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Artficial star distance?

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seucamthepo:
Looking to begin star testing my 80mm f5 and need some clarity for the distance of my artificial star. I'm guessing a 5mm eyepiece and approximately at least 60ft minimum according to my figures but isn't that too close? Thanks

Duane Berhane:
<sub>50 feet or so is about right, if you got a long hallway the length of house can do it inside. My last house couldsetup scopeby front door aimed down hall, leave garage door open and put the hubble flashlight in dark on far wall, still needed an extension tube to get to focus. Stellarvue has a 4.75" for a reasonable price.</sub>

taibedaha:
Thanks. It just sounds too close. Am I correct in the 5mm eyepiece?

Tim Massey:
Are you testing the telescope's colour correction, its objective's spherical aberration, or just trying to see how well it performs?

For an optic designed to operate at infinity so close an artificial star might throw up a completely false result under test.

I keep meaning to put a Christmas tree bauble or shiny ball bearing on a post at 100 yards in sunlight to see if it can work for optical testing of achromats.

Even this distance is considered much too close for some compound instruments.

A brilliant Cree LED could be used to illuminate the shiny ball at night if daylight swamps the results in sunlight.

Very distant electrical insulators on overhead wires used to be favorite telescope targets in sunlight.

Eric Guffey:
It does in fact depend on what you are trying to do with that artificial star.
 If you are just collimating, then such short lengths work. If you are evaluating the quality of the optics you'll get some spherical aberration at that distance.

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