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Secondary obstruction

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#1 Rob N

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Posted 16 July 2019 - 10:38 AM

I have an 18” f/4 dob and can see the secondary with the Ethos 21mm when looking at the full moon. Does this imply that it’s not the optimal low power eyepiece for me when looking at DSOs?

#2 TOMDEY

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Posted 16 July 2019 - 10:51 AM

For DSOs (Deep Sky Objects) under decent Dark Sky conditions... you should not notice that effect. The full moon is undoubtedly constricting your eye's pupil way smaller than the 5mm pupil provided by the eyepiece. You should use the darkest moon filter (13%) that you can get your hands on... and may still get that effect. Or, alternatively, this may be one situation where a nice 7-inch off-axis stop might really give great views of the moon!

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#3 Jon Isaacs

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Posted 16 July 2019 - 10:52 AM

I have an 18” f/4 dob and can see the secondary with the Ethos 21mm when looking at the full moon. Does this imply that it’s not the optimal low power eyepiece for me when looking at DSOs?

That's normal and has no relevance when viewing DSOs.

 

When you look at the full moon, your pupil contracts to daylight levels, maybe 2 mm or less.  Your pupil acts as an aperture mask.  The exit pupil is 21/4 = 5.25 mm but you're only admitting 2 mm. Meanwhile, the secondary shadow has not changed in size because it's centered.

 

The net result is the O becomes effectively large and its visible.

 

With a dark adapted eye, it's not a problem.

 

Jon


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#4 Redbetter

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Posted 16 July 2019 - 11:05 AM

No.  The full Moon is enough to dilate your pupil to a much smaller diameter, probably 2 mm or so.   DSO's are not going to dilate your pupil like that, it should be near maximum with low illumination.

 

The 18" f/4 has something like a 4" secondary mirror.  So the obstruction will take up 4/18 = 2/9 of the exit pupil.  The exit pupil for the 21mm will be 21/4 = 5.25mm, so the obstruction is 1.17mm.  With a 2mm pupil the effective obstruction is 1.17/2 = 58.5%.  I would expect to see an obstruction that large.   


Edited by Redbetter, 16 July 2019 - 11:06 AM.

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