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How to remove clips shadows in a newtonian?

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#1 astroby2

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Posted 06 April 2017 - 07:48 PM

I'm not very lucky with the telescopes I've got so far. Probably my best one is the Orion 8" F/5 Newtonian, but it has one issue which really spoils otherwise not very bad images I'm getting with it. There are pronounced weird shadows around bright stars, which make them look like radioactivity symbols (quite applicable to stars but doesn't add to image quality). I think the shadows come from both focuser AND the clips which hold the primary. Here is an example:

 

Pleiades_final_8N_dark_res.jpg

 

And some explanation:

 

Pleiades_expl.jpg

 

Here is a link to the full size image:

 

https://www.dropbox....N_dark.jpg?dl=0

 

Does anybody know how to get rid of the clips shadows? I'm pretty sure if I move the primary up the tube, it will eliminate the focuser shadow as it will not be protruding into the tube anymore, but would it also help with the clips issue? I think maybe, as less area of the primary will be reflected in the secondary.

 

Thank you!

 

 

 

 

 

 



#2 telfish

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Posted 06 April 2017 - 08:19 PM

This is my image of the same object in my 8 inch f4 scope. I don't get any clip or focuser spikes though the scope is standard.

 

 

If possible you could try a shorter drawtube in your focuser. Look down the tube when you are at the normal focus position and see if the drawtube is over the primary.

Attached Thumbnails

  • M45_final_DBE small.jpg

Edited by telfish, 06 April 2017 - 08:22 PM.


#3 astroby2

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Posted 06 April 2017 - 09:47 PM

This is my image of the same object in my 8 inch f4 scope. I don't get any clip or focuser spikes though the scope is standard.

 

 

If possible you could try a shorter drawtube in your focuser. Look down the tube when you are at the normal focus position and see if the drawtube is over the primary.

I have another 6 inch Newt and it doesn't have this issue, it happens only with that particular 8" one. Definitely drawtube is over the primary at the normal focus position, that's why I wanted to move the primary up.

 

Nice image telfish, what coma corrector are you using?


Edited by astroby2, 06 April 2017 - 09:50 PM.


#4 telfish

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Posted 07 April 2017 - 08:07 AM

You could try making a circular ring mask maybe start an inch wide and fit that to the front of the scope. Or make one narrower to cover the clips.

 

Are you sure it's the mirror clips causing these spikes?

 

I am using the ES Field flattener that came with the scope currently. This image was taken with the TS Quatro flattener. 

 

After I took this which was the first image with the scope I took the mirror out and made sure there was no striction, also loosened the clips so the mirror had room to move. That got rid of the slightly triangular star shapes.


Edited by telfish, 07 April 2017 - 08:08 AM.


#5 Campos

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Posted 07 April 2017 - 09:44 AM

You need to do two things, move the primary up the tube using longer screws and springs (make sure to check the focus point) and also mask a little bit of the primary, just enough to cover the clips, you'll loose a tiny bit of apperture but it will be hardly noticeable on the images :)

 

With my 200mm f/4 newt. I'm using a 175mm mask, as this brings me a much better field correction and much improved stars and still a fast f/4.6 system..

 

Cheers,



#6 pbkoden

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Posted 07 April 2017 - 10:39 AM

Campos, is the mask something you made or something you bought? Can you add some details?

#7 telfish

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Posted 07 April 2017 - 11:02 AM

Campos, is the mask something you made or something you bought? Can you add some details?

Just get a piece of card, draw a circle a little larger than the mirror. Draw another inner circle to just cover the clips. Cut out the resulting ring. Attach the ring to the clips using double sided tape, making sure it's centralized. 

 

If you have a lamination machine you can laminate the card to make it more durable. You can also paint it with flat black paint to avoid reflections.


Edited by telfish, 07 April 2017 - 11:03 AM.

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#8 Campos

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Posted 07 April 2017 - 11:42 AM

 

Campos, is the mask something you made or something you bought? Can you add some details?

Just get a piece of card, draw a circle a little larger than the mirror. Draw another inner circle to just cover the clips. Cut out the resulting ring. Attach the ring to the clips using double sided tape, making sure it's centralized. 

 

If you have a lamination machine you can laminate the card to make it more durable. You can also paint it with flat black paint to avoid reflections.

 

+1 :)


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#9 Bizzidy

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Posted 07 April 2017 - 01:26 PM

I too have heard about making a ring mask to cover the clips. You image btw is really great!!!  Post a pic after you make the modification so we can see the changewaytogo.gif

 

i may be wrong but i dont think putting the mask over the opening of the tube will work. It must be placed right adjacent to the clips. 


