Here's a little set of slides I made that helped me understand backfocus. Maybe you'll find them educational, too!
Basic prime focus imaging relies on placing the imaging device's sensor at the focal plane of the optics. The position of the focal plane depends on the properties of the optics. In a refractor or reflector the focal plane distance can be measured from the objective or, since the objective is often hard-mounted in a mechanical system, the distance from a reference point somewhere else is given. Figure 1 might help visualize all this by using my FSQ-106EDX4 as an example.
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| Figure 1. Definition of backfocus |
The FSQ reference point is at the rear end of the focus tube when it's fully retracted. I will call the distance between the reference point and the focal plane the True Backfocus; it's 178 mm for the FSQ (Figure 1, top). I call it this because it is usually the only number you're given by the telescope maker. The FSQ focuser travel is a modest 30 mm (Figure 1, bottom). Notice that when the end of the drawtube is at the middle of its travel range the distance from it to the focal plane is 163 mm (Figure 1, middle).
Now examine Figure 2. The purple adapters are supplied by Takahashi; you may have something analogous to them for your optics--read your scope's documentation! For the FSQ, the Tak adapters always sit on the camera side of the drawtube, so their backfocus is included in the total backfocus. The green adapters are optional: they may include step-down adapters, an EFW, an OAG, a rotator, a tilt plate, and who knows what else you may need. The individual backfocus of each item sitting between the end of the focus tube and the sensor sums to be your setup's backfocus. This will be the distance between the end of the drawtube and the sensor regardless of the drawtube's extension, as shown in Figure 2.
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| Figure 2. How Practical Backfocus positions the sensor |
Figure 2 (middle) shows the sensor at the focal plane and the drawtube half extended. This will be the case when the fixed backfocus plus half the focuser travel equals the True Backfocus. I'll call the True Backfocus minus half the focuser travel the Practical Backfocus. The following two statements now apply:
Practical Backfocus is typically found by subtracting half the focuser travel from the True Backfocus.
The total backfocus of all the items sitting between the end of the drawtube and the sensor should equal the Practical Backfocus.
It's that easy! Just add up all the backfocus values of your adapters and spacers (and camera, too) and make sure it equals the Practical Backfocus.
This is exactly how Takahashi calculates best backfocus (see steps 1 and 2):
Imagers should supply adapters with the following total backfocus (The values for my FSQ imaging in prime focus mode are in brackets):
- Maximum focuser back focus [the distance from the end of the fully retracted metal drawtube to the focal plane, 178 mm]
- Minus ½ focuser travel [half of 30 mm, or 15 mm]
- Minus necessary telescope accessories [the four adapters (645 RD, CAA Rotator, Aux Ring (S), and Coupling) provided by Takahashi that have a total backfocus of 110.2 mm]
- Minus Camera backfocus [17.5 mm for the 2600's with their tilt plate attached, 12.5 if it isn't]
Why do you want focus at the midpoint of focuser travel? If you're using an autofocuser, it will move the focuser in and out. Being in the middle of travel makes it unlikely those motions will ever try to take the focuser past a travel limit.
This raises an interesting point: if your autofocuser movements are smaller than half the focuser travel, you can make the Practical Backfocus larger by adding an extension tube. The drawtube will then be less extended when focus is reached, possibly reducing the dread "drawtube droop." Again using the FSQ as an example, if I know my autofocuser will never move the focuser more than 3 mm in and out, I could add a 3 mm spacer. This effectively pretends the Practical Backfocus is 165 mm (True Backfocus minus 3 mm). That will mean the drawtube is extended only 3 mm when the sensor is at the focal plane. I could also reduce the Practical Backfocus to 151 mm (148 + 3), causing the camera to sit farther back from the OTA, but I can't think of why that might be desirable.
If you're using a focal reducer, tele-extender, or flattener, things are different (Figure 3). The reference position is now a mark on the reducer, or perhaps the plane of its back plate; be sure to check its documentation.
