31 replies to this topic

[Bonco2]

I'm considering masking my 8 inch f/6 because I think it suffers a turned edge. I thought I'd reduce its diameter by one inch. Does it matter whether I mask it at the top of the tube are at the mirror. My guess is that it doesn't matter. Would appreciate a recommendation.

Bill

[sg6]

At the mirror.

Otherwise the "turned" edge still is part of the optics and you get the same result.

And you could likely find out by simply using a mask 5-6mm smaller diameter and so lose less of the aperture. Just get it central.

[Eddgie]

I would be shocked if the TE was so large as to require a 7" mask.  TEs are usually very near the edge and you can do more damage with an oversize mask than the TE would do.

See, when you mask the scope, you may cover up the TE, but you also increase the size of the central obstruction (as a percentage of the remaining aperture).  For this reason, you really want to be positive that you have a turned edge, and you want to see the severity of the TE.  A shallow and narrow turned edge is probably best left alone. 

 

My advice is for you to try to accurately measure the intrusion and severity of the TE.  A 7" mask may do more harm than good.

 

How to you tell the size and severity? Well, and optical bench is the best way to go, but barring that, you can perhaps see it using a star test. Do you know how to test for TE using star test?  I mean you think you have one you must have used a star test, Ronchi, or Roddier.

 

Why don't you step back and tell us why you think you have a turned edge and how you came to settle on a 7" mask?  I would be really shocked if you had a TE that required such an aggressive action as to reduce the aperture by a full inch and increase the obstruction size correspondingly.  

 

If you do mask, do it at the mirror. 

Edited by Eddgie, 28 November 2020 - 04:57 PM.

[Eddgie]

And I may never come back to this post without prompting so if you do respond with details (and better yet, images of the star test) please PM me to let me know so I can come back and get up to date. 

[Pinbout]

The easiest and quickest is to make a mask on front of the tube. That way, if you know how to star test, you don’t have to take anything apart.

 

The light coming into the scope is already straight so a mask upfront works well. That’s what people do for off axis masks.

 

so do it and let us know how it goes.

 

people also use anodizing masks upfront also.

 

dont touch the mirror till you have to.

Edited by Pinbout, 28 November 2020 - 05:35 PM.

[Bonco2]

Good posts and thanks.

I'm thinking I have a turned edge due to the fact that my star tests are poor. Inside focus shows a VERY rough fringe on the edge of the diffraction disk. Outside of focus is much better but far from perfect. Members here suggested it might be a turned edge. So I want to do some simple tests like a mask to see if that's true. I know what a good star test should show and this optic appears to be poor. Also on close double stars I cannot get the sharp separations and airey discs  that I'm used to seeing in a 10 inch and 6 inch reflectors that I've owned. Actually my 8 inch is the poorest performer for double star and high power planetary. My Zhumell 10 inch was fine but I needed to downsize due to my age. I know what a quality 8 inch f/6 should provide. But mine is not up to par.

Bill 

[Pinbout]

 

. Inside focus shows a VERY rough fringe on the edge of the diffraction disk. Outside of focus is much better but far from perfect. Members here suggested it might be a turned edge

Sketch what it looks like

 

the 2ndry shadow size on both sides of focus is the most important data to collect

 

what your describing isn’t a tde. Sounds more like an overal overcorrection.

 

with a tde the outside image would be very hairy outer ring.

[Bonco2]

The easiest and quickest is to make a mask on front of the tube. That way, if you know how to star test, you don’t have to take anything apart.

 

The light coming into the scope is already straight so a mask upfront works well. That’s what people do for off axis masks.

 

so do it and let us know how it goes.

 

people also use anodizing masks upfront also.

 

dont touch the mirror till you have to.

Seems others disagree but your thinking is much like mine. Not only turned edge I want to see if minor masking improves things. It might compensate the mirror's imperfections just by changing the f/ ratio from 5.9 to 6.7. If it does I'll not be concerned about light loss.

Thanks, Bill

[Pinbout]

Seems others disagree but your thinking is much like mine. Not only turned edge I want to see if minor masking improves things. It might compensate the mirror's imperfections just by changing the f/ ratio from 5.9 to 6.7. If it does I'll not be concerned about light loss.

Thanks, Bill

Where do people install off axis mask? Nuff said 

[Bonco2]

Sketch what it looks like

 

the 2ndry shadow size on both sides of focus is the most important data to collect

 

what your describing isn’t a tde. Sounds more like an overal overcorrection.