Edited by Bizzidy, 07 April 2017 - 01:29 PM.


#10 sharkmelley

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Posted 07 April 2017 - 02:53 PM

I reckon what is going on is the following:

 

The 4 main spikes are caused by the spider holding the secondary.

The myriad of other spikes are caused by a rough edge to the silvered surface of the mirror.

The gaps (wedges) in the myriad of spikes are where the clips are obscuring that rough edge.

The stars away from the centre don't have the myriad of spikes going around the star 360 degrees.  This is where that star is not being illuminated by the whole mirror but part of the mirror is vignetted by the front opening of the scope ( which probably has a smooth edge ).

 

It will be very interesting to see a photo of the main mirror and its clips. In any case, as others have said, you need to mask off the rough edge of the silvered surface by a mask with a sharp but smooth edge (so it doesn't create its own myriad of diffraction spikes).  The sharper the edge, the less you will have in the way of unwanted diffraction spikes.

 

It's a manufacturing or design issue for which there is no excuse in my opinion.

 

Mark


Edited by sharkmelley, 07 April 2017 - 03:17 PM.

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#11 astroby2

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Posted 07 April 2017 - 05:18 PM

I reckon what is going on is the following:

 

The 4 main spikes are caused by the spider holding the secondary.

The myriad of other spikes are caused by a rough edge to the silvered surface of the mirror.

The gaps (wedges) in the myriad of spikes are where the clips are obscuring that rough edge.

The stars away from the centre don't have the myriad of spikes going around the star 360 degrees.  This is where that star is not being illuminated by the whole mirror but part of the mirror is vignetted by the front opening of the scope ( which probably has a smooth edge ).

 

It will be very interesting to see a photo of the main mirror and its clips. In any case, as others have said, you need to mask off the rough edge of the silvered surface by a mask with a sharp but smooth edge (so it doesn't create its own myriad of diffraction spikes).  The sharper the edge, the less you will have in the way of unwanted diffraction spikes.

 

It's a manufacturing or design issue for which there is no excuse in my opinion.

 

Mark

Thank you Mark,

 

It is a very good description of what's going on. I certainly didn't know that a rough edge could cause those myriad of spikes. If the mask solution will solve this issue as well, I will be really happy.

 

I don't have an up-close image of the mirror yet until I get it out, here is an image I've got by placing a camera in front of the tube, probably not very helpful:

 

mirror.jpg


Edited by astroby2, 07 April 2017 - 05:20 PM.


#12 astroby2

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Posted 07 April 2017 - 05:33 PM

telfish and Campos,

 

Thank you for proposing the masking solution and providing a lot of details on possible implementation! I'm going to try it first as it should be pretty easy to do and should solve a bunch of issues (including multiple spikes from the rough edge Mark mentioned). Then probably I will try to move the mirror up to resolve the focuser issue as well.



#13 astroby2

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Posted 07 April 2017 - 05:46 PM

I too have heard about making a ring mask to cover the clips. You image btw is really great!!!  Post a pic after you make the modification so we can see the changewaytogo.gif

 

i may be wrong but i dont think putting the mask over the opening of the tube will work. It must be placed right adjacent to the clips. 

Thank you Bizzidy!

 

I will try to post some pics when the weather cooperates. Currently we have strong winds in the forecast which are definitely not favorable for 8 inch newt.



#14 calypsob

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Posted 08 April 2017 - 09:52 PM

I think aligning the clips with the venes can help some as well.  3d printing an aperture mask on a large format printer would be super easy. Some redesigned wide low profile clips would also be a good option.  



#15 pbkoden

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Posted 23 April 2017 - 12:11 PM

Reporting back after making a mirror mask. I purchased some flat black matte board from Amazon (made for mixed media art), and cut out a circle just small enough to cover my mirror clips. I attached it with some double-sided foam tape to the top of the clips. Below you can see the difference between a pre-mask integration on the left and a post-mask integration on the right. Much better! Thanks for the tips here guys.

 

3UV7bgS.jpg


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#16 Alex McConahay

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Posted 23 April 2017 - 01:33 PM

Interested in a rescue of your beautiful Pleaides data?

 

In Photoshop, make a spherical copy of a star you want to change. Be sure to feather a bit around the edges.

 

Paste that copy to a new layer immediately above.

 

Transform that copy with a rotation of 90 degrees.

 

Then use "lighten" as the blend mode for the copy.

