Jump to content

Stargazers Lounge Uses Cookies

Like most websites, SGL uses cookies in order to deliver a secure, personalised service, to provide social media functions and to analyse our traffic. Continued use of SGL indicates your acceptance of our cookie policy.

sgl_imaging_challenge_banner_through_the-_eyepiece.thumb.jpg.cb85f690376dcb3053c747827de6bf9e.jpg

Ross1204

Would a small scratch on the primary mirror show up in a DSO image?

Recommended Posts

Hi, 

Does anyone happen to know whether a small scratch on the primary mirror of a newtonian would show up in a DSO exposure? 

Was trying to clean the mirror and noticed a small scratch which I don't believe was on there before the cleaning. Not really a clear night in sight to check so just thinking if anyone could shed some light on the situation. Most of the googling I've done has shown posts where people have suggested it shouldn't be a problem but they are predominantly referring to visual astronomy. I am thinking that the greater sensitivity of the camera sensor is more likely to show up the problem. 

Thanks 

Ross

Edited by Ross1204

Share this post


Link to post
Share on other sites

I very much doubt it will show up.

  • Like 1

Share this post


Link to post
Share on other sites

No. It will be fine. I've used a terrible scratched & fungus ridden mirror before, and still managed very good observing!. So for imaging you should perfectly ok.

Edited by Rob
  • Like 1

Share this post


Link to post
Share on other sites

Depends on type and size of scratch.

Two possible things can happen - light scatter and diffraction effects. If you place any sort of obstruction on primary mirror it will show up as a sort of diffraction artifact - secondary spider diffraction is one example of it. Any straight sort of blockage on primary will throw perpendicular (to it) spike like artifact. Size of blockage (it does not have to be real blockage - just a place on mirror where there is less reflectivity), and amount of blocked light determine size of artifact.

Circular blockage will behave differently - it will not cause spike like effect but rather "aperture diffraction" type of effect - light will be spread from each point in radial way like halo - this has similar effect of central obstruction - lowers resolution.

Above happens due to lack of light bouncing off that part of mirror. Second thing that scratch can do is randomly bounce light off (simply because it is not smooth in surface) - this will increase background brightness somewhat (you can think of it as randomly distributing photons that would otherwise be reflected to a single point).

How large effect can this be? It needs to be seriously large to show up in images - you will have trouble guessing if scope used for DSO image has 25 or 33 percent central obstruction - so fact that it blocks some light (circular form) for small surfaces is going to be extremely small. If it is straight - compare it to surface of secondary spider - if it is much less in surface (and I believe it is) - it will have proportionally smaller effect.

If it is under secondary (in its shadow) - there will be no difference at all (no light will bounce of it anyway so no problem there).

In the end - if you are likely to see any artifact from small scratch it will be in long exposure of very bright star - and it will be rather small effect compared to other things.

Look at this to put things in perspective:

image.png.96fc9daad7569d5cb039073feafa872b.png

This is crop of one image in top part where I did not pay careful attention to placement of OAG prism - part of it ended up in light path - so light was diffracted of edge of the prism (around 7-8mm long) - and mind you this is very close to focal plane so light is already "very concentrated" - much more than it is when on primary (more light to diffract when "concentrated").

It created very small vertical diffraction spike in the image - you can barely see it. Scratch shorter than this is probably going to be undetectable on all but very brightest stars if it shows at all.

  • Like 2
  • Thanks 1

Share this post


Link to post
Share on other sites

 

A photo of the scratch would help, but I’m agreeing with those that are saying it won’t be a problem, imaging or visual.

To be a problem, a defect like a small scratch needs to be at the focal plane - either the eyepiece field stop or the the imaging sensor.

If it were me, I’d relax about it.

Ed.

  • Like 2

Share this post


Link to post
Share on other sites

As already said the scratch should have little or no effect, if it is deep or wide then filling it with a black marker pen or paint may reduce scattering effects.

Alan

  • Like 4

Share this post


Link to post
Share on other sites

I know we're dealing with a mirror in this particular instance but this article helps give insight and perspective into the effect of damaged optics on IQ. Not AP I know, but you get the idea! 😊

  • Like 4

Share this post


Link to post
Share on other sites

Unless it is straight then it is very unlikely to matter. In the unlikely event of it showing I would just black it out as suggested above.

Olly

  • Like 1

Share this post


Link to post
Share on other sites

In 1970 the McDonald 107 inch suffered a gun and hammer attack with small chunks removed.

Opticians smoothed out the edges and carried on using the mirror. 

As above you may get some small effects but I doubt you will notice them.

Regards Andrew

  • Like 3

Share this post


Link to post
Share on other sites

Firstly wanna thank everyone for taking the time to comment and help me with this question.

I manged to luckily get the scope out and test the mirror since the scratch when cleaning and I don't believe there is any noticeable difference thankfully.

The only thing I have noticed is that there is a difference in the diffraction spikes when you compare a picture from before cleaning and collimation to after. I have posted two pictures below to show the difference of the center point of the star, the one after cleaning has a noticeable cross in the middle which I am thinking is down to bad collimation. Is that correct?

Ross

138704408_BeforeCC.thumb.jpg.da473c94fe33a5b5ec917f3dfca5cb2f.jpg322327644_AfterCC.thumb.jpg.823d693c4d6782a793f2826feaf30b50.jpg  

Share this post


Link to post
Share on other sites

Collimation has to be checked separately, telling the bahtinov effect from the collimation effect is too complicated.

And about that scratch issue: this is discussed very often in forums, and there is a simple answer to that. If a tiny scratch made a visible defect in the image, the thousands of dust grains in the air you observe through, the secondary mirror and/or its spider, the spacers in some refractors, and the protruding screws in some newtonians would ruin the view but they don't.

The process of polishing optics consists of scratching them finely all over the surface till the scratches are small enough to not show, but there are millions of them. They always scatter a little light, but the amount is negligible in good optics. A bigger but lone scratch does vastly less than millions of smaller ones. You see, putting things in perspective is the solution to most worries. 😀

Edited by Ben the Ignorant

Share this post


Link to post
Share on other sites

I would check the star image at high magnification without the mask to see if there is now a small amount of astigmatism. If there is, it could be due to the primary mirror being held firmly following replacing after cleaning. The mirror should be free enough to move easily under its retaining clips.   😄

Share this post


Link to post
Share on other sites

good stuff Ross.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Recently Browsing   0 members

    No registered users viewing this page.

×

Important Information

By using this site, you agree to our Terms of Use.