Lessons Learned

Dark Spot in Centre Eyepiece

Submitted by Jim Eaton - Nov 2nd, 2005
Learned: Copenhagen, Denmark

I purchased a LXD75 8"SN last week and having put it all together, proceeded to align the view finder.

To my surprise I found that the supplied 26mm plossl showed up a distinct dark/black spot clearly visible in the centre of the eyepiece.

Although I fully understood the principle that certain large eyepieces would cause this phenomena (i.e. that a short focal ratio reflector of only f4 would indeed show up the secondary in the eyepiece as a dark spot)

I still could not understand why Meade had supplied an eyepiece that would throw up this problem.

My first thought was that perhaps the scope was not collimated properly and thus began to consult the manual to see if it would enlighten me in anyway. However, I found no reference to this "dark spot" issue in the manual.

I then proceeded to peruse the web and found several references (in the Yahoo forum for LXD75 scopes by the way) that the problem is related to the size of one's eye pupil.

During the day, your pupil is smaller and will therefore show up a dark spot in certain eyepieces - although this problem decreases when using higher power eyepieces.

However, at night this is not a big issue (with the exception of the moon and only then if one does not take the precaution of using filters to tone down the brightness) as one's pupil increases in size and the dark spot will therefore disappear.

Now, I think that this issue should have been mentioned in the manual as it will no doubt cause some anxiety for those, such as myself, who have not come across this issue before - especially as the manual recommends aligning the viewfinder during daylight by focusing on a distant land-based object, such as a telephone pole.

Anyway, thought this topic was worthy of mention as I'm sure there will be others who come across this problem.

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