Mirror cleaning

[tripleped]

How do I know when secondary mirror needs cleaning? When I shine a bright light on it see tons of tiny closely packed whiteish dots but with regular room light don’t see them and mirror looks good. Problem? Need cleaning? Have only had scope for a couple months. Did have condensation on mirror recently after coming inside after viewing on a chilly night. Would that  cause it? Am I being silly worrying about this? 

[steppenwolf]

Chances are, no cleaning is required - shining a bright light at optics will give a confusing result, you should be led by the view of the mirror in ambient light. Even then, you need a lot of dust and debris to make a difference to the quality of the view.

[tripleped]

Thanks! That’s very helpful. Figured I was overreacting. Appreciate the help!

[tripleped]

Another question: will condensation hurt the mirror long term? If so what’s the best way to prevent it after chilly night viewing?

[randomic]

If you have to ask whether or not the mirror needs work, it doesn't. Remember that every part of the mirror focuses light from one point to another, it takes a significant amount of dirt/damage to manifest in reduced performance. You can do a lot more harm than good to a mirror by trying to clean it.

I think I read somewhere that you need something like an inch squared of black paint on an 8" mirror before you can start to notice degradation. 8" circle has an area of ~200 square inches so even an entire square inch is only 0.5% of the surface.

[tripleped]

Thank you!

[steppenwolf]

1 hour ago, tripleped said:

Another question: will condensation hurt the mirror long term? If so what’s the best way to prevent it after chilly night viewing?

Condensation is part of astronomy! However, it can be harmful if not managed so the best way of dealing with it is to bring the instrument into your home after your observing session, place it on a flat surface with the dust cap and eyepieces/camera removed with a towel either side of the tube to stop it rolling anywhere and allow it to dry naturally. When dry, cap the telescope and plug the eyepiece holder with a dust cap.

[Captain Scarlet]

I clear my secondary mirror of dust by using a gardener’s water spray. I fill it with distilled water, set the nozzle to “jet”, point the scope tube slightly down so the dripped-off water runs out through the front, and fire the water jet through the open focuser tube onto the secondary. Being distilled, it dries clean.

Edited October 7, 2020 by Captain Magenta

[randomic]

13 minutes ago, steppenwolf said:

Condensation is part of astronomy! However, it can be harmful if not managed so the best way of dealing with it is to bring the instrument into your home after your observing session, place it on a flat surface with the dust cap and eyepieces/camera removed with a towel either side of the tube to stop it rolling anywhere and allow it to dry naturally. When dry, cap the telescope and plug the eyepiece holder with a dust cap.

I've found pointing it down onto a piece of cardboard does the trick as well. Best thing to do is just prevent it in the first place with dew shield/dew heater but it's nothing to freak out about, the important thing is your view/image quality.

[ozzy21]

Is an Air Duster a solution? (it is supposed that it's guarandeed that they are moisture free(

[Starwatcher2001]

When I first started observing I'd been looking at Saturn for a while, nudging my dob to keep it in the eyepiece. After a while I looked through the finder and realised I'd spent the last few minutes watching the planet though our privet hedge! It taught me that a few flecks of dust on the mirror just aren't going to affect the view.

[randomic]

3 hours ago, Starwatcher2001 said:

When I first started observing I'd been looking at Saturn for a while, nudging my dob to keep it in the eyepiece. After a while I looked through the finder and realised I'd spent the last few minutes watching the planet though our privet hedge! It taught me that a few flecks of dust on the mirror just aren't going to affect the view.

I had a similar experience watching Mars drift behind my neighbour's house. Rather than cutting off part of the disk as you might intuitively expect, the disk instead gradually got fainter and blurrier until it vanished completely.

[Captain Scarlet]

Yes I've had that exact experience too. With my Skymax 127 I looked up to see how long I had before Saturn went behind the big Magnolia, and found it'd been behind it for a good few minutes!

[Ships and Stars]

Regarding condensation, one thing I try to do with eyepieces is place them in a waterproof roll-up kayak/canoe bag or similar before I bring them inside. That way they can warm up without becoming a moisture magnet. My worst nightmare is to look through one the next day and see internal moisture stains, should it bead up inside.

To note, once they've been inside long enough to warm up a bit, I will remove them as soon as possible to naturally air dry without caps. 

I've cleaned the primary on my big dob twice in the last year because it's so easy to remove (single locking collar underneath, off in about 20 seconds). The first time it just needed a good clean, the second time was after a windy night and a ton of light dust settled on the mirror - I used a rocket blower first to remove the bulk of it, but gave it a very gentle rinse afterwards at a 45-60 angle in the bathtub. All good. 

Otherwise as mentioned above, I'd leave it unless it's obviously mucked up or you're feeling bothered enough that it stays on your mind.

Again, shining a torch on any mirror and you will instantly see tiny particles here and there - that's happened within minutes of reinstalling a perfectly clear mirror, nothing to worry about. I don't think any mirror looks spotless when a torch hits it at night.

One last thing, I do hear after use in the Spring, pollen can build up on the mirror and this can affect the coating if left for an unspecified period of time, so I will rinse mine at the end of observing season here in Scotland in May for summer storage. 

Edited October 9, 2020 by Ships and Stars

[noah4x4]

I have just had my Celestron 8" SCT serviced by high end carbon fibre OTA manufacturer Orion Optics UK (OOUK) located near Stoke on Trent, which cleaned my mirrors in mild acid, realigned all optics including corrector plate and re-centered secondary mirror, replaced my Phillips screws with Allen bolts, re-collimated and returned my OTA in a better than new condition.

I am now convinced that my mass produced and assembled Corrector Plate and secondary mirror had never previously been perfect. The difference this adjustment has made is astoninishing. I used to blame 'poor seeing' for lack of detail in planets. Now I am getting the performance I desire. I had earlier spent £500 on a Hotech Laser Collimator which after using it for collimation led me to question the alignment of my optics but I didn't think there was an easy DIY solution. Cost of the OOUK service was £120, well worth the investment. The scale of charges for different OTAs is listed on their website.  The adage, if it ain't broke don't fix it is sensible, but if convinced your optical performance isn't great, this service is worth considering, if your scope is out of warranty. 

Edited October 12, 2020 by noah4x4