I have previously cleaned my corrector in distilled water and with a little alcohol but have never done more than dusted the mirror with a rocket blower..Not keen to do it again.
Now it seems having the reducer left on too long and some surprising condensation has led to a dirty mirror and perhaps the beginning of growth.
There are new tiny white spots and when looking in with a torch at night some very tiny shiny pinpoints. Condensation? Dust?
Worth cleaning or just go back to shorter exposures.? I also wonder if alcohol misted on the mirror would kill everything and leave no marks.
Perhaps when cooling down I should leave the corrector off and use a heater at the back near the mirror.
I cleaned my 10" Newt couple of times in the past.
Basically, I left the tap water (in thin stream) to soak mirror for couple of hours (overnight), then I applied diluted dishwash liquid and removed dirty spots by gently massaging mirror with clean fingers.. and then flushed it with distilled water.
All water will simply drop from vertical mirror surface, remaining small droplets (if any) will evaporate without trace.
Well I removed the mirror assembly and cleaned it as per recommendations. Adding that I destroyed the potential fungus with a little isopropyl alchohol first. Used four litres of distilled water.
Developing pinholes I think as you can see in this view from the back. Thin spots all over. So either stick with exposures less than 2 minutes (where I did not notice too much) or refinish the mirror. But now I know....
Those pin-holes shouldn't degrade your image and may well have been there since new. In the unlikely event that they get worse by the next time you clean it, then you can consider a recoat. It's the front surface that matters most
I think the pinholes are in the front surface mirror coating. That is why you can see them from behind with a strong light under the mirror.
They can be seen in a five minute exposure as tiny dots but not in the refractor sub .Both cropped down to 1/5 width around 1000x700
They are partly eliminated in stacking but not completely as shown in the first image
Quote:
Originally Posted by AstroJunk
Those pin-holes shouldn't degrade your image and may well have been there since new. In the unlikely event that they get worse by the next time you clean it, then you can consider a recoat. It's the front surface that matters most
Take an equivalent dark frame and see if those dots line up. If they do, then it's camera noise. If you rotate the camera and the dots remain in the same position with regard to the target object, its the scope.
Dots on the mirror can't be seen on the image taken with that mirror.
Theoretically they will reduce contrast.. but the effect is negligible.
However if there were streaks (as produced by rubbing the mirror surface with rough material) they would be visible as diffraction spikes (and they would be perpendicular to streaks on the mirror).
As Jonathan suggested, you are looking at hot pixels.
I don't know enough about optics to tell if these particular defects are definitely caused by the mirror coating. I do know enough about the DSLR camera dng files and PS to tell that these are not hot pixels. In this DSLR I have updated the hot pixel list in camera and this information is passed to the software which automatically replaces the hot pixel . These images are from single unstretched raw DNG files in PS. The second image is from a Tak F8 refractor with the same camera for the same time and does not show the same amount of defects.
There are other types of sensor noise generated by the heat of long exposure and which you can see in the dark frame taken at the same time, if you take the time to look at the middle image.
However the Centaurus A image defects are more numerous and different to the sensor noise and so are not removed by the dark frame removal process in DSS or the automatic hot pixel list removal also used on camera raw DNG files imported into DSS.
It is good to imagine that a mirror coating would last for 25-30 years used night after night with no deterioration visible in a 20mp prime focus image or visible pinholes, but I doubt that is the case. Internal condensation must play a role. The question is whether deterioration causes a problem in a final image. Anglo Australian recoat their huge mirror every year for a reason I suppose.
If the coating is damaged I'd avoid wetting it. You might also chip some more by cleaning it. The only thing I can think of is the edge of the chips might catch the light so I'd blacken them with a texter. Otherwise if you have the funds just PM saintech and get a cost for recoating it.
Ray, I am not sure what I am looking at - could you mark those details that bother you on your image?
How about taking flats (clear sky, short exposure so histogram is at 1/3rd)?