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| Figure 3. Reducer backfocus |
When focus is correct the focal plane of the reducer will be the manufacturer-specified distance from the reference position. The total backfocus of your adapters and camera will need to equal or be very close to that.
For reducers et al., Practical Backfocus = True Backfocus.
The CR 0.73X reducer I use has a required backfocus of 72.2 mm.
This is also the situation for imaging with a camera lens, where the reference point is the back plate. My Canon EF lenses have a True Backfocus of 44 mm.
Note that in this case we can't play around with the Practical Backfocus as we did for prime focus imaging. When using a reducer/flattener/teleextender/lens with its own True Backfocus, you will need to get the total backfocus as close as possible to that -- within practical limits. Most people aim for getting within +/- 1 mm. But given the software that now exists (I'm looking at you, BlurXTerminator) it's possible to correct star distortion caused by being at an errant backfocus. Up to a point, anyway.
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No, I'm not getting lazy in my advancing years. When I started CCD imaging it was almost all narrowband because that made it possible to image from my Bortle 7.5 backyard. Back then I had sufficient open sky above me, but in the last fifteen years the trees have eaten up the sky. All I have now is the area around the north celestial pole, and I can tell you --- in narrowband, it's not exactly an area that's rich in narrowband targets!
I now travel to open sky sites where the sky is usually Bortle 5 or darker and I can LRGB image. Technically I could be using OSC! OSC data is easier to process than combining three or four channels. So, ok, maybe I'm a little lazy.
So shortly I'll be diving into OSC by buying a ZWO ASI𑽈2600MC, the almost-identical twin of my 2600MM. I am going to hold onto the MM, though. I really do like its higher sensitivity and greater effective resolution, particularly when imaging with my FSQ at f/5. I'll definitely do some L-OSC imaging, and may want to augment some OSC images with H alpha.
As in past years my primary imaging optics will be the Takahashi FSQ-106EDX4 in its native f/5 mode, the FSQ with a 0.73X focal reducer, and my Canon EF compatible lenses (at focal lengths of 200, 135, 70, and 50 mm).
And now on to the adapters! First, some notes:
- The FSQ modes need new adapters because (a) I won't use the 2600MC with an EFW and (b) When I did this last time I missed adding the Takahashi Rotator Adapter's 10 mm backfocus. This error seemed not to affect image quality in the least, but I'll correct it now.
- I have an urge to reduce the vignetting when using the FSQ with the reducer's wider light cone so I'll use M48 when possible instead of M42 adapters. Even better would be to use M55 extensions if I could find them.
- I've removed the M42 adapter from my Samyang and restored its original Canon plate, making it compatible with my other lenses. This will let me use the 2600MC with all of them.
- Modes that use the EFW have an added backfocus of 1/3 of the filter thickness, about 0.6 mm
Here are my adapters for each mode:
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| Figure 4. Adapters and backfocus |
That rightmost mode (2600MM + EFW + Canon lens) is a very tight fit. I found the thin Canon-M48 adapter on AliExpress; the thin (2 mm backfocus?!) M48 to M42 adapter is coming straight from Astromania Optics. Will it work? And if it doesn't, will BlurXTerminator be able to correct the distortions? Oh, the suspense!
Optics vary slightly between two examples of any telescope or lens. There probably will need to be some tuning of all this to cope with those variances! Field testing will be high priority when the weather finally warms up.
And finally, yes, that's a lot of different adapters/extenders/spacers!
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An little fiscal side note: When I needed to connect my old SBIG ST-8300 to a Canon lens in 2012, the SBIG adapter cost $295 (this is also the current price!). this year I'll pay only $59 for the ZWO Canon lens adapter, and the AliExpress adapter was even less at $35. Am I justified to feel as if the SBIG adapter was a bit overpriced?
I'll test all these adapter sets as soon as the weather allows.
Next time, maybe some old data reprocessed!
