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Edge Darkening?

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#1 astrogeek64

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Posted 27 May 2023 - 01:57 PM

Hi, All!

 

I've seen comments about darkening the edges of mirrors to increase contrast and wondered if this is worth the effort? And if so, what does one use to perform this task? Paint? What sort of paint would be best and how do I avoid getting any on the reflective face of the mirror?

 

Also, would you do this to both mirrors or only one?

 

Any constructive comments are welcome.

 

Dave


Edited by astrogeek64, 27 May 2023 - 01:58 PM.


#2 star acres

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Posted 27 May 2023 - 07:05 PM

With my Sky Watcher, removing the primary mirror is the easiest chore, but darkening it's edges would be a complete waste of time. The mirror surface is in the front, buried under a micro thin layer of silicon dioxide spray. The side reflection, in the tube, of the angle secondary is very trivial. Save your soldier paints.
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#3 areyoukiddingme

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Posted 27 May 2023 - 09:36 PM

As well as being a likely waste of time, if you need to get your mirror re-coated, getting any black paint off is like to be a big PITA.


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#4 astrogeek64

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Posted 28 May 2023 - 08:06 AM

With my Sky Watcher, removing the primary mirror is the easiest chore, but darkening it's edges would be a complete waste of time. The mirror surface is in the front, buried under a micro thin layer of silicon dioxide spray. The side reflection, in the tube, of the angle secondary is very trivial. Save your soldier paints.

 

 

As well as being a likely waste of time, if you need to get your mirror re-coated, getting any black paint off is like to be a big PITA.

Okay, thanks, Dudes. After pondering it for a while I reached the same conclusion, that it makes no sense and the risk of getting paint where I don't want it is too great. It might make some sense on a lens, depending on cell design, but a surface coated mirror?
 



#5 MarMax

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Posted 28 May 2023 - 10:48 AM

I've been working on a fix for flaring with bright stars and a secondary mask seems to work. If it's the reflective beveled edge of the secondary causing the flaring then blackening may also work (based on Don P's comment below, painting is a bad idea).

I'm very hesitant to attempt blackening and prefer the 3D printed mask. The scope is an ES N208CF.

 

EDITED to revise and add a picture of the mask and link to the last post in the N208CF discussion thread.

 

SecondaryMirrorMask_V2s.jpg

 

I had to have three versions 3D printed to get one that fit properly and masked the outer 1mm of the secondary's flat surface. The mask overhang is greater than 1mm because of the bevel on the edge but the net masking is about 1mm. I'll also paint the top and outside edge flat black.


Edited by MarMax, 28 May 2023 - 06:16 PM.

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#6 Starman1

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Posted 28 May 2023 - 11:22 AM

Quite often the biggest error in a mirror's figure is right at the edge.  Masking the mirror down to a slightly smaller diameter might improve image quality.

There is an easy way to check.

Cut a disc out out a black plastic or carboard that has an I.D. about 1/4" smaller than the reflective surface of your primary mirror.

Secure the disk to the mirror clips with, say, 2-sided tape, not touching the mirror.  The outer edge of the primary mirror will be completely masked off, including the bevel at the edge.

Then, evaluate the star images.  Less in the way of 'hairy edges", fewer spikes, generally sharper star images at higher powers?  Then it is likely you made a positive change.

If it makes no change at all, and you see the same issues you currently see, AND you have addressed collimation and cooling issues, then the secondary would be the next place to check.

A similar mask might be hard to cut, and it needs to be thinner, covering only the outer 1/16" of the optical surface.  If the secondary is in a holder, you can attach the mask to the holder.

If the secondary is glued to its stalk, I wouldn't bother with a mask for the secondary.

 

Never paint the edges of a mirror unless it is junk and you just want to make it serviceable and you never intend to recoat it.


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#7 star acres

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Posted 28 May 2023 - 05:56 PM

I haven't so much as burnished the end of a screw and my small Newt works perfectly.
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#8 MitchAlsup

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Posted 28 May 2023 - 06:02 PM

I've seen comments about darkening the edges of mirrors to increase contrast and wondered if this is worth the effort? 

The side of the primary cannot reach focal plane, so there is no reason to blacken the sides.

The bevel on the primary can allow scattered light to reach focal plane, so there is reason to blacken that 1mm-ish wide bevel. {Be careful here}

There is no reason to blacken the back of the mirror.

 

The top side of the secondary can scatter light into the focal plane and needs some blackening.

The bottom side of the secondary does not.

The front must not be blackened (except for the bevel) {Be careful here}

The back needs not be blackened.

The basic rule of thumb is if you place your eye at the focal plane and you can see anything that is not black, blacken it if possible.


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#9 fmendes

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Posted 28 May 2023 - 06:23 PM

On contrary of the most here, I did some stuff in this subject with moderate success, and improved contrast.