This will at least eliminate the dust on the sensor (which will not be visible on darks)
Marc is right - and actually having already wet cleaned it the coating will deteriorate quickly from now on. While it will be ok for now it will produce scattered light much the same as a dirty corrector does. At some point you’ll have to decide whether to recoat it (Saintech).
You can see from the attached dark subtraction from the image that the dark frame noise (shown as dark red groups of pixels removal) only removes some of the noise with which it lines up with perfectly.The other bright spots are single pixel spots all over so I am not sure what they could be.
I am not sure I would do this due to the palaver of getting the mirror off the glue to the support and then getting it all back in the exact same location to match the corrector. The secondary also should be involved which would be another headache.
Good to know it could be done if required and could be still much cheaper than another second hand scope.
Can dust or condensation cause bright reflection points? I could look more closely at the edge as you suggest.
Do you mean it would be better to simply use a blower or use an isopropyl clean as the mirror makers do.?
Will scattered light produce those single pixel bright spots?
Quote:
Originally Posted by Wavytone
Marc is right - and actually having already wet cleaned it the coating will deteriorate quickly from now on. While it will be ok for now it will produce scattered light much the same as a dirty corrector does. At some point you’ll have to decide whether to recoat it (Saintech).
The single bright pixels indicated are not part of the dark frame image. So they must be in the large opticals to be that small.
Some of the sets of bright pixels indicated in a group are removed in the dark frame and some were removed as part of the hot pixels list.
The flats from the same session show nothing. There is no dust on the sensor as it would be much bigger and is very visible. I often blow it off with a puffer which keeps the sensor clean as well as the shakes on start up..
Quote:
Originally Posted by bojan
Ray, I am not sure what I am looking at - could you mark those details that bother you on your image?
How about taking flats (clear sky, short exposure so histogram is at 1/3rd)?
This will at least eliminate the dust on the sensor (which will not be visible on darks)
Pixel peeping will drive someone insane...if you need to zoom that much, all fun is gone
I am not sure I can see how pin pricks in a coating would manifest as pin pricks in an image. Then again, what I know about mirrors could be written on a pin head. (and I have NO intention of learning)
Those are definitely not caused by dust or pins on primary mirror.. They are the size of individual pixels. NOT produced by imperfections of the mirror.
I think you shouldn't worry too much about re-coating at all.
I had something similar: residuals of hot pixels (visible on attached very-stretched image), because I was in a hurry and I used master dark from previous night, so sensor temperature did not match.
They look like this (extended) because the alignment was not ideal, and DSS stacker stacked stars, not hot pixels - so they are "smeared".
I would simply ignore details like that - adjusting dark level will take care of them/
The single bright pixels indicated are not part of the dark frame image. So they must be in the large opticals to be that small.
You won't see any bright pixels in your images as a result of pinholes in your primary mirror. If your mirror had let's say a chunk the size of a soft drink can missing, provided you have blackened any edge as not to get reflections you'd get a drop in luminosity and possibly contrast. Even a coin size chip wouldn't make much difference. So you'd be hard pressed to see any degradation with the pinholes you've mentioned.
Reassuring regarding chips in the mirror. I had no option but to clean due to start of fungus growth but using water was a pain, and perhaps counterproductive.
I will try again with the cleaner mirror and more dark frames and the dew heater on full
So I suppose the result of all those pin holes and thin spots which are everywhere visible at close spacing may be simply some loss of brightness over time.
Interesting that these particular image defects are all single pixels where most of the visible dark frame noise is multiple pixels, visible as tiny coloured lines in the original dark frame . Seen at over 200% in a PS layer the noise subtracts as an asymmetric blob of pixels similar to the hot pixel list automatic subtraction.
But then perhaps not all dark frames from the same session can show identical noise and will not catch all defects.
Thanks
Ray
Quote:
Originally Posted by multiweb
You won't see any bright pixels in your images as a result of pinholes in your primary mirror. If your mirror had let's say a chunk the size of a soft drink can missing, provided you have blackened any edge as not to get reflections you'd get a drop in luminosity and possibly contrast. Even a coin size chip wouldn't make much difference. So you'd be hard pressed to see any degradation with the pinholes you've mentioned.
