Last December for my son's 7th birthday I bought him an orion XT4.5 Dobsonian telescope. I have nothing but good things to say about the telescope. The views through the scope always seem pretty good.
Using the star test and comparing the results to what I saw in the Aberrator star test simulation program, I thought the correction was somewhere between 1/2.5 and 1/3.5 waves PTV.
When I became aware that Roddier analysis software is available for free (thanks to Roland Christen) and can charaterize the correction of an optical system easily using nothing more than a CCD camera and a star, I decided to do it on all my telescopes.
I had already captured some star test intra/extra focal images of the XT4.5, and while they were not ideal for use in the Roddier analysis, they weren't bad. It was a bit of a hassle getting the into FITS format, but IRAS helped.
The results are that his telescope is 1/2.7 PTV and 1/14.0 RMS for a strehl of 82%, a bit better than the Raleigh criterion. I am going to repeat the images with them a bit better suited to the Roddier analysis.
What is interesting is that the predominant error is spherical, to the tune of 1/4 wave PTV and 1/16 wave RMS. There is no doubt the mirror was left spherical, which is a good choice for a f/8 114mm newtonian. The next predominant error is astigmatism, followed by trefoil, high order spherical, coma , supplimental, etc. (there are many to etc.).
Philip
Using the star test and comparing the results to what I saw in the Aberrator star test simulation program, I thought the correction was somewhere between 1/2.5 and 1/3.5 waves PTV.
When I became aware that Roddier analysis software is available for free (thanks to Roland Christen) and can charaterize the correction of an optical system easily using nothing more than a CCD camera and a star, I decided to do it on all my telescopes.
I had already captured some star test intra/extra focal images of the XT4.5, and while they were not ideal for use in the Roddier analysis, they weren't bad. It was a bit of a hassle getting the into FITS format, but IRAS helped.
The results are that his telescope is 1/2.7 PTV and 1/14.0 RMS for a strehl of 82%, a bit better than the Raleigh criterion. I am going to repeat the images with them a bit better suited to the Roddier analysis.
What is interesting is that the predominant error is spherical, to the tune of 1/4 wave PTV and 1/16 wave RMS. There is no doubt the mirror was left spherical, which is a good choice for a f/8 114mm newtonian. The next predominant error is astigmatism, followed by trefoil, high order spherical, coma , supplimental, etc. (there are many to etc.).
Philip
