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Ceph and Cass

chill out focus failure ?

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hi Oh great source of knowledge... I have a recently acquired celestron nexstar SLT 127 which is new and is very pleasant to use. I think another 2 year learning curve and I may find my way around the heavens. I am on the coast near Sydney Australia and have the lat and long for there encoded in the unit. It is quite cool at nite to about 10-12 degrees . I had no problem sighting stars and Jupiter and Saturn and the moon. I have a large finder scope too. That said , I travelled inland with the scope well protected-in the car- and the new destination is much colder falling to about 5 celsius with lots of dew and moisture in the air. I recalibrated the lat and long and looked thru the finder on a clear nite. All good until I looked thru the scope and I could see nothing. Not a single star ! I tried moving the fine tuning and ended in frustration. I have established in the day that the optics are working fine. What happened ? regards impactcrater

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My first quess would be dew forming on the corrector plate. Do you have a dewshield for your scope? Here in the UK they are pretty much mandatory owing to our damp climate.

It has always been a mystery to me why manufacturers always supply refractors with dewshields but never supply them with Maks and SCTs.

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dear steve....happily not so intoxicated to forget to do the basics...when I took the scope back into the warm hotel room I was aware immediately of the moisture on the 'entry lens' and could see it reflected in the mirror at the end...I was initially worried that the moisture was inside and that would be that. happily the warm room removed all the moisture dewlets from the entry lens and the mirror looked mirror like again. No-one ever mentioned I'd need a dewshield and I have never heard they exist...How do they work ? kindest to all...impactcrater. ps what is the corrector plate....

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They look like this. You can buy them ready made, or quite simply cut up a small amount of something like an old camping mat and make your own. It prevents dew formation by shielding the front corrector plate from being completely exposed to the elements and cooling to the dew point where moisture will then form.


More sophisticated solutions such as heated strips can be used but then you have to worry about additional power and control units and as you will only occasionally suffer it is probably more sensible to go for the dew shield.

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ps what is the corrector plate....

Your telescope is of a design known as a catadioptric. That means that it combines both refractive and reflective elements to get an image. A refractor is lenses (so refraction only) and a reflector is mirrors (so reflection only). Your SCT has three optical surfaces (meaning 3 surfaces that bend light in some way):

1. The primary mirror, which has a spherical cross-section and a focal ratio of about f/2 or f/3

2. A convex secondary mirror, which has the effect of increasing the telescope's focal ratio (which is why the focal length works out to longer than twice the tube length).

3. A refractive corrector that's placed over the front of the telescope and to which the secondary is mounted.

As stated above, the primary mirror is spherical. A spherical mirror cannot bring light from different parts of the optic into focus at the same location. This is a "defect" known as spherical aberration. In a Newtonian telescope, the mirror has a parabolic shape, which does bring light to focus in the same location. The purpose of the corrector plate is to correct the spherical aberration introduced by the SCT's spherical mirror. The result is that the whole telescope in effect has a parabolic mirror. Why go to all this trouble? A spherical mirror is substantially easier (and so cheaper & faster) to make. Furthermore, manufacturers have figured out a way to quickly and automatically grind the corrector plates (some cool machinery involving suction and spinning disks, IIRC). As a result, SCTs are relatively cheap to mass-produce. Nowadays we also have fancier designs, such Celestron's Edge series and Meade's ACF series, that also correct for coma, which is a different aberration that's inherent event to parabolic mirrors.

There are other variations of the design available on the commercial market. One is Vixen's VISAC system, which also corrects an aberration known as field curvature; here the tube is open and has a Newtonian-like spider and a corrector directly in front of the secondary: http://www.vixenoptics.com/reflectors/VC200L.htm It's even arguable that the, fast, Newtonians we now use are in effect a catadioptric system since they're unusable without the refractive elements of a coma corrector.

Hope that's useful.

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  • 2 years later...

I found that even a standard dew corrector may not be enough for an all night session. However there is a solution to this that I'm testing & it seems to work well. Buy a camping mat which has the silver reflective shield. Glue the camping mat to your existing dew corrector with the silver side out. You may now a sexy looking new dew shield which appears to be much better at dealing with the dew. I've had the telescope & dew shield wringing wet with this arrangement & my optics have stayed dew free.

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