Calculate Back Focus Error?

[PRejto]

I've recently set up a new system using my TEC140 + new TEC FRC (a .9 reducer/corrector) that has a back focus of 115 mm. Initial tests may be indicating a slight error in getting the 115 mm correct though it isn't at all obvious as to where or how the calculation might have been in error. (The camera is an SX-46 with a KAF 16200 CCD. Resolution is 1.43 arcsec, FOV 104 x 85 arcmin) TEC indicates that the corrected circle of the TEC140 is 52mm. The 16200 CCD is 22 x 27mm so I think my results should be better....

I seem to remember reading somewhere that it is possible to calculate how the distance might need to vary by comparing focus at the center and at the edges of the FOV. I presume knowing the distance between the two focus points could reveal the correction amount and direction of the correction. Can anyone point me to some written instructions?

Many thanks,

Peter

[Atmos]

What you could do is make three plastic tabs out of a milk bottle lid. You then unscrew the camera from reducer, insert the tabs and take another test shot. It’ll move the camera back maybe 2mm and you can see if it gets better or worse.

[PRejto]

Thanks Colin,

Certainly a "method" to try! If anyone would know the "calculating" method I suppose it might be you. Is this just wishful thinking on my part?

Peter

[Atmos]

Effects are kinda linear so what you want to do is compare two images with different back focus.

Plate solving the two images plus knowing the change in back focus can allow you to roughly calculate things. I did it a few years ago, there is a bit of guess work but I managed to get it accurate to within 1mm. I was doing something different though. The back focus of the reducer changed the amount of reduction so it wasn’t about correction but getting the most out of the reducer. Happened to be that when I got the most out of the reducer the correction would shocking :lol:

[PRejto]

Here finally is the document I remembered reading a long time ago, but could not remember where I saw it.

This will prove to be helpful to many! Thanks to Roland at AstroPhysics.....

Peter

Determining what spacing change is needed:

Step 1: focus exactly on a star in the center of the frame. Note the exact focus position.

Step 2: focus on a star at the corner of the frame and note the exact focuser position.

If the focuser moves out (away from the scope) in Step 2, then the field flattener is overcompensating and the focal plane of the camera must move inward - shorten the distance from flattener to the camera.
If opposite occurs, then lengthen the distance.

This can be done in steps of 1 - 2 mm at a time until the field is flat. This is the only way to get precise flat field.

The Quad TCC was made for 35 mm format, but it can be used with larger 16803 KAF sensors if the spacing is reduced slightly. The amount of reduction is typically 1 - 3 mm

[The_bluester]

I should do that with mine. I am pretty close but it would be good to do it by better than trial and error.


Not before I finish a project I am doing at the moment though.