 2 flatteners I have (Orion Flattener and WO FF4). It's looking like the issue is the spacing between flattener and sensor.
According to QSI website and YahooGroup, the backfocus for my model is 35.5mm nominally. I understand this to be the distance from the front of camera, where you screw in T thread adapters etc, to the sensor. I also understand that there are construction tolerances, and also an adjustment to be made for light path through the filters. If I take all of those into consideration, it only works out to 1-2mm difference.
Now most flatteners I've tried specify 55mm backfocus, which I understand is adopted from DSLR backfocus of 45mm nominally plus about 10mm for the typical camera adapters. I take it that this is the distance from where you screw it in to the sensor.
So breaking that down, I've been using a 20mm spacer between the flattener and the camera (55-35) nominally, and figuring I'm within a couple of mm of where I should be.
I wondered of there's something drastically wrong with my calculations or assumptions, because at this theoretically correct spacing, I wasn't getting the flat fields I expected. So I thought I'd test with a 10mm spacer instead of 20mm. Stars got worse.
Ok, I'll go the other way, add 10mm spacer to the 20mm to give me 30mm. Et voila! Pretty good stars right across the field! Yeehaa! But that's 10mm and 50% more than the theoretical!
And I know Peter recently found a similar thing with his MPCC and 10". Theoretical backfocus and what he really needed was 8mm more!
I'm thinking I'll go back to check my 10" and MPCC spacings and make more drastic changes to see if that is where I'm going wrong with it too.
So what's the point of this post? I guess 2 things. First, what's your experience? Have you found you need to play with the spacings a lot, or was the advertised/theoretical correct? And second, bit of a heads up that if you're not getting the stars you expect from your flattener, maybe play with the spacings drastically, test and re-test, and maybe that'll get you where you want to be.
Last edited by troypiggo; 12-04-2012 at 12:41 PM.
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12-04-2012, 12:36 PM
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Dazzled by the Cosmos.
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Join Date: May 2005
Location: Brisbane
Posts: 11,811
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A very helpful write up of your experience and conclusions Troy, thanks for the details.
Cheers
Dennis
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12-04-2012, 12:57 PM
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Really just a beginner
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Join Date: Jun 2009
Location: Brisbane
Posts: 3,045
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I guess the other thing to consider is has the reducer been designed to that particular scope. The distortion for each scope will differ, so the amount of correction required will vary - adjusting the distance between the reducer and chip might help you achieve an optimal reduction/flattening.
DT
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12-04-2012, 01:12 PM
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Bust Duster
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Join Date: Nov 2008
Location: Brisbane, Australia
Posts: 4,846
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G'day David,
The specs of my scope, f/5.5 506mm FL sit right in the middle of the advertised range that the flattener (not reducer) are said to work for - 400-650mm FL and f/5 to f/7.5
http://www.bintel.com.au/Astrophotog...oductview.aspx
That's what's deceptive and frustrating. You'd think, reading the ad, that it should work at 55mm backfocus.
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12-04-2012, 01:24 PM
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Really just a beginner
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Join Date: Jun 2009
Location: Brisbane
Posts: 3,045
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Still think your suffering the consequences of a "mixed marriage"!
DT
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12-04-2012, 01:37 PM
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Bust Duster
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Join Date: Nov 2008
Location: Brisbane, Australia
Posts: 4,846
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Yeah, could be. I'm still testing some other flatteners as well, so hopefully that'll highlight the good and bad.
How's this, talking about mixed marriages. On the Astronomics website, their recommended flattener to go with this scope is the Astro-Tech 2" flattener. Ad for this flattener says it's for f/6 to f/8 refractors, and also works on the AT RC reflectors! Magical. (skeptical)
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12-04-2012, 02:03 PM
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Registered User
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Join Date: Aug 2005
Location: Hahndorf, South Australia
Posts: 4,373
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Troy - when I was considering this scope I stumbled upon a test with the TV reducer among others. It came out on top. Think it was on Craig Stark's site but it must be nested somehere as it doesn't fall readily to hand!
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12-04-2012, 02:58 PM
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Bust Duster
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Join Date: Nov 2008
Location: Brisbane, Australia
Posts: 4,846
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Thanks mate. Is that the TRF-2008? I was reading up on that one last night.
