After each reassembly my Edge11 seems to have a slighty different focal length (according to AsiAir’s plate solving solution).
This is probably driven by the position of the camery in the Baader Diamond Steel focuser and the main mirror setting.
So I was wondering if a perfect focal length (plate solve equals scope sepcification) would be equvalent with a perfect back focus.
Any thoughts?
The focal length of an SCT will change as the primary mirror moves for focusing.
Yes, for sure - but my question is if focal length is a quality indicator for backfocus?
Arny · Sep 7, 2025 at 04:10 PM
Yes, for sure - but my question is if focal length is a quality indicator for backfocus?
No, as most telescopes will vary in terms of the actual focal length vs what is advertised in addition to the SCT being a moving target. Star analysis through focus can help determine if the center and the four corners come into good focus at the same point. This would be an indicator of proper back focus.
After each reassembly my Edge11 seems to have a slighty different focal length (according to AsiAir’s plate solving solution).
This is probably driven by the position of the camery in the Baader Diamond Steel focuser and the main mirror setting.
So I was wondering if a perfect focal length (plate solve equals scope sepcification) would be equvalent with a perfect back focus.
Any thoughts?
NO! No, no, no…
This incorrect notion seems to have 9 lives no matter how hard I’ve tried to kill it. The back working distance is a fundamental optical design parameter and it’s the spacing used to test and figure the optics during fabrication. Small differences in the radius of each component are what cause the EFL to vary slightly from telescope to telescope. Set the back working distance to precisely match the specification and then measure the focal length of your system. The focal length is the variable; not the BWD.
John
Thanks for trying to kill that ghost for me again, John :-)
Let me rephrase what I am trying to understand:
As I use a Baader Diamond Steel Focuser, I can vary both main mirror focus position AND the Baader focus position. Depending on where I set these, my focal length determined by plate solving varies between 2700-2800mm. Within that range, image distortions are quite ok.
So I wonder, whether and how the 2800mm result differs from the 2700mm result - thats where my question came from, whether in this situation the correct focal length would indicate anything?
Arny
So there is definitely NO difference in the imaging quality of the EdgeHD whether my primary mirror is all the way in or out, as long as the backfocus is the exactly as specified?
Interesting - even though I don‘t understand why the invariance is soo great
I’m a little slow to get back to this but Andrea is doing a great job of explaining it. When the sensor is located at the correct BWD, that will determine the mirror spacing and the focal length of the system. That will also determine where you get the best imaging performance.
- John
I understand the acronym BWD, but lost on IOW and wrt?
John Hayes · Sep 8, 2025, 04:11 PM
When the sensor is located at the correct BWD, that will determine the mirror spacing and the focal length of the system.
And can I turn logic this around, that whenever I meet the (actual) systems focal length, my backfocus must also be correct?
Arny
John Hayes · Sep 8, 2025, 04:11 PM
When the sensor is located at the correct BWD, that will determine the mirror spacing and the focal length of the system.
And can I turn logic this around, that whenever I meet the (actual) systems focal length, my backfocus must also be correct?
Arny
No…and that’s my point. The BWD is a design spec and it is what’s normally used to figure the optics. When a two-mirror reflector is made, there are a couple of ways to figure the optics to achieve a high quality wavefront. In almost all cases, it is easier to make a concave surface than a convex surface, which means that the primary mirror is typically figured first to meet the shape defined by the design (parabola, oblate spheroid, sphere, whatever). Then the secondary is roughly figured. The final figuring is then done to match the primary mirror. That’s accomplished by testing the two mirrors in double pass against a master flat. The secondary is mounted so that a point source placed at the precise back working distance is imaged back through the system on top of itself—at the correct back focal position defined by the BWD. That’s where a knife edge tester, a star tester, or interferometer test beam is located. The figure of the secondary is then adjusted to minimize optical errors through the system. This step is NOT used to adjust the radius of curvature of the secondary mirror—that’s done much earlier in the process. This step is solely used to fine-tune the shape of secondary mirror to optimize the optical performance in terms of image quality. That’s why the effective focal length varies slightly between telescopes. No one cares that it precisely matches the design value. It will be close but not exact.
To be complete, I should add that there are other ways to figure the components using either master optics or holographic components, but the basic process that I’ve described is a very common way to make two mirror telescopes. For example: I’ve visited the US Celestron manufacturing facility and they perform final figuring on their secondary mirrors using a similar process. They use manufacturing jigs that insure the proper BWD spacing is achieved and maintained for each test.
The only way to guarantee the optical performance in your telescope is to place the sensor at the correct BWD. Then measure the EFL to see what you get. It does not work the other way around.
John
What matters is the image quality across the field, and while the BWD provides a guide, its exactly that - nominal - and some variation is to be expected.
The actual focal length as determined from platesolving is not important.