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DSalters
sage
Reged: 03/03/07
Posts: 389
Loc: High Ridge, MO (St. Louis)
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I recently purchased a Celestron 5" f/5 OTA (minus the "T"). In the picture below you will see the secondary mirror stuck to the spider assembly with a thick, double-sided foam-tape. Anyway, I am struck by the amount of what looks like offset of this mirror. If this is offset, why is it so extreme? Newt calculates an offset of less than 1/10".
I will be redoing this spider and remounting the mirror. I don't plan on offsetting the new configuration though. I just want to know if there is a good reason that it currently looks the way it does.
Thanks,
Daniel
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5" f/5 Celestron Rebuild Grab-and-Go (under construction)
6" f/8 Bushnell Voyager Dob. Rebuild
10" f/5 Highe Dob. variant (under construction)
EPs: Antares Modified Erfle (30); Baader Hyperions + Finetuning Rings (17, 13, 8); BO/TMB Planetaries (4)
"The sun has one kind of splendor, the moon another and the stars another; and star differs from star in splendor." -1 Corinthians 15:41
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Houdini
sage
Reged: 07/13/07
Posts: 352
Loc: Europe
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For an f/5 scope the offset should be about 1/20 of the secondary minor axis.
On your picture it looks like about 1/12 of the minor axis, probably a bit too much.
Can you measure it exactly?
Robert
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16" f/4.9 motorized alt-az, 25" f/5 Dobson, 43" f/4 alt-az in construction
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Jason D
Pooh-Bah
Reged: 10/21/06
Posts: 1442
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Daniel,
It does look like the offset is somewhat exaggerated; however, it is not a big deal as long as your scope does not use DSC. Regardless whether the secondary was mounted with an offset, no offset, over-offset, or under-offset, by following the proper collimation steps you will indirectly compensate for the secondary over/under/no/offset.
Jason
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XT10 classic with premium optics
Tri-knob CR2 with compression rings
Round Table Platform
4.5" StarBlast
6" StarBlast6
TV EPs
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DSalters
sage
Reged: 03/03/07
Posts: 389
Loc: High Ridge, MO (St. Louis)
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In Newtonian telescopes when the secondary is placed out of the light path (such as a Herschelian telescope), you have to have the primary's focal length be long enough to make the aberrations unnoticed. In that case, isn't having an extreme offset going to eventually introduce these aberrations even if the scope is properly collimated?
Daniel
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5" f/5 Celestron Rebuild Grab-and-Go (under construction)
6" f/8 Bushnell Voyager Dob. Rebuild
10" f/5 Highe Dob. variant (under construction)
EPs: Antares Modified Erfle (30); Baader Hyperions + Finetuning Rings (17, 13, 8); BO/TMB Planetaries (4)
"The sun has one kind of splendor, the moon another and the stars another; and star differs from star in splendor." -1 Corinthians 15:41
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DSalters
sage
Reged: 03/03/07
Posts: 389
Loc: High Ridge, MO (St. Louis)
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Robert, from what I remember, it was around 1/4" and the mirror's minor axis is 1.4". I cannot remeasure it because I already removed it from the spider and cleaned off the sticky tape. This would mean that it is nearly 1/6 of the minor axis.
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5" f/5 Celestron Rebuild Grab-and-Go (under construction)
6" f/8 Bushnell Voyager Dob. Rebuild
10" f/5 Highe Dob. variant (under construction)
EPs: Antares Modified Erfle (30); Baader Hyperions + Finetuning Rings (17, 13, 8); BO/TMB Planetaries (4)
"The sun has one kind of splendor, the moon another and the stars another; and star differs from star in splendor." -1 Corinthians 15:41
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Vic Menard
Carpal Tunnel
Reged: 07/21/04
Posts: 2586
Loc: Bradenton, FL
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Quote:
...from what I remember, it was around 1/4" and the mirror's minor axis is 1.4"...
The correct offset at f/5 with a 1.4-inch minor axis secondary mirror is about 0.07-inch. If you had left the offset at 0.25-inch, and used a standard offset alignment procedure--the optical axis would have ended up about 0.18-inch offset away from the focuser. OTOH, if you remount the secondary mirror so that it's mechanically centered, and you use a standard offset alignment procedure--the optical axis will end up 0.07-inch closer to the focuser. Either way, I doubt you will notice any difference in the optical performance.
If you choose to offset the secondary mirror correctly, and the mounting stalk is 1.25-inch--the 0.07-inch offset (with a 1.4-inch minor axis) will reduce the overhang on the side closest to the focuser to less than 0.01-inch and increase the overhang on the side away from the focuser to almost 0.15-inch. It will look a little strange... but it does deliver the optimal placement for the secondary mirror. Of course, you could also decenter the primary mirror and/or the spider and arrive at an almost identical placement.
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