What is the correct distance from field flattener to CMOS for WO 132mm scope?

[g__day]

I am trying to ensure I have set my WO scopes up with the correct separation from field flattener to imaging chip. My 110FLT was missing a 16mm tube extender to take it to the 55mm seperation from flattener to cimaging chip (now sorted and images look a lot better).

My 132mm scope has coma too - but from the coma charts I have used (refer below) it indicates the camera is too close to the focuser - whilst I suspect its about 30mm too far separated instead!

My original set up has the field flattener run into a ZWO OAG (with a guide camera focuser add on - moving the imaging chip about 50mm from the pick off prism - so I required about 50mm of tube extensions to bring both the imaging chip and guide chip into focus - but this creates my coma. This all connect into a ZWO 7 position filter wheel and then into a 1600MM-c.

I suspect the correct focus separation is 55mm (an almost industry standard). So the offset dimmensions I am work with sum to be 6.5mm for the camera chip to the body + 20mm for the width of the ZWO EFW + 22.5mm for the width of the ZWO OAG means I need to connect the ZWO OAG directly to the ZWO filter wheel which connects directly to the imaging camera and have only a 6mm extension tube (not 50mm) from OAG to field flattener.

Does this sound right? The OTA I bought here a few months ago (very pleased with it) - it came with the old original field flattener if that helps.

[Camelopardalis]

The model of the field flattener is crucial in determining the correct back focus for best results. Look it up on WO’s website as they usually list the specs for discontinued products as well as their current offerings.

55mm isn’t so much a “standard” as an adoption encouragement, based on the back focus requirements of a Canon DSLR plus a T-adapter. This is purely a “convenience”, depending on your point of view (or an inconvenience for those of us who dare using something more). Many flatteners provide more than this - including most of WOs recent flatteners.

Anyways, from your image of M20, you need to add more space in front of your main imaging camera. Consequently, you’ll need to add more to your OAG too…the same amount, assuming they are currently both in focus in your rig pic. I’d suggest putting this scope side of the OAG, as you’ll then only need to use one adapter instead of two ;)

[g__day]

I agree - I am searching the WO website and all I can see so far is the https://williamoptics.com//william-optics-flattener-68-ll but I am wondering if my model is even older than that - will try and see if there is any model or post a picture of it.

The spec of the model I found seems to say only 51.5mm is needed from the rear of the cell - and I am way past this distance!

[g__day]

Pardon my ignorance - I just presumed the attachment I fitted was a field flattener - I better check that assumption that it wasn't just the rear end with from memory a glass element in it!

I don't see any markings on it at all - flattener of any type - so maybe I presumed wrong and have to buy one. Can anyone with knowledge of old WO scopes confirm does this look like a flatterner or not?

I also notice the scope is alsmot racked all the way in - only about 15mm of space left - so not much space to add any further extenders!

[g__day]

I think what I have - looking at all the pictures in the Archives for folk who have ever sold one of these is the original 68 flattener (the one on the left below) - before even the discontinued 68II model (I think they are up to 68III model now - its slimmer so can allow for more tube lenght behind it can comes in no reduction slight reduction of faster reduction variants).

So now if I have the oldest of all the flatteners I just have to find its backfocus documented somewhere - as I couldn't find it on the WO site itself!

[ronson]

Hi Matt,

Could it be this one: https://www.google.com.au/search?q=William+Optics+TMB+Designe d+Field+Flattener+for+FLT-110&ie=UTF-8&oe=UTF-8&hl=en-au

Seems the name is P-FLAT68, but on some photos looks different. Maybe the cone shaped part is added separately?

As per https://www.astroshop.eu/field-flatteners-other-photo-accessories/william-optics-field-flattener-for-flt-series/p,5034, Lens-to-chip suggested distance: 68mm.

But then, there is this link https://www.opticsplanet.com/william-optics-tmb-field-flattener-for-flt-we-flat-68.html to this diagram suggesting what it looks like different distance: https://op2.0ps.us/static/lib/supplementary/william-optics/TMBflattener_ChiptoLens.gif, 46.45mm with the cone adapter, else total distance 113.13mm.

Confusing…

Another link I found, http://www.astro-photography.net/FLT110-field-curvature-problem.html, but seems this person doesn’t use the cone adapter, so he had to add distance to get it it to the recommended total distance of 113.13mm

Might be a matter of playing with the distances and see if any of them works.

[g__day]

I have added another 10 mm extender - so the camera to the flattener is about 113mm - 120mm separated (the flattener has a threaded M48 tube with a lock nut). I only have about 5mm of separation between the focuser and the OTA - so tonight I will try focusing and seeing if I can improve coma - removing extenders about 5 - 10mm at a time!

I wonder if any of 113, 68, 55, or 46 mm (plus 1mm for filters) will end up being the correct separation!

[g__day]

Interesting - the folk at Bintel say they have the same Optec chart - they question it is right - they have never seen the camera is too far from the flattener star pattern - and reckon the correct distance should be somewhere between 40 - 60mm.

Off to buy more gear from them...

[g__day]

So I took a 2 min shot of Rigel Kenatrus at 120mm spacing versus 55mm spacing a combined 6 minute shot - much happier with the coma now! I would guess I am within 1-5 mm of the ideal now!

[Camelopardalis]

Nice one :thumbsup: the second one is much better.

[g__day]

Many thanks - I am happy!

Final imaging training set up for the night - I have posted in the Williams Optics FB page asking if anyone knows the precise measurement for sure!