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optics problem

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pox's picture
optics problem


I have just come in from a usable clear night extremely fed up. For a while now, looking through the eyepiece shows a 'milky' sky background with awful light-grasp. I could just about see AV Cyg at around magnitude 11 tonight (36cm dob x 70, usual limiting mag with this eyepiece about 12.7)

The whole thing is like looking through water at the sky beneath. Does anyone recognise this? Is it the optical surfaces that need realuminising?
The phenomenon has been getting worse for a year or so now, and was more evident at higher powers, but the trusty 1" focal length eyepiece is now suffering too.

WGR's picture
Optics Problem


Do you have a friend with a telescope that you could look thru?  You could take your eyepieces and quickly separate telescope, eyepieces, observer eyes. 


lmk's picture
Not that many possibilities

Hi Mike. Well, Dobs are not that difficult to diagnose (compared to goto or CCD systems!!) "Hazyness" can be caused by the primary coating, the secondary, the eyepiece, or your vision. A slowly worsening problem seems to imply coating deterioration on one hand, but usually that doesn't cause "haziness" or loss of contrast, rather just slight dimming. The coating would really have to gone very bad to star noticing major changes though! Thats easy to determine by just looking at them. Eyepieces can grow fungus internally.

The most common cause I have for loss of performance is buildup of dew, usually on the secondary, or the eyepiece fogging up. Do you have a heater?

And thirdy, maybe your vision is going, aka. "cataracts" ?

Mike LMK


PYG's picture

Have a look down the tube Mike (and at the flat) and see if the surface has gone 'milky'.  If so you need new coatings.  If you can easily remove the primary, have a look through the bottom of the glass and see if you can see light  through the coated side.  If you can you'll need a new coating.  My guess is it's a damp problem (the bane for UK visual observers).  GIve it a blast with a hairdryer and see if your visual inspection improves. I do this after every session with my 20 inch.  Warm air on the primary from a distance of about 50cm, keep it going for about two minutes then pop the cover on.  

If it's the same effect through various eyepieces, then it's the mirror or flat.

If you do need a new coating, always ask for a silicon overcoat and always have both mirrors done at the same time!.  We need an overcoat in our damp atmosphere. If you take care of your mirrors, then a coating here should last around 10 years or longer depending on how careful you are.  Your in cleaner air than I have, so 10 years should be a reasonable target.

Good luck.  It's depressing when this happens.  Drop me an e-mail if you want to discuss further.


pox's picture
Hi Gary et al, So many thanks

Hi Gary et al,

So many thanks for the many kind suggestions. I have a skeleton tube but the main mirror is at the bottom of a roughly 15-inch deep aluminium surround, and I have not known it to dew up. The night before last was largely dew-free (it was quite windy!). The flat and finder are both heated by one of those Kendrick gubbins, and I always collimate with a laser collimator after moving the tube (I carry the tube out to the mount, which is in the back garden). The main mirror was silicon overcoated when it was last aluminised, about 3-4 years ago. Both mirrors are slightly dirty, but certainly not enough to cause the problems I've been having. Maybe it is an eye problem, though I don't have one as such; I wear glasses occasionally (TV and so on) but not all the time. No astigmatism or anything like that.

The nearest observer to me is Mark Thompson, about 4 miles away. But he might well be too busy with his new-found media career, hobnobbing with Brian Cox! ;-)

HNL's picture
hazy sky

Hi: I am not into optics problems but, I can say that I have noticed that My sky has gotten hazy.  It looks OK except when looking through the scope  I see milky sky.  Best HNL




WGEinWVA's picture
Although it does sound like

Although it does sound like it is in your equipment, whether the scope or your eye, complaints about poor transparency lately abound on the Internet forums.  I feel like it is rarely truly clear where I live here in the mid-Atlantic states.  One night, July 3, was truly clear and the difference was unbelievable.  Most dark, clear nights are still rather murky through the eyepiece.

What you are experiencing sounds similar to when you use too long of a focal length eyepiece.  Maybe your pupil in your eye is to blame?  Perhaps you could measure it when dark adapted.


pox's picture
measuring pupil

Not quite sure how I would measure my pupil! But from what has been said, it could actually be a sky problem. Actually the problem gets worse - or certainly doesn't improve - when I use a high power (i.e., short focal length eyepiece) probably because definition is more critical then. It's pretty dark where I am - I have occasionally bumped into the scope because I can't see it! But even so, my ZLM seems to have gone from about 16 to 13. It's terrible, and the stars are such a part of me that not being able to see them properly is a 'spiritual' issue.

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