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Field flattener and newtonian?


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I get backfocus distance to camera, however this question is really about the other side and more so in relation to the secondary and focuser being where it needs to go???

Also I appreciate imaging newts have larger secondary mirrors for the larger chip size compared to pupil size outside the focuser.

So where do the field flateners need to go in relation to the secondary and focuser?

If a camera is positioned at the correct distance from the secondary without a field flattener, then if a field flattener (with no affect on focal ratio) is added, firstly would the camera stay in the same place in relation to secondary?

If a field flattened requiring 55mm backfocus was required it would need to go 55mm in front of camera, so would you need a secondary and focuser that allows the camera to be more than 55mm past the focuser?

Or is it possible with some focusers to mount field flattener on the secondary side of the focuser?

I appreciate other aspects including filter wheel etc but not getting into that yet as just want to understand first please.

I need to get a new secondary and want to make sure it's big enough but small as possible to take a flattener and all works with focuser.

I couldn't find what I was after searching, so would appreciate your expertise, hope this makes sense, thanks.


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Posted (edited)

Realised part of it I think - so it typically screws on front of camera with spacers and looks to slide inside 2" focuser. So doesn't need extra space between focuser and camera.

However any filter wheel obviously won't fit inside focuser, so part of backfocus distance will have to stick out of focuser for filter wheel.

I'm guessing the field flattener doesn't change distance from secondary to camera if it doesn't change focal ratio.

So I need secondary mirror that gets camera enough past focuser for larger gear like filter wheel... Need to make sure I get simple field flattener that fits in as some look to have adjusters that won't fit...

Interesting to see different sizes of secondaries from various manufacturers, some use same size in 8/10/12" f4 versions presumably to reduce cost so 8" would likely get camera furthest out and 12" may struggle to fit large gear like filter wheel between...

Edited by Demon
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For Newts its coma correction you need. Most CCs have 55mm backfocus which most commercial camera+filterwheel combinations can accommodate - that may be a challenge for a DIY system.


The backfocus distance is set for the best star shapes. Some CC designs specify the optimal backfocus per scope focal length. Most CCs change (shorten) the distance of the camera to secondary. For my old 250mm newt I had to swap out the focuser to get a low profile one, I could not achieve focus otherwise. That was with the Skywatcher CC (x0.95) and the Lacerta KomakorrF4 CC( x1.0).


One thing to note - even though my 150mm newt was advertised as suitable for astro-imaging it wasn't perfect. At focus with a CC fitted the focus tube protruded into the light path by 18mm. I hacksawed that bit off to remove the D shaped stars.


The larger secondaries for dedicated imaging scopes allow for shorter tubes and to push the camera further away from the OTA to avoid both the issues I mentioned.

Edited by paul
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Thanks Paul,

Not sure why your reply didn't pop up on my messages so apologies for late reply.

I think I've got my head around it, was mainly where the cc's fit and if they alter the optical distances. The main thing I've needed to do is draw it out in relation to my primary and secondary. I thought that my secondary would be small and I finally drew it and was right.

So I've been stopping down my scope by moving secondary to close to primary.

I just ordered a new secondary, both new so no dinks and going from 63 to 76.

The larger ones are a good bit more money but hoping to work with my small diagonal camera and moving focuser inside tube - should still miss optical path so fingers crossed.

I may try cc in future but for now getting good enough results at the moment.

I need to star test to see just how good my primary is - have no idea as second hand and diy...

Thanks again and appreciated.



Edited by Demon
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Old 63mm and New 75mm 1/15. Looking forward to trying this - mounting ASAP. Checking calcs on exactly where it needs to go including offset. Also altering focuser inward at same time. Both spider and focuser currently off for work... 

Ps the old secondary had the name B. C. Watts on the back if anyone knows who that may be? 


Edited by Demon
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