troypiggo
12-04-2012, 09:32 AM
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.
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.