 

First I tried to flock the back and sides of the secondary. It is trickier than it seems as the sides of the mirror need a perfect flocking to prevent the stars from getting a weird shape. So I removed the secondary and painted with black matte spray. Make sure you use a paint that takes at least a few minutes to dry - I tested the one I had at home and it would dry to the touch in a few seconds - not good for cleaning the potential mess. Keep alchool and cotton balls at hand, as you'll paint the back and as soon as you're finished, lift the mirror through the center threaded hole and clean potential droplets of paint.

 

When I did this job I also flocked the region around the focuser. Needed to disassemble the spider for that. If you do, do yourself a favor and measure accurately the length of each vane. Mark the one that goes toward the focuser. Do not flock anything on the spider, it won't help.

 

My primary mirror has only the reflective coating and all the rest is matte glass. A lot of stray light come from the back as I usually image from Bortle 8. To block the light in a reversible way, I put electric tape around the primary, taking care not to cover the edges. Now I want to find a way to block light through the mirror cell. Still thinking how to do it, a candidate is door gasket.


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#10 astrogeek64

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Posted 28 May 2023 - 07:36 PM

On contrary of the most here, I did some stuff in this subject with moderate success, and improved contrast.

 

First I tried to flock the back and sides of the secondary. It is trickier than it seems as the sides of the mirror need a perfect flocking to prevent the stars from getting a weird shape. So I removed the secondary and painted with black matte spray. Make sure you use a paint that takes at least a few minutes to dry - I tested the one I had at home and it would dry to the touch in a few seconds - not good for cleaning the potential mess. Keep alchool and cotton balls at hand, as you'll paint the back and as soon as you're finished, lift the mirror through the center threaded hole and clean potential droplets of paint.

 

When I did this job I also flocked the region around the focuser. Needed to disassemble the spider for that. If you do, do yourself a favor and measure accurately the length of each vane. Mark the one that goes toward the focuser. Do not flock anything on the spider, it won't help.

 

My primary mirror has only the reflective coating and all the rest is matte glass. A lot of stray light come from the back as I usually image from Bortle 8. To block the light in a reversible way, I put electric tape around the primary, taking care not to cover the edges. Now I want to find a way to block light through the mirror cell. Still thinking how to do it, a candidate is door gasket.

On the mirror cell end I use an Astrozap Dust Cover to keep stray light and dust out. (Actually, I use them on both ends.)
 


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#11 Starman1

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Posted 28 May 2023 - 08:15 PM

On contrary of the most here, I did some stuff in this subject with moderate success, and improved contrast.

 

First I tried to flock the back and sides of the secondary. It is trickier than it seems as the sides of the mirror need a perfect flocking to prevent the stars from getting a weird shape. So I removed the secondary and painted with black matte spray. Make sure you use a paint that takes at least a few minutes to dry - I tested the one I had at home and it would dry to the touch in a few seconds - not good for cleaning the potential mess. Keep alchool and cotton balls at hand, as you'll paint the back and as soon as you're finished, lift the mirror through the center threaded hole and clean potential droplets of paint.

 

When I did this job I also flocked the region around the focuser. Needed to disassemble the spider for that. If you do, do yourself a favor and measure accurately the length of each vane. Mark the one that goes toward the focuser. Do not flock anything on the spider, it won't help.

 

My primary mirror has only the reflective coating and all the rest is matte glass. A lot of stray light come from the back as I usually image from Bortle 8. To block the light in a reversible way, I put electric tape around the primary, taking care not to cover the edges. Now I want to find a way to block light through the mirror cell. Still thinking how to do it, a candidate is door gasket.

Blocking light from getting into the tube from the bottom, around the outside of the primary mirror is best done with a black cloth under the scope.

Anything blocking light around the primary can interfere with a fan used for cooling (and 8" needs a fan), where the air is supposed to go around the mirror and up and out of the tube.

Many have installed the fan on a back plate that blocks back-flow from the fan and also blocks light from getting into the tube.  That works, though attaching that plate may require some creativity because collimation bolts have to go through it as well.



#12 fmendes

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Posted 28 May 2023 - 11:56 PM

On the mirror cell end I use an Astrozap Dust Cover to keep stray light and dust out. (Actually, I use them on both ends.)
 

Would work most times, but when umidity is high, I put a fan behind the primary. But then, if I block the mirror cell, there will be no air flow... Yes, maybe a shower cap will work for me (my telescope is a 8").



#13 areyoukiddingme

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Posted 30 May 2023 - 01:18 PM

I use shower caps as dust covers on my 8" F7 (it's a 10" OD tube).


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#14 fmendes

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Posted 30 May 2023 - 08:08 PM

Blocking light from getting into the tube from the bottom, around the outside of the primary mirror is best done with a black cloth under the scope.

Anything blocking light around the primary can interfere with a fan used for cooling (and 8" needs a fan), where the air is supposed to go around the mirror and up and out of the tube.

Many have installed the fan on a back plate that blocks back-flow from the fan and also blocks light from getting into the tube.  That works, though attaching that plate may require some creativity because collimation bolts have to go through it as well.

Yes, I realized that. I do have a fan but only use it when temperatures drop below dew point. In that case, I already admitted that something that blocks air flow permