Back Focus and Bahtinov

[Dmelv]

ok here's my confusion - still not understanding this fully although I am shooting deepsky photos and getting some results

 

if back focus distance is not quite "Right" will I be able to still line up the diffraction spikes using a Bahtinov mask to the exact centering on a star by using the refractor drawtube focuser - but still not have optimum focus for my photos?  Is one focus setting, say on Arcturus, good enough to continue imaging the Lagoon nebula without touching the drawtube focus.  Does one focus fit all objects?   Does one back focus fit all objects?

I am using a refractor with a field flattener - so the draw tube focus does not affect the camera image sensor in the same way with focusing movement as it would without the flattener   is this a true statement?  I think the back focus is measured from the flattener but where?  from the end or from the flange shoulder  I.E. do I include the threads or not?

The final dilemma.  Can one focus on a distant object in the daytime and adjust their back focus by either adding a mechanical helical focuser between the camera and flattener or adding threaded cylinder spacers?  If the telescope drawtube will focus on an object in the daytime with room to spare on either side, is this ok to use this backfocus for the stars?  I added spacers until I have 55mm from the flattener shoulder to the camera sensor.  The sensor is recessed back from the edge of the camera front by 17.5mm so I subtracted it from 55mm to get 37.5mm.  Is this right or am I doing it all wrong?  My photos seem to be in good focus but I am actually not certain if I could do better.  I want a high degree of perfection or else why would I even try to take a photo.  So that's the epistle.  I am learning as I go here

 

[Clarkey]

20 minutes ago, Dmelv said:

Does one focus fit all objects?   Does one back focus fit all objects?

Yes and Yes... But you would probably need / want to refocus or check the focus after a while as it will drift with temperature / filters etc. But if you are focused at infinity (stars) that focus is fixed.

 

22 minutes ago, Dmelv said:

I am using a refractor with a field flattener - so the draw tube focus does not affect the camera image sensor in the same way with focusing movement as it would without the flattener   is this a true statement?

No - the drawtube focus will still affect the 'normal' focus. The difference will be that the stars right across the field should be in focus and sharp. Without the flattener you would only be able to get part of the image in focus. If you have a very small sensor you might get away with it, but any larger sensor would show distortions.

The total 55mm distance is from the shoulder of the reducer / flattener to the sensor. So as you say in your case the 37.5mm from the front of the camera to the shoulder of the reducer should be correct. This figure is approximate and you might find to get a 'perfect' result that you might need to deviate from this value slightly. For example in my set up I have had to add about 0.5mm extra to get a completely flat field.

You can focus during the day, but you will be better off focusing using the Bahtinov mask at night. Focusing at infinity in daylight leads to a lot of atmospheric distortion which makes accurate focus difficult - but you can get close.

[rickwayne]

"Backfocus", as it's customarily used, is an annoyingly misleading term. When referring to the required distance between a flattener and the sensor (the most common use of the term), it actually has nothing whatever to do with focus!

Our telescope optics produce a curved field by their nature. So a flattener is used to ensure that the stars in the corners are in focus at the same time as the ones in the center. But the flattener works at a specific distance -- if you move the sensor closer or farther away, the corner bits will be distorted. That spec is what's commonly referred to as "backfocus" and is fixed for a given setup. E.g., flatteners often have a distance of 55mm because most DSLRs with a T-thread adapter happen to measure 55mm from the sensor plane to the front of the adapter.

So think of the flattener-to-sensor part of the train as one unit, usually the distance is maintained by some sort of spacer. In the case of the DSLR and a 55mm BF flattener, the adapter is all you need; other cameras might require extension tubes, and there are some which are adjustable. Usually fixed tubes are preferable because they can't creep out of spec.

That whole unit moves in and out to achieve focus, depending on the distance to the target and the temperature (which affects the length of the tube and other components). As Clarkey notes, focusing during daylight probably won't produce optimal nighttime focus, not least because the temperature will almost certainly differ. The depth of the zone of critical focus depends on your particular setup, but it's probably in the 100-micron range (mine is 74 microns). A 1℃ change will easily change the length of your optical path by that much.

So: Establish the correct "backfocus" to spec*, then with that locked in, find a rough focus point during the day if you like, but always finish up just before imaging with a Bahtinov mask. And in fact, as the night progresses, best practice is to refocus every so often as the scope cools. I use one hour but your mileage may vary.

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*Note that some flatteners don't actually perform exactly to spec -- the label may say 55mm but your particular unit might be 56 or 54.5. If the stars at the corners are elongated while the center is round, you might have to tweak it. But start on-spec and work from there. Here's a diagram of what the errors look like:

 

Edited June 24, 2021 by rickwayne

[Dmelv]

Thanks for your speedy and courteous responses - fellow astrophotographers

Duane