Focal reducer back focus vs camera BF

[Amaranthus]

The basic definition of back focus is this: "Back focus is the distance from some telescope component to the focal plane, or point where the telescope reaches focus."

This concept is simple enough to understand when considering a focal reducer. For instance, the back focus on my TeleVue 0.8 FF/FR is 56 +/-4 mm, which means that the camera chip needs to be this distance from the end of the FR in order to achieve focus (and the designed FR).

What I'd like to seek clarity on is the concept of camera back focus. How does this relate to the above, in terms of designing an imaging train? Let's say a CCD camera reports a back focus of 40 mm. Does this mean that the minimum back focus needed for this camera is 40 mm (i.e. the distance from the chip to where the T-thread connects to other equipment), and thus:

(i) In the above example with the TV FR back focus of 56 +/- 4mm and the camera with a 40 mm BF, you would need a spacer of 12 to 20 mm in order to achieve focus - correct?

(ii) If you had a FF with a BF of <40mm, it could not be used with the above camera - right?

Or am I missing something? It's important for me to get this concept of camera back focus crystal clear for when I purchase my next AP setup!

A related question - some equipment specifies extremely precise BF distances (e.g. 53.2 mm, whereas others give an integer and a tolerance range [e.g. the TV FF noted above]). Is it always the camera that requires the precise distance, and if you have a FF with a range, then as long as the range includes the camera BF distance you will be okay?

[Merlin66]

barry,
You're mostly correct in your assumptions...
The focal reducer spacing is driven by the need to present "acceptable" images...some designs are very sensitive to the spacing. Incorrect spacing can cause significant aberrations.
For the camera - the back focus is a physically fixed distance from the front flange to the plane of the CCD. Obviously different cameras have different distances.
The "standard" for a DSLR is 44mm, 55mm when a standard T2 adaptor is fitted.
Hope this helps.

[RickS]

A spacer of 12 to 20mm will give you the correct distance from the FR to the camera sensor assuming that the 40mm distance specified for the camera is the optical distance. The optical distance and the physical distance won't be the same if there are filters or other optical windows in the light path, e.g. a 3mm thick Astrodon filter reduces the optical distance by about 1mm.

Note also that getting the correct spacing between a field flattener or focal reducer and the camera sensor is to guarantee that the FF or FR operates correctly. It doesn't necessarily guarantee that you can reach focus and you may need to make adjustments in the spacing in front of the FF/FR to make this happen.



Correct modified by comments above.



The spacing specifies where the sensor needs to be in relation to the FF or FR for it to perform best. If the specs include a range then hopefully anywhere inside that range will give good performance. FRs tend to perform OK if the spacing isn't perfect but the size of the image circle and the amount of focal reduction may vary. FFs are typically more fussy and if you don't get the spacing right then you'll probably get a less flat field and some visible aberrations at the edges of the image circle.

Cheers,
Rick.

[Camelopardalis]

This is all good stuff chaps, keep it rolling :D

When I started taking snaps a couple of months back I wasn't sure about the diagrams for my kit. My refractor specifies 68.32mm or something ridiculous, so naturally I added a 12.5mm spacer into the optical path, but the stars are all funny out to the edge. Not using the spacer gives acceptable results (to me!)

Same for my Edge 8...I recently took some shots from Bretti and kicked myself when I saw them on the computer because I'd left the extension tube on by accident which meant my optical path was about 30mm too long for my Edge 8 + reducer, so there's ugly coma outside of the centre circle :sadeyes: For this scope, Celestron specify 105mm +/- 0.5mm - how practical is that!?!?

[traveller]

Dunk, according to Celestron, a C8 with f6.3 has backfocus of 3 inches (75mm) http://www.celestron.com/c3/support3/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=2210
I am only raising this as I am attempting to put my QHY8 on my C8 with a f6.3 FR. The distance from top of the camera body to the chip is 7 mm, so I need to have 68 mm clearance between the FR and the camera body (I hope I am correct in this :shrug::question:)
Bo

[Camelopardalis]

Thanks Bo, but I'm using the Edge HD version with 0.7x reducer which gives a little more backfocus but less than at f/10 :sadeyes: (which is why the T-adaptor has the extension tube)

[Amaranthus]

Thanks for the confirmation and clarification Ken & Rick - very useful.

[Camelopardalis]

Is it safe to assume that when the manufacturer specifies a given distance from, say, the outer piece of glass that the thread that the T-adapter screws onto fully meets that spacing requirement?

Sorry for the mini-hijack Barry :help:

[RickS]

Usually you don't measure from the glass but the end of the adapter: http://rainerehlert.com/Takahashi/MetalBack-00.jpg

Also a tip from Ashley @ Precise Parts: "Incidentally for your information a male thread does not contribute to the effective length while a female thread is an integral part of the length."

Cheers,
Rick.

[Merlin66]

Hmmm
For the Celestron x0.63 reducer, I've always used a spacing of 105/110mm.....the reduction factor varies with the distance....
I made up a spreadsheet a few years ago to assist....
(This has been updated to reflect the NEW Meade reducer focal length)

[Camelopardalis]

Great info, thanks chaps :thumbsup: