How Flat does a Flattener get things ?

[AlistairW]

Hello,

Recently been struggling with elongated stars. I think I have now got to the point that the last thing I need to tweak is the Field Flattener back focus spacing, as my stars are slightly radially elongated from the centre. My question is should a flattener achieve near 0 percent curvature. I have a CCD Inspector plot from my most recent attempts, should I be aiming for 0 curvature, or is that physically impossible ?

Thanks

Alistair

 

 

 

[ronin]

In the "perfect" sense I doubt they can give a flat fiels and I would say that they will be defined as being to provide an image circle of some duameter. Say this as if I recall the Star 71 Mk2 is now specified as 30mm and so I presume that on that WO are saying flat/sharp/whatever within a 30mm diameter circle at the sensor plane. So if you have a bigeer sensor the outside this 30mm they are not going to necessarily produce as good an image.

The problem is that a flattener often does not have a specified circle, but your sensor is 22mm diagonal so I would have thiught that you would get a flat image over the sensor without great trouble.

Are you using the flatenner for the GT81? They had a dedicated flattener when they came out, no idea how good or bad it is however. If it is then you may find it useful to ask WO what the image circle size is that you can expect.

I would take it that a flattener will improve things, perfection is another matter, more a case of how close.

[AlistairW]

Thanks Ronin. 

I am actually using a Hotech FF only as the WO FF/FR gave me issues with compression rings (another story). I think I can actually get a flatter field with the Hotech, but I am in search of the optimal setting at the moment.

Thanks