Protect Window and Back Focus Calculations

tbhausen

@Tech@ZWO Do the specified back focus distances for ZWO cameras include refraction caused by their sensor protect windows, or are these specifications the actual physical distance from camera body (or T-ring) to the sensor?

w7ay

tbhausen Do the specified back focus distances for ZWO cameras

Cameras do not have back focus. If you mean "flange focal distance" (the distance from the flange to the sensor, see Wikipedia page) then ZWO diagrams appear to state it in physical and not optical distance.

In any case (when you dig through enough ZWO's spec sheets), ZWO cameras' flange focal distances have a tolerance of ± 0.5mm (I don't know if this has improved over time). That tolerance is much greater than the effect from the glass.

By comparison, a typical good DSLR camera has a tolerance of ± 0.01 mm, but they also have to deal with much shorter focal lengths.

When you are down to the brass tacks, it is QA that matters, not what is written on paper. Caveat emptor.

Chen

tbhausen

I always appreciate your detailed answers. I'm having PreciseParts make a custom adapter and I'm just trying to get a good nominal as a basis for shimming from, so I'll ASSuME it's actually 17.5mm with another ~.66mm for the AR protect window.

Just to exonerate myself, I was parroting ZWO's terminology from the ASI585MC spec's:

w7ay

tbhausen I was parroting ZWO's terminology from the ASI585MC spec's

As usual, ZWO is pulling it out of their arse.

Before getting PreciseParts to make you a custom part (and they machine really beautiful parts, the threads are really smooth), just perform a star test using washers to space out the camera (with the intended filters and filter wheel/tray).

Use the "Camera sensor too close" and "camera sensor too far" diagrams (not originally from OPTcorp) here about a third down the page ) under the "How to tell if your equipment's Back Focus is off" section:

https://optcorp.com/blogs/deep-sky-imaging/how-to-set-the-correct-back-focus

Chen

tbhausen

Thanks... I've read that article. Good stuff. I'm afraid I'll need a bigger camera before fine-tuning will offer real benefits. I just ordered the custom spacer about 1mm short so I can shim it later if I get an APS-C cam and it proves necessary.

tbhausen

Anyone know the thickness of the AR protect window on the ASI585MC?

Tech@ZWO

tbhausen
If you are referring to ASI585, the thickness of the protective window is 2mm. and 6.5mm/17.5mm back focus length of this camera is optical distance.

Hara

It can be unattached and measured without any harm. You need only very basic tools and skills.

tbhausen

Hara I understand that, but they publish this specification for some of their older cameras. It’s either 1.1 or 2 mm (I suspect 2 mm). I assume these cameras are built in a clean room; no sense opening it to possibly introduce dust.

w7ay

tbhausen I assume these cameras are built in a clean room; no sense opening it to possibly introduce dust.

If so, their clean room must not be so clean.

I just this past week got delivery of a backordered ASI220MM mini (ASIAIR can actually drive that to 2 FPS at Bin 1 guiding, so it looks promising when I use ASIAIR. Once I can do some SNR measurements with real stars, I will see if this can replace my ASI178 that I had to use Bin2 guiding to get 2 FPS, in spite of a slightly smaller sensor area in the newer camera).

Right out of the box, there are two black dirt spots. Cleaned the outside of the camera window, and the spots remained. Opened up the sensor chamber and blew at the sensor itself -- the dark spots went away.

QA had gone to lunch, obviously. It would have been easily spotted [sic] by QA.

Chen

Hara

w7ay
Same experience with my 294MC. I had to open it several times to get out all particles inside the casing. And ZWO also advise us to change the moisture pills now and then, and this requires also opening the case. So opening it is not an issue, if you are careful and think twice what kind of room you do it. I use air purifier in my study to rid out of most particles and do not do dusting just before opening the camera. So far there has been less particles inside the casing afterwards than before.

tbhausen

Tech@ZWO Thank you... Is this true for all ZWO cameras (that the back focus is the optical, rather than physical, distance--taking into account the thickness of each camera's protect window)?

Tech@ZWO

tbhausen Yes, back focus length of all ZWO cameras is optical

w7ay

Tech@ZWO Yes, back focus length of all ZWO cameras is optical

When there are other spacers in between the OTA and the camera, what you are referring to is not called back focus. It is usually called Flange Focal Distance.

Back focus is the property of a lens (like the back focus of a reducer), while flange focal distance has to do with the camera.

Back focus is the required distance of the metal back of an OTA to the sensor, while the flange focal distance is the distance from the camera flange to the sensor. They are only the same if there is no spacer in between the OTA and the camera, e.g., using a Canon EOS lens with 44mm back focus, and a Canon EOS camera with 44mm flange focal distance.

http://camera-wiki.org/wiki/Flange_focal_distance
https://briansmith.com/flange-focal-distance-guide/

The formal name of back focus is "back focal distance." See for example, page 15 of John Greivenkamp's [RIP] Field Guide to Geometrical Optics, SPIE, ISBN 978-0-8194-5294-8. I think that if you are an SPIE member, you can get the book at a small discount.

Front focal distance of an astronomical telescope is at infinity.

Chen

Tech@ZWO

w7ay Got it, thanks for your clarification, Chen.