 

Use the eraser to get rid of as many of the distinctive features of that particular level as you can. (Remove the diffraction spikes, remove the stars, etc. while keeping the general radiating glow.)

 

Copy that layer, rotate it a bit, and paste it. Copy the newest layer again (drag the layer to the "new layer" icon), and rotate, or flip/horizontal--vertical.

 

As you add each layer in a lighten blend mode, the general "starriness" of the topmost layer hides the "holes" in your lower layers. You are "Painting" over your previous stars using. (more accurately, you have made a "rubber stamp" and are stamping out your previous holes.

 

Do not try this with Pixinsight. They will never forgive you.  

 

It will not be as easy as I just described, but it is a pretty powerful technique.

 

Alex

 

Check out the top star in this re-work of your data, and see that the clip holes are gone. This was a quick and dirty attempt.....with a little care, it can be done much better.

 

 

 

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  • M45-star-fix.jpg


#17 astroby2

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Posted 23 April 2017 - 06:23 PM

Interested in a rescue of your beautiful Pleaides data?

 

In Photoshop, make a spherical copy of a star you want to change. Be sure to feather a bit around the edges.

 

Paste that copy to a new layer immediately above.

 

Transform that copy with a rotation of 90 degrees.

 

Then use "lighten" as the blend mode for the copy.

 

Use the eraser to get rid of as many of the distinctive features of that particular level as you can. (Remove the diffraction spikes, remove the stars, etc. while keeping the general radiating glow.)

 

Copy that layer, rotate it a bit, and paste it. Copy the newest layer again (drag the layer to the "new layer" icon), and rotate, or flip/horizontal--vertical.

 

As you add each layer in a lighten blend mode, the general "starriness" of the topmost layer hides the "holes" in your lower layers. You are "Painting" over your previous stars using. (more accurately, you have made a "rubber stamp" and are stamping out your previous holes.

 

Do not try this with Pixinsight. They will never forgive you.  

 

It will not be as easy as I just described, but it is a pretty powerful technique.

 

Alex

 

Check out the top star in this re-work of your data, and see that the clip holes are gone. This was a quick and dirty attempt.....with a little care, it can be done much better.

Thank you Alex,

 

This is definitely a pretty powerful technique, but the bright stars in the image have so many issues (including excessive bloating with myriad of spikes because of the rough edge of the mirror) that I would prefer to reacquire the data. If I remember correctly it was just a little bit over an hour of it - probably processing in PixInsight took several times more than getting the data.

 

I'm also pretty sure that people can use ideas from your post in many situations.



#18 astroby2

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Posted 23 April 2017 - 06:28 PM

Reporting back after making a mirror mask. I purchased some flat black matte board from Amazon (made for mixed media art), and cut out a circle just small enough to cover my mirror clips. I attached it with some double-sided foam tape to the top of the clips. Below you can see the difference between a pre-mask integration on the left and a post-mask integration on the right. Much better! Thanks for the tips here guys.

 

3UV7bgS.jpg

Thank you for sharing Phil!

I couldn't try the solution myself yet due to the weather.

In addition to removing the clips shadows, it looks like it eliminated numerous bright spikes caused by the mirror edge (as explained by Mark above) so the star is now much more compact. Very good result overall!



#19 nathang123

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Posted 23 April 2017 - 06:46 PM

How much mirror area was lost by masking those tabs by blocking off the whole perimeter? 



#20 pbkoden

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Posted 23 April 2017 - 06:57 PM

The inside diameter of my mask is Ø7.75" on an Ø8.00" newt. Doing some math, I lost about 6% of the light-gathering capability and dropped the f-ratio from 4.0 to 4.12. Not nearly as noticeable as the uneven diffractions are.



#21 spokeshave

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Posted 23 April 2017 - 07:02 PM

pbkoden, on 23 Apr 2017 - 7:57 PM, said:pbkoden, on 23 Apr 2017 - 7:57 PM, said:

The inside diameter of my mask is Ø7.75" on an Ø8.00" newt. Doing some math, I lost about 6% of the light-gathering capability and dropped the f-ratio from 4.0 to 4.12. Not nearly as noticeable as the uneven diffractions are.

Since most Newts typically suffer from some degree of turned-down edge, a thin mask often improves overall performance quite noticeably in addition to masking off clips and a bright edge.

 

Tim


Edited by spokeshave, 23 April 2017 - 07:03 PM.


#22 nathang123

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Posted 23 April 2017 - 10:01 PM

Good info, thanks. I'll look into doing this.




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