refractor image

[lhuchison]

when i look through my 100mm refractor i sometimes notice the image is blotchy? specially the edges . Like i,m peering through a fog. Is this light pollution, dewed up optics, dirty objective lens , any ideas? the 8" reflector does not suffer from this.

[John]

It could be the things you mention but also it could be a dirty star diagonal mirror / prism or eyepiece lens. Does it go away when you change eyepieces ?.

The view should be crystal clear of course.

Try peering through the scope with no eyepiece in place - you should see the full outline of the objective lens with it's 3 little foil spacers showing equally spaced around the edges. This can be done indoors and in daylight. It would also show if there is anything in the tube that should not be there eg: cobwebs !

[NGC 1502]

Perhaps the best way to answer this will be a process of elimination.

Do the optics look very dirty ? (it takes a great deal of dust & crud to noticeably degrade the image)

It's usually easy to see if the optics are dewed up by taking a look.

Is the view dodgy with all your eyepieces ?

Perhaps try it during the day on terrestrial objects ?

Hope you sort it, Ed.

[lhuchison]

more noticeable on higher power lenses, 25mm not a problem,18mm sometimes but 7mm always. is it trying to push the objective lens smaller aperture, too much with the ambient light available?

[John]

7mm in a TAL 100 (I assume this is the scope) gives 143x which should be well within the capacity of the scope. If you were getting this with your other scopes at similar power then I'd be thinking it might be something in your eye but clearly thats not it.

Another thought is that, at some time, someone has taken the objective lens apart, to clean it, and put one of the lens elements back the wrong way round.

[Mr Q]

when i look through my 100mm refractor i sometimes notice the image is blotchy? specially the edges . Like i,m peering through a fog. Is this light pollution, dewed up optics, dirty objective lens , any ideas? the 8" reflector does not suffer from this.

The key word here is "sometimes". If this is true, the only thing that can cause this is a dewed up eyepiece or the same with the diagonal. The next time it happens, pull out the EP and look at the diagonal mirror. Foggy? If not, look at the inner and outer lens surfaces of the EP. Foggy? If so, DON'T wipe it clean! Put either in your pocket to warm them and in a few minuets they will clear up. With no diagonal (or EP in the diagonal), keep the opening covered to keep night moisture from getting into the inside of the scope.

Also check the outside face of the objective the next time it happens. If there is any fog on the glass, again, DON'T wipe it clean! An electric hair dryer can warm the lens and disipate the fog, otherwise, if a dew cap (a tube extension a few inches long that screws onto the end of the scope) dosen't work, you will have to quit for the night.

Let us know how your testing came out - I'm sure we all want to know the reason for the bad views.

[Neil_F]

I had a similar experience the other night. During my observing session the air pressure must have dropped quite suddenly. Within the space of around 30 minutes my secondary mirror had fogged over and it was like trying to see through soup! I had to give up and come inside for a warm cuppa. Very strange.

[astroeddie]

Dew Strap heaters eliminate this problem of fogging. refractor lenses are prone to this, especially this time of the year.

These are available in all sizes. Or get an hair dryer !!

[lhuchison]

it must of fogged as a couple of nights later all perfect. i kept checking the objective and eps all clear.

dont panic Mr Mainwaring!