Field flattener to camera distance, positioning the filter wheel correctly & what do I need?

Equipment Flattener Filter Wheel CMOS
2 replies•189 views
shaun mckiernan avatar
shaun mckiernan avatar
Getting correct (55mm) distance between field flattener and Camera CCD with ZWO EFW inbetween the two...

Hello everyone, I'm new to the forum so please forgive my lack of knowledge as I am relatively new to astrophotography & setting up equipment.

I wonder if anyone could help with this...

I am trying to get the correct distance between my 0.8x field flattener and the Altair 183m camera with a ZWO 8 position EFW fitted with 1.25" filters inbetween the flattener and the camera.

Now I do have a 1.25" nose piece and the piece that this slides into. These two pieces assembled had M42 male each side, which fitted perfectly between the camera and the EFW, meaning (to my mind at least) that all I needed was an M48 female to M42 male adapter ring.

Sadly though, now knowing that the EFW needs to be as close to the camera as possible to reduce any halo effect, I can't make use of this setup anymore, or can I?

If I can't, how would I go about achieving the correct distance?

I have a Starwave Ascent 80ED field flattener/reducer 0.8x (M48 male) , a ZWO 8 position EFW (20mm depth, M42/T2 female each end) fitted with 1.25" filters and an Altair 183m camera (M42/T2 female).

If anyone could advise, I would be most grateful.

Thanks,
Shaun
Respectful
andrea tasselli avatar
andrea tasselli avatar
You need a male to male M42 adaptor (a.k.a gender changer). The order of insertion would thus be: corrector -> spacers (if needed) -> FW -> male to male adaptor -> camera

The camera flange to sensor distance is 17.5mm and the FW optical depth is 20mm (+1/3 filter thickness), so let's say 38 mm all told. There should be around 12mm spacing required and thus ample space for a M42 to M42 gender changer.
Well Written Helpful Insightful Concise
shaun mckiernan avatar
shaun mckiernan avatar
andrea tasselli:
You need a male to male M42 adaptor (a.k.a gender changer). The order of insertion would thus be: corrector -> spacers (if needed) -> FW -> male to male adaptor -> camera

The camera flange to sensor distance is 17.5mm and the FW optical depth is 20mm (+1/3 filter thickness), so let's say 38 mm all told. There should be around 12mm spacing required and thus ample space for a M42 to M42 gender changer.

Thanks for your response Andrea. That makes sense now. I've checked the flange to sensor distance on Altair's website and it is actually 12.5mm, so that would be 12.5mm + 20mm + 1/3 * 1.9 + 2mm (could be 1.5mm) M42 male to male connector, m48 female to M42 male connector (1.5 - 2mm), so that would be approx 36mm, so would need approx 19mm M48 extension, give or take a mm for not knowing the extra added due to the M42-M42 & M42-M48 ! 

Just looking at a set of extension tubes now, they come in a set of 3 5 7 10 12 15 20 30mm, so a 12 & a 7mm should do the job .

Just out of curiosity, how accurate does this have to be? I mean, if it was +/- a milimeter out from 55mm, would that be an issue?

Regards,
Shaun
Related discussions
ESPRIT 100 + RICCARDI 0.75 REDUCER BACKFOCUS ADJUSTMENT. CRAZIEST!
Hi i´m a owner of esprit 100 with sw flattener and i love this combo . In last week i get the riccardi 0.75 x reducer but I'm trying to adjust the backfocus. I've seen in the documentation that for this reducer, the distance for this scope va...
Discusses backfocus adjustment with field flattener and reducer spacing issues.
May 29, 2025
Flattener uneven correction on FF camera
Hi all, I have juste received a 0.8x flattener for my new 360mm refractor (the 1x flattener is still on backorder :/ ) and I was not expecting stellar result as the flattener is made for APS-C and I have a FF (corrected field is 33mm vs 43mm for my s...
Field flattener positioning and camera distance critical to correction quality.
Feb 15, 2022
Anyone using this with a 120mm f/7.5 OTA?
I can't figure out what the adjustment value for this focal length should be. The OTA I'm using this on is a Sky-Watcher Equinox 120ED Pro f/7.5, with a 900mm focal length. This is the list that Teleskop Service provides with the required, bu...
Addresses focal length adjustment values for optical configurations similar to yours.
May 26, 2025