 

with a tde the outside image would be very hairy outer ring.

Next time I have a chance I'll check it out. My personal opinion has always been it's over corrected. Never knew about the secondary shadow.

Thanks, Bill

[Keith Rivich]

Good posts and thanks.

I'm thinking I have a turned edge due to the fact that my star tests are poor. Inside focus shows a VERY rough fringe on the edge of the diffraction disk. Outside of focus is much better but far from perfect. Members here suggested it might be a turned edge. So I want to do some simple tests like a mask to see if that's true. I know what a good star test should show and this optic appears to be poor. Also on close double stars I cannot get the sharp separations and airey discs  that I'm used to seeing in a 10 inch and 6 inch reflectors that I've owned. Actually my 8 inch is the poorest performer for double star and high power planetary. My Zhumell 10 inch was fine but I needed to downsize due to my age. I know what a quality 8 inch f/6 should provide. But mine is not up to par.

Bill 

All of my scopes show this difference between inside and outside of focus. Since my in-focus stars look fantastic I don't worry about it...

[SteveG]

Good posts and thanks.

I'm thinking I have a turned edge due to the fact that my star tests are poor. Inside focus shows a VERY rough fringe on the edge of the diffraction disk. Outside of focus is much better but far from perfect. Members here suggested it might be a turned edge. So I want to do some simple tests like a mask to see if that's true. I know what a good star test should show and this optic appears to be poor. Also on close double stars I cannot get the sharp separations and airey discs  that I'm used to seeing in a 10 inch and 6 inch reflectors that I've owned. Actually my 8 inch is the poorest performer for double star and high power planetary. My Zhumell 10 inch was fine but I needed to downsize due to my age. I know what a quality 8 inch f/6 should provide. But mine is not up to par.

Bill 

These charts support your diagnoses.

 

I would try masking only the outer 1/8" of your primary. I don't think the top of the tube is a good idea, as it will trap warm air in the tube. Okay for testing purposes I suppose.

 

Can you return it? I would not be happy with a dud when I down-size, which is on my horizon. I'd be striving for the best optics possible.

 

[izar187]

Another vote for mask at the top of the tube, if it is a solid tube.

Or at the top of the tube section, if it is some kind of half tube/half strut.

Best to have a gap to counter tube currents percolating out thru the mask.

To kinda allow them to vent before the mask.

If tube currents are even an issue. 

[mjgillen]

Not to hijack this thread however how do you mask a mirror at the mirror? Pictures helpful.

 

My mirror is also turned at the edge and it was recommended to mask it.

 

Clear skies,

Michael

[Pinbout]

Not to hijack this thread however how do you mask a mirror at the mirror? Pictures helpful.

 

My mirror is also turned at the edge and it was recommended to mask it.

 

Clear skies,

Michael

You can get a sheet of black plastic and cutout the shape and attach it to the mirror clips.

you can put that same shape under the mirror clips 

dont tape it to mask it, that’s so destructive.

Edited by Pinbout, 29 November 2020 - 09:06 AM.

[Pinbout]

Here’s a tue, a bad tue, if yours is tde your outer ring would be opposite this 

 

i stated incorrectly earlier bout being on outside hairy, it would be hairy inside focus 

 

https://youtu.be/tAD-2r6gW9A

Edited by Pinbout, 29 November 2020 - 09:16 AM.

[Bonco2]

These charts support your diagnoses.

 

I would try masking only the outer 1/8" of your primary. I don't think the top of the tube is a good idea, as it will trap warm air in the tube. Okay for testing purposes I suppose.

 

Can you return it? I would not be happy with a dud when I down-size, which is on my horizon. I'd be striving for the best optics possible.

 

StarTest1.jpg

My image is most like the image in your post of a turned down edge. I'll be doing a test with a slight mask at the end of the tube.

Thanks, Bill

[SteveG]

Not to hijack this thread however how do you mask a mirror at the mirror? Pictures helpful.

 

My mirror is also turned at the edge and it was recommended to mask it.

 

Clear skies,

Michael

I had a metal ring laser cut. It's attached to the top of the mirror clips.

 

 

[SteveG]

Not to hijack this thread however how do you mask a mirror at the mirror? Pictures helpful.

 

My mirror is also turned at the edge and it was recommended to mask it.

 

Clear skies,

Michael

I had a metal ring laser cut. It's attached to the top of the mirror clips.

