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ZEISS DTI thermal imaging cameras. For more discoveries at night, and during the day.

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  1. H

    Star Testing

    Sorry for the late reply. Determining how good a scope is just by looking through it can be tricky unless it's really bad. It's much easier if you have a reference scope of known high quality set up next to it to establish the best image that can be expected under whatever the existing...
  2. H

    Star Testing

    Any lens can be hand aspherized. Glenn LeDrew at Cloudy Nights described hand aspherizing the objectives of a cheap Bushnell binocular to improve their correction (at the cost of removing some of the AR coating). For the last 20 years I've used one of Roland Christen's hand aspherized scopes...
  3. H

    Star Testing

    Hi Joker, The 90 degree shift in the ovals on opposite sides of focus happens whether the astigmatism is in the scope or your eye. Of course it could be in both so that the severity changes as you rotate your eye to positions that either tend to cancel or reinforce the scope’s astigmatism. At...
  4. H

    Star Testing

    If you can't decide whether the optics are under or over corrected you almost certainly don't have a problem. A real star test often shows a little softer rings outside focus from air turbulence somewhere in the atmosphere in front of the infinity focus point and correction also shifts with...
  5. H

    Star Testing

    If the astigmatic shape is just an artifact of unsteady air in the photos and you see circular rings at the eyepiece then you have a winner. Everything else including coma and spherical aberration looks plenty good enough. What scope is this? Henry
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