Optical design

The optical design

GENERAL OVERVIEW

This is not an obvious design since the optics has a pretty low aperture. The main mirror has a diameter of 257mm at F3.5, hyperbolic b=-1.4, the secondary mirror is a 89mm minor axis. This yield to an obstruction ratio of 0.35, which is not so important because this telescope aims at making large field of view imaging. Moreover, the largest illuminated field was a critical point in this design. The field of view is 1°48' x 1°48' using the 2x2K CCD camera.

IMAGE QUALITY

Without a field corrector, this setup would be useless, and according to optical simulation (ZEMAX), the field corner (CCD2x2K) would hexibit elongated star of 323µm (i.e. 23 pixels wide !! ). The pixel size is 3.15 arcsec (and focal length is 915mm).

Without field corrector system..The actual pixel size is drawn for comparison.

In order to overcome this issue, a field corrector must be inserted between the focal plane and the main mirror. A first simulation has been carried out using a two lenses field corrector (which is far from being optimal, but better than nothing).

Top : spot diagram using a two lens corrector, bottom is the focal plane optical layout including the 5 star locations used to compute the previous spot diagram and a 2048x2048 CCD (15um pixels). Up to 8.3 mm stars of less than 16um can be acheived.

With a 2 lens field corrector, up to a radius of 0.87 degree from the center, the shape of the star is still "usable". The main hyperbolic mirror has helped to get these results. The wavelengths are 500,700 and 900nm. The corrector elongate the focal length from 915 to 944.5 mm. The circle of 1.75 degree is usable with 14um pixel, the extremes corners.

The corrector is a 80mm diameter, located at 812 mm from the main miroir, 23 mm thickness, focal plane is located at 89.7 mm from last lens surface, yielding to a 9.7mm focal back shift.

The corrector is a 80mm diameter, located at 812 mm from the main miroir, 23 mm thickness, focal plane is located at 89.7 mm, yielding to a 9.7mm focal shift. Total equivalent focal length is 944.5mm, F3.7 system.

On the sky, tests have been carried out, and showed the following results :

Left :center field (black cross): FWHM is 1.6, right : stars at 15mm from center (55'), FWHM is 2.5x2.9, zoom is 4X, one pixel is 3.05 arcsec. Since this field corrector has been retrived almost from trash (!), the only way to optimize its use, is to adjust the distance between the two lenses and the focal plane. The radius of the field corrector could not of course be changed ! Anyway, this is not so bad ....

In the future, if everything works well, a new custom-made field corrector will be designed and build to get better performance.

Field distortion is an important issue and has been reduced here to 0.94% in the corners

SYSTEM TUNNING AND PHYSICAL SET-UP

The system is not easy to tune because of the low F ratio and the fact that a field corrector in front of the CCD doesn't make same stars patterns like it would do it without field corrector when it is untunned. For instance, this is the consequence of a 10 and 5 mm shift of the main optical axis wrt to the field corrector :The stars FWHM is degraded more or less like it would be without no corrector field, but leading to lower effects.

The best way to tune the optical axis is :
1. tune the M2 with naked eye and no camera
2. to tune the 3 M1 cell screw, so as to move smoothly the optical axis toward the desired direction and overcome residual coma. The 3 screws are depicted here and the way they behave when they are screwed.

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By Cyril CAVADORE, 10 June 2001



Top : spot diagram using a two lens corrector, bottom is the focal plane optical layout including the 5 star locations used to compute the previous spot diagram and a 2048x2048 CCD (15um pixels). Up to 8.3 mm stars of less than 16um can be acheived. With a 2 lens field corrector, up to a radius of 0.87 degree from the center, the shape of the star is still "usable". The main hyperbolic mirror has helped to get these results. The wavelengths are 500,700 and 900nm. The corrector elongate the focal length from 915 to 944.5 mm. The circle of 1.75 degree is usable with 14um pixel, the extremes corners. The corrector is a 80mm diameter, located at 812 mm from the main miroir, 23 mm thickness, focal plane is located at 89.7 mm from last lens surface, yielding to a 9.7mm focal back shift.

The corrector is a 80mm diameter, located at 812 mm from the main miroir, 23 mm thickness, focal plane is located at 89.7 mm, yielding to a 9.7mm focal shift. Total equivalent focal length is 944.5mm, F3.7 system. On the sky, tests have been carried out, and showed the following results :

Left :center field (black cross): FWHM is 1.6, right : stars at 15mm from center (55'), FWHM is 2.5x2.9, zoom is 4X, one pixel is 3.05 arcsec. Since this field corrector has been retrived almost from trash (!), the only way to optimize its use, is to adjust the distance between the two lenses and the focal plane. The radius of the field corrector could not of course be changed ! Anyway, this is not so bad .... In the future, if everything works well, a new custom-made field corrector will be designed and build to get better performance.