 

 

[SkybumMN]

A much simpler way to mask the mirror to check the degree of TDE is to use Scotch black electrical tape. It is somewhat stretchy and can be wrapped around the circumference, masking say 3 to 5 mm of the top mirror edge radius/surface, with the remainder on the sides of the mirror. After a star test you can either increase or decrease the width of masking, looking for the least amount of mask that greatly improves the performance. And yes the tape will come off cleanly, and will hold in place even in cold weather.

I have just used this masking on an 8" mirror to greatly improve planetary viewing and detail resolution. Best performance in my case required just 3mm loss on the mirror radius to correct my mirror TDE.

[SteveG]

A much simpler way to mask the mirror to check the degree of TDE is to use Scotch black electrical tape. It is somewhat stretchy and can be wrapped around the circumference, masking say 3 to 5 mm of the top mirror edge radius/surface, with the remainder on the sides of the mirror. After a star test you can either increase or decrease the width of masking, looking for the least amount of mask that greatly improves the performance. And yes the tape will come off cleanly, and will hold in place even in cold weather.

I have just used this masking on an 8" mirror to greatly improve planetary viewing and detail resolution. Best performance in my case required just 3mm loss on the mirror radius to correct my mirror TDE.

 

Agreed. I was going to suggest 1/8” from the radius. You typically don’t need much.

[Bonco2]

I've made a 7 inch mask out of black poster board and placed on the end of the tube. I'm masking more than recommended because I want to see if masking improves the star images. If it does the I'll trim it larger in diameter and evaluate. Only took a few minutes to make the mask. Another reason I chose 7 inch is the f ratio will be increased  from 5.9 to 6.7. If there are other optical errors, this change alone might improve the optics. I previously had a superb 6inch f/8. I'll be happy if I can at least equal its performance and maybe better it. We'll see.. Weather prevents star tests now but will eventually write results. 

Bill

[Eddgie]

The "Test" use used is not a valid test for turned edge.  It does not really tell you anything about a turned edge.  A turned edge will not show in this way.  It does nothing to make the the star look rough. Nothing at all. 

 

The test for turned edge is done at between maybe 10 to12 waves of defocus.

 

When you defocus to this level, what you are looking for is the difference in fineness between the interior rings.  If there is a turned edge, you will see that the rings are kind of fat and soft on one side of focus and thin and fine on the other side of focus. 

 

 

This is why I used to urge people to read a book on star testing. It is very powerful but if you listen to people that don't know how to do all of the tests properly, you get bad advice.

 

You have given yourself a lot of anxiety by not understanding what you were doing.  Sorry to sound so curt about it, but I have seen far to many people go down rabbit holes for no good reason other than bad advice.

 

If you do the proper test for a turned edge and you see that you actually do have a turned edge, the next step would be to use a Ronchi grating. While the Ronchi grating is not really useful for Spherical Aberration without using it in a double pass, if you can see a turned edge using the Ronchi in a single grating test, then that says you absolutely have a turned edge, and you can use the result of the test to estimate where you want to mask.  You want to preserve as much aperture as possible, so this means that you want to mask no greater than the break, but sometimes it is best to stop somewhere between the break and the edge depending on the width of the TE.

 

But I am talking as if you actually have a turned edge and until you do the proper test for a turned edge, you really don't know if you have a turned edge or not.

 

Surface roughness is more likely but even here, without seeing an image of the problem, it is impossible to say. 

 

[Eddgie]

Again, what you did was in no way a useful test for diagnosing a turned edge and even if it were,, it would tell you nothing about the severity or how much you need to mask. 

 

You can read more here, which all is basically straight out of Suiter's book on star testing.

 

https://www.astronom...rned-edges.html

 

I have spent years encouraging people to read Suiter's book. Here is another example of spending time on a problem that is not even been verified to be a problem. 

 

I am not saying you don't have a turned edge. I don't know that because the way you tested has nothing to do with how a turned edge affects the image.  What I am saying is that there is a test for the turned edge, and you should do that test before doing anything else. 

[Bonco2]

Eddgie,

Maybe you missed it but I've done multiple star image tests and all give the indications you described and as depicted in your post. So I'm masking and will adjust the mask based on observations. I have zero anxiety and enjoy tinkering. This is a hobby so I don't mind experimenting. For my use on double stars I'd sacrifice some aperture if it gave me a significantly better image. A good performing 7 inch would serve my double star observing needs. If my experimentation doesn't improve things I'll just return to base line and I'll be fine. This is just one of many scopes that I own so I have many other choices.  Thanks for your posts, keep up the good work.  

Bill