Here's a set of several slides I made that helped me understand backfocus. Maybe you'll find them useful, too!
Basic prime focus imaging, where the objective directly forms an image on the camera sensor, relies on placing the sensor at the focal plane of the objective. To do this we need to know where the focal plane is relative to the optical tube assembly. In a refractor or reflector the focal plane location can be measured from the camera end of the focuser's drawtube when it's fully retracted, also called the reference point (Figure 1, top). For my FSQ-106EDX4 the focal plane is 178 mm beyond reference point.
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| Figure 1. Definition of backfocus |
I will call the distance between the reference point and the focal plane the True Backfocus. I call it "True" because it's commonly a number you get from the manufacturer, so you know it's realiable. The FSQ focuser travel is a very 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, where I've added all the hardware that sits between the end of the drawtube and the sensor. The purple adapters are supplied by Takahashi; they are four adapters that connect the drawtube to the manual rotator and extend a nice wide optical path out to a female M54 thread. Their total backfocus is 110.2 mm. These adapters are between the end of the drawtube and the camera sensor, so their backfocus must be included in the total backfocus. You may have something analogous to them for your telescope, so check your scope's documentation!
The green adapters are the items I add: In my case they're mainly working the M54 down to an M42 male thread that will connect with the EFW or camera's tilt plate. In your case you may also have an OAG, a electronic field rotator, or a filter tray in this green area. Again, since these all sit between the drawtube end and the camera sensor, their individual backfocuses must count toward the total backfocus.
The total backfocus 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 drawtube half extended, and because I've chosen the total backfocus "correctly" the sensor is at the focal plane. So what's the numerical value of the correct total backfocus? It's going to be the True Backfocus minus half the drawtube's travel. I'll call this value the Practical Backfocus. The following two statements now apply:
Practical Backfocus equals True Backfocus. minus half the drawtube travel
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.
Imagers should supply adapters with the following total backfocus (Numbers for my FSQ 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 is the Practical Backfocus the "best?" Mainly for safety. If you're using motorized focusing and autofocusing software, the drawtube will move in and out during autofocus. Being in the middle of travel makes it unlikely those motions will ever carry the drawtube to one of its stops, making the autofocus fail and possibly damaging your motorized focuser.
Let me be perfectly clear about the subjective nature of the Practical Backfocus. Any backfocus that allows your sensor to get to the focal plane is perfectly fine, although it may not work well with autofocusing.
This raises an interesting point: if your autofocuser movements away from focus are smaller than half the focuser travel, you can increase backfocus beyond Practical Backfocus by adding an extension tube. As a result the drawtube be less extended when focus is reached and possibly reducing dreaded "drawtube droop." This might be an adjustment that's worthwhile if your focuser has a great deal of range that you don't need.
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 can "live dangerously" by adding an extension tube that has a backfocus of 12 mm (half the focuser travel minus 3 mm). This effectively makes the total backfocus 175 mm, and the drawtube now needs only extend 3 mm for the sensor to be at focus. (3 mm + 175 mm = 178 mm, the True Backfocus. Effectively I've brought the sensor 12 mm closer to the OTA.)
I could also reduce the Practical Backfocus 12 mm giving me a total backfocus of 151mm. In this case the drawtube would need to extend 175 mm for focus, but I can't think of why that might be desirable.
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If you're using a focal reducer, tele-extender, or flattener, things may be different (Figure 3). The reference position is now a mark on the reducer or is 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. Most imagers try to get within 1 mm of the correct value. Given the software that now exists (I'm looking at you,
BlurXTerminator) it's possible to correct star distortion caused by backfocus error if it's not too great.
For reducers et al., Practical Backfocus = True Backfocus.
The CR 0.73X reducer I use has a required backfocus of 72.2 mm.
If you image using any of the following you may need to match a specified backfocus:
- Camera Lenses
- Telescopes with built-in flatteners or reducers
- Coma Correctors
- RASA astrographs
Anytime you put something that bends light into the optical path, read its documentation to see if it requires a specific backfocus.
Note that in these cases we can't play around with the Practical Backfocus as we did for prime focus imaging. When using a reducer et al. with its own True Backfocus, you will need to get the total backfocus as close as possible to that -- within practical limits.
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Several years ago I went into gory detail about what adapters were needed to connect my ASI𑽈2600MM and Canon DSLR to my FSQ-106EDX4 and Canon EF lenses. I've been imaging only LRGB since then and the adapters have worked fine. I'm going to switch to OSC this year despite this.
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, and was possibly even beneficial (see earlier discussion)
- I have an urge to reduce the vignetting when using the FSQ so I'll use M48 when possible instead of M42 adapters. Even better would be to use M55 extensions with the CR 0.73X reducer -- if I could find any
- 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 0.6 mm (about 1/3 of the filter thickness) backfocus
- The FSQ prime focus modes are within 1 mm of their Practical Backfocus value. They differ from each other by 0.6 mm so that they'll share the same focuser setting
- There's a huge number of ways to combine adapters to get these results; what I show here is only one of them
- Most (if not all) the items in "My Adapter" green come from AgenaAstro; The exceptions are noted below. A few are so old they may have been found at an archeological dig
Here are my adapters for each mode:
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| Figure 4. How I'll achieve the desired 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 as advertised? And if it doesn't, will BlurXTerminator be able to correct the distortions? Oh, the suspense!
Of course my optics may not exactly match Takahashi or Canon specs, which will only be important when imaging with the reducer or lenses. There may be some tuning necessary, so 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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A 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-M42 lens adapter, and the AliExpress Canon-M48 adapter was even less at $35. Am I justified to feel that the SBIG adapter may have been a bit overpriced?
