Field Flattener focal reducer question

[Astroboffin]

Hi all,

Can someone explain this to me please, I was under the impression that as my scope has a focal length of 389mm I assumed, (maybe wrongly) that this means my camera will come to focus 389mm behind the scope lens.

So as my FFR reduces the focal length to around 312mm I again assumed that my camera will come to focus 312mm behind the scope lens.

I have done some tests with my DSLR fitted to my scope to work out exact distances for focus with and without the William Optics III 0.8x FFR, and I am a little confused by my findings and wanted some feedback from you guys.

The difference in back focus between using and not using the FFR is only approx 10mm, (my camera comes to focus 10mm closer with the FFR fitted) does this sound right to you, I would have thought the difference would have been a lot more.

From the back flange of my WOZS66 focuser, where the focus tube comes out is 137mm with FFR fitted to the camera sensor, and 147mm without the FFR.

One other thing I kept the correct 55mm spacing from reducer to sensor when using that.

Thanks in advance

AB

[steppenwolf]

The key word here is 'apparent' focal length. The native focal length of your telescope may well be 389mm but using a focal reducer gives an apparent focal reduction not an actual so the image on your camera is the size that you would have got if the focal length was actually 312mm but this has only been produced by an optical 'distortion' of the light cone.

[Astroboffin]

The key word here is 'apparent' focal length. The native focal length of your telescope may well be 389mm but using a focal reducer gives an apparent focal reduction not an actual so the image on your camera is the size that you would have got if the focal length was actually 312mm but this has only been produced by an optical 'distortion' of the light cone.

Hi,

Thanks for that

I understand that.....I think, lol

So would you say that a 10mm difference in back focus could be normal when using the focal reducer compared to without it?

As I would have expected a much larger difference in back focus distance.

Regards

P.s have almost finished you book, and have learnt a shed load of stuff, but am still trying to get my head around some of it, but a valuable aid.

AB

[jambouk]

It's the revere of a barlow or powermate; a 2x barlow gives an apparent focal length which is twice that of the native focal length of the scope, and the point at which focus is achieved won't be massively different than without the barlow, but the field of view will be reduced and the apparent speed of the optics will be reduced.

James

[ronin]

A reducer reducesthe size of the image, depending where it sits in the optical train and how it performs this makes the final result.

They could use a positive lens set further back from the prime image by more then 2x the focal length of positive lens.

In which case the image is reduced but the image plane is pushed further out so extended in a way not shortened.

[jambouk]

Why do you think the focal point should be so different? The optics in the focal reducer are so designed and manufactured so that focus can still be achieved using the stock focuser and using standard cameras etc. there would be no point having the focal point too close to the objective lens or too far away if the user could then never achieve focus.

James

[ecuador]

Take a look at this page (scroll down to Focal Reducers).

[Astroboffin]

Why do you think the focal point should be so different? The optics in the focal reducer are so designed and manufactured so that focus can still be achieved using the stock focuser and using standard cameras etc. there would be no point having the focal point too close to the objective lens or too far away if the user could then never achieve focus.

James

Yes that makes a lot of sense....

Because when using my FFR with the small refractor there is only 6mm of inward focus travel left when in focus, so very close to the inward limit.

I was just under the impression that wit a FR in the imaging train the back focus will be quite a bit shorter, it certainly is different on my 8" SCT when using the f6.3 reducer, I have to dial the focus in quite a bit, it seems.

Regards

AB

[Astroboffin]

Take a look at this page (scroll down to Focal Reducers).

Thanks for that

According to that the difference in focus position should be around 13mm so pretty close to what I found in my tests.

Thanks again

AB

[jambouk]

With an SCT or Mak, there is so much travel available on the focus that maybe the manufacturers can use cheaper materials/methods/designs as they know where ever the focal plane is, the user will be able to achieve focus, though they do not have this freedom with a refractor... I've just made that up by the way, have no idea if it is true.

James