Cleaning SCT - what to do

[joecoyle]

Hi

If you look at the master flat below, you will see a lot of dust bunnies. Quite a few were on the outside of the corrector of my SCT, which are easily removed with a rocket blaster. 

Some of them however, like the really dark ones are either on my primary mirror or the inside of the corrector. How on earth am i supposed to clean them?

I shone a torch down the focusser and can see the dust particles. 

Has anyone got experience of cleaning an SCT internally?

Is it best to leave it and correct with flats until it falls off naturally or try and rocket blast it out? 

My astrophotography career is not going to be stopped by a few dust bunnies!

Cheers

Joe

[damnut]

Hi There,

The outside of the meniscus can be cleaned with baader wonderfluid. This is not recommended for mirrored surfaces..

Are you sure these are dust bunnies?? Some look perfectly circular, the reflection of the secondary??.

If you must clean it :--

1. Mark the outer ring that holds the corrector in several places on the circumference.

2. Loosen the screws holding the outer ring in place.

3. Remove the ring, being very careful to note if any pieces of packing are there.

4. Clean as required.

5. Re-assemble being careful to put back any packing material and the meniscus in EXACTLY the same position.

There are some vids on youtube too.

[joecoyle]

Isn't a meniscus something to do with fluid in a measuring cylinder?

What does it mean in a lens sense?

Could it just be a focus problem then?

[Joseki]

from what I've read dust has to be quite close to the focal point/sensor to show up.  The large central obstruction of a sct or regular newt could be considered the mother of all dust bunnies if it had an impact.  - so it doesn't sound likely that you'll need to dismantle your scope (and probably introduce more dust which will then settle on the optics) and I'd look closer to the sensor for the culprit (rotating different components might help identify where they are) .  Taking flats should sort it out anyway.  

[damnut]

The meniscus or corrector is the term for the front element on an sct. If you take this off, it must go back in exactly the same place, as it is matched to the primary mirror

[Mr Spock]

A meniscus lens is a Maksutov corrector, an SCT corrector is aspheric plate corrector not a meniscus.

Anyway, any dust bunnies will appear out of focus. The 'doughnut' shape is due to the secondary mirror. It's normal to have dust on mirrors and flats should eliminate them.

Make sure they are on the mirror before attempting remedial action - you can remove the corrector plate, but must put it back the way it was - an SCT will have the primary and corrector matched to complement each other.

[MarsG76]

I wouldn't recommend "cleaning" the inside of your SCT unless you plan on doing it in a clean room. The chance is that you'll end up with more dust inside then you started with. I think completely unnecessary and you won't get much dust in there unless you leave the visual back open in a dusty area over time.

Those dust bunnies look like dust on or near your sensor. On the sensor, Barlow lens or filters you use. Dust on the corrector plate would not be visible in your pictures any more then you can see the secondary mirror. Shining a light into you corrector will make your dust problem inside look a lot worse then it actually is. A bit of dust will not effect you SCT performance.

If you will be taking the corrector place off, make sure you mark the position it is in, it has to go back in exactly the same position or your SCT will never perform right again.

As far as cleaning your corrector. I use a roscoe lens cleaner and after blowing all loose dust off, very carefully wipe the corrector down first with lens cleaner wet lens tissue then dry lens tissue, again, very gently.

I only cleaned the SCT twice in the 6 years I owned it, you don't need to do it often and only when the corrector is very dusty or dirty with marks left after some dew dried up.

[Davey-T]

Defo look like they're very near the sensor, I've got an equation somewhere to work out exactly where they are in the imaging train based on their size, can dig it out for you if you're not mathematically challenged 

Dave

[damnut]

Only cleaned my 10" LX200 once too, there was a dog hair in it. Didn't affect the image, just annoyed me... Bloomin setters Also worth keeping the sct pointed upwards else  the corrector plate can fall out!

[JamesF]

I think people panic far too much about taking SCTs and Maks apart to clean them.  It's no big deal as long as you're methodical and patient and you're set up to realign and collimate everything after reassembling it.  Collimation isn't a nightmare either, though it can be a bit of a slog.

That said, I'd agree that those dust bunnies are probably far closer to the camera sensor than you realise.

To tie down the exact location you might start by rotating the camera, say.  If the dust bunnies stay in the same place in the frame than they're on the bits that rotated.  If they don't then they're somewhere else.

My suspicion is that they're on the camera's optical window or a filter.  You should be aware that they can be too small to see, even with a magnifying glass.  What looks clean may well not be.

James

[joecoyle]

Thanks guys - ill try rotating the camera and bits, to track down.

If it is the secondary... why would they be reflected in so many different positions?

Cheers

Joe

[JamesF]

The distance of the dust bunny from the sensor is allegedly the focal ratio of the optical system multiplied by the diameter of the dust bunny.

Assuming a few things (you're using the OTA at f/10, the image is not resized and your camera is the 414EX) the bunnies look to be around 60 pixels in diameter.  At 6.45um per pixel that's 0.387mm diameter and multiplying by 10 gives you 3.87mm.  That would put the dust on the camera's optical window I reckon.

Obviously this will be completely wrong if my assumptions are faulty.

James

[joecoyle]

It was on the sensor window. Nothing to be seen of course

[JamesF]

Good that you've found it.  I'm really not that surprised that you couldn't see it though.  It's amazing how tiny particles of dust can be and still look like they ought to be a huge lump of rock from the camera image.

James