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13-04-2012, 01:35 PM
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Registered User
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Join Date: Aug 2005
Location: Hahndorf, South Australia
Posts: 4,373
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13-04-2012, 03:05 PM
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Bust Duster
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Join Date: Nov 2008
Location: Brisbane, Australia
Posts: 4,846
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Yep. That's the TRF-2008. It's on my shortlist. 
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08-05-2012, 03:02 PM
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Registered User
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Join Date: Jan 2011
Location: Perth
Posts: 34
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Troy,
I have a TMB92ss and have been using the Astro-Tech field flattener recommended by Astronomics. I have found that it does the job nicely.
Due to the recent inclusion of an OAG in my imaging train, the Astro-Tech is no longer suitable as the OAG adds too much back focus for it.
Annoyingly, I can't put the OAG before the flattener (relative to the focuser) because the flattener's nosepiece is not removerable!
I am therefore considering purchasing the WO FF4 as it allows for considerably longer backfocus.
How have you experiences been with the TMB and WO FF4? Does the flattener perform well with the scope?
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08-05-2012, 03:51 PM
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Newtonian power! Love it!
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Join Date: Jul 2008
Location: Mandurah
Posts: 2,597
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Something that everybody has failed to mention is glass.....
I have a mixture now of 4mm FLI LRGB filters and Astronomik NB filters (I think 2mm?) the focus shift is quite substantial!
I will have to find the formula for it but in my system according to my FT scale its put nearly 5mm extra back focus required.
I know its not 10mm but if you added all your mechanical tolorances together your within the flatteners tolerances and hence your stars are happy again.
Thats my experience with Filters, Mono cameras and Flatteners/Reducers
BM.
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08-05-2012, 07:27 PM
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Bust Duster
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Join Date: Nov 2008
Location: Brisbane, Australia
Posts: 4,846
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Quote:
Originally Posted by thefrogulox
Troy,
I have a TMB92ss and have been using the Astro-Tech field flattener recommended by Astronomics. I have found that it does the job nicely.
Due to the recent inclusion of an OAG in my imaging train, the Astro-Tech is no longer suitable as the OAG adds too much back focus for it.
Annoyingly, I can't put the OAG before the flattener (relative to the focuser) because the flattener's nosepiece is not removerable!
I am therefore considering purchasing the WO FF4 as it allows for considerably longer backfocus.
How have you experiences been with the TMB and WO FF4? Does the flattener perform well with the scope?
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G'day mate. Funnily enough, I was going to test the WO4 just the other night, but guide camera failed on me so didn't get it done. I will some time this week I reckon. Certainly doesn't achieve focus with that OTA extension on it, so have to remove it. ie have FeatherTouch connected direct to white OTA, not the black extension.
By the way, I've ordered the Televue TRF-2008, and if it delivers what I'm after, I'll be selling the WO4 
Quote:
Originally Posted by bmitchell82
Something that everybody has failed to mention is glass.....
I have a mixture now of 4mm FLI LRGB filters and Astronomik NB filters (I think 2mm?) the focus shift is quite substantial!
I will have to find the formula for it but in my system according to my FT scale its put nearly 5mm extra back focus required.
I know its not 10mm but if you added all your mechanical tolorances together your within the flatteners tolerances and hence your stars are happy again.
Thats my experience with Filters, Mono cameras and Flatteners/Reducers
BM.
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Pretty sure the Astronomik filters are 1mm thick. And I think the rule is that they add 1/3 of their thickness to the light path, ie increase focus distance by that. My Astrodon LRGB filters are 2mm thick IIRC and as you mention, there's noticeable focus distance between them. Haven't measured if it's the 0.33mm difference based on above calcs. Would be an interesting exercise to see if it is around that.
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08-05-2012, 10:37 PM
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Registered User
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Join Date: Jan 2011
Location: Perth
Posts: 34
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Quote:
Originally Posted by troypiggo
G'day mate. Funnily enough, I was going to test the WO4 just the other night, but guide camera failed on me so didn't get it done. I will some time this week I reckon. Certainly doesn't achieve focus with that OTA extension on it, so have to remove it. ie have FeatherTouch connected direct to white OTA, not the black extension.
By the way, I've ordered the Televue TRF-2008, and if it delivers what I'm after, I'll be selling the WO4 |
Haha alright, let me know how the test goes. Although can I trust the results given there may now be a commercial element to them...? 
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