There is quiet a lot of confusion about Back Focus Distance (BFD), which is basically when the telescope is connected with other equipment/ units (like ; AO, OAG, focus reducer or simply the CCD) and the focus point must reach the camera chip. The focus point of the telescope is vital to be known or estimated so that the image is shape and clear either when visualing or taking an image.
Now, the confusing part is that if we take the telescope BFD we will have to adapt the imaging train to meet this point, but is NOT that simple. Why??
Well, the image train can be composed of many units or only the CCD. You will have noticed that the majority of CCD manufactures will include in their CCD specification a Back-Focus distance, or plane or length (they are all the same).
For example, A Celestron 9.25" will have a 146.05mm BFD, meaning that we need to reach this point from the Rear Lock Nut (look inside and you will see a lenses-its from there) to the chip of the CCD.
This is the first step, for the second step we need to check all the units BFD, for example my CCD a QSI 632 has a 50.17mm, my filters are 1mm, the SX-AO is 50mm and my Focus Reducer 0.63x is 105mm!!!
The above is quite confusing given that many astrophotographers that I have seen uses the CCD (like QSI cameras) connect to reducer using the 105mm, and believe me, their images are great.
So, where do we stand??, I believe that we need to stay with the telescope BFD, in this case 146.07mm, and adapt all the units on this final figure.
Below I am showing a set of diagram to show the layout of a imaging train.
Diagram 1 : In this diagram we have the CCD & the Filters, so the measurements are:
Telescope = 146.05mm
CCD = 50.17mm
Filter = 1mm
Spacer = 146.05-49.17 = 96.88mm
Now the distance that I have posted is the CCD to rear end of the scope, but you will need to include an extension drawtube or spacer of 96.88.mm to make the 146.05mm
So, In the diagram 1 below, the 49.17mm is what I have seen in many blogs and websites, placing the camera to 50.7mm less 1mm filter to the rear end of the scope lenses, if you put the CCD to this length you wouldn't have a lot of space for nothing, the QSI has a physical distance of 44mm from the sensor to the front plate of the camera, the T Mount plate on the camera has 0.225, so 44 + 0.225 = 44.225mm, you would need an extension of 4.95+/- 5mm.
No space for the OA which BFD is 50mm to the sensor.
CCD = 50.07
Filter = 1.00
Distance = 50.07
For me this the Telescope BFD is correct
In Diagram 2: I have included the SX-AO has a 50mm to reach Back-Focus, given that the used the distance of 50.7mm -1mm = 49.7mm (which is the leftover to put the AO), the AO has an BFD of 50mm!!!
So, 49.7mm of leftover - 50mm = 00.03mm. This means that you can put together both units in the focus train connecting to the Celestron rear end. This setup is standard and I seen it many times disrigating the Celestron BFD.
If we use what I believe should be using the Telescope BFD of 146.05mm the calculation is different
Celestron 9.25" BFD = 146.05 - 49.17 = 96.88 mm
So in between the SX-AO and the Celestron there should be a spacer / extension drawtube of 96.88mm
The third diagram