How critical is back focus?

[cfpendock]

I understand that without more or less correct back focus (i.e. distance from the imaging camera sensor to the image plane) that focussing can be impossible.  

I also understand that it is possible to calculate the precise position for the sensor depending on the optical path length of filters, filter housings, additional lenses, focusers etc, taking into account the back focus of the camera itself.

All of these seem to be given to around 1mm.  

My question is:  if it is possible to achieve "focus" even though the optical path length is out by 5 or even 10mm, is this "focus" truly in focus?  If you see what I mean....

Or when you are focussing, and there is plenty of adjustment to pass through the focus point, then is this focus point actually in focus?  Of course normally this could be tested on the appearance of stars in the final image, but unfortunately we don't seem to have stars these days - just clouds or haze.

i would be grateful if somebody could enlighten me, if possible without too much detail about the optical characteristics of the telescope.

Thanks,  Chris

[PhotoGav]

I have no idea, but that's a great question and I look forward to the answer!

[pete_l]

Th

My question is:  if it is possible to achieve "focus" even though the optical path length is out by 5 or even 10mm, is this "focus" truly in focus?  If you see what I mean....

There's only one point of focus. If your imaging train achieves focus at a point that disagrees with the theory, I'd go with the position you find in practice.

Since you're imaging, you'll know when you hit focus: either by finding the point where the stars are smallest (and going through that point from both sides and finding the optimum position). Or by using a focusing aid such as a Bahtinov mask.

Short focal legth telescopes have a much more critical focus-point than longer focal lengths: MUCH more critical. The focal point will also change with the temperature: as the telescope tube expands and contracts lengthwise as temperature changes. You can also find that different wavelengths of light will focus on your CCD at different positions: some further in, some further out. And if you don't have an I.R. filter, your CCD will detect that light, too and it's focal point is much different from that of visible light.

The only real answer is to focus on "real world" stars and to keep rechecking your focus precision at times during your imaging session. Just be aware that on some nights atmospheric turbulence, or even high and thin cloud,  will make everything look blurry: whether you've focussed perfectly or not.

[Steve 1962]

Chris.

My experience with a variety of flatteners is that the spacing is pretty critical.

This is not because the spacing affects the imaging train's ability to reach focus, but rather because it affects the curvature of the image.

The spacing specified by the manufacturer is normally the distance at which field curvature is at a minimum (I.e. The image is at its flattest with distortion of stars at the edges / corners at a minimum).

Steve

[cfpendock]

There's only one point of focus. 

Thanks, Pete.  Yes, I agree.  But is this a true point of focus, or is it a point of best focus?

you'll know when you hit focus: either by finding the point where the stars are smallest (and going through that point from both sides and finding the optimum position). Or by using a focusing aid such as a Bahtinov mask.

Again, is this point true focus?  I use motorised focussing (a) because I find it possible to focus far more accurately, and ( for exactly the reason you said, i.e. temperature changes over the session.  In practice I focus either with each filter change, or every hour or so.

This is not because the spacing affects the imaging train's ability to reach focus, but rather because it affects the curvature of the image.

Thanks Steve.  In reality, when fitting reducers or other bits to the imaging train I have often been in the situation when I simply cannot achieve focus at all.  That is easy - get the backfocus length approximately correct and focus can be achieved.  But if this is only "approximately" correct, is the focus the very best possible?

I have in mind trying to bring an out-of-focus image into focus. For example, I am taking a photo of an out-of-focus photograph, even though my camera is in perfect focus, the image will always be out-of-focus.  But I can certainly achieve best focus.  I know this situation is different, but it serves to illustrate my confusion....

Chris

[michael8554]

Any extra glass (filters, coma correctors, barlows, reducers etc) you introduce into the light path is going to degrade the image to an extent.
So you will achieve best focus for that particular setup, but it may not be as sharp as the setup without that extra glass.
Also, many reducers have to located a fairly accurate distance from the ccd to give the best image.

Michael

[cfpendock]

Thanks Michael.  I was pleased to see you used the word "best" for focus and image.  Yes, my reducer has to be positioned very precisely (+/- 5mm) in order to be able to achieve any focus.  But what if it is out by say 3mm?  I can certainly focus at this position, but is the image as sharp as it would be if the position was more precise, e.g. +/- 0.5mm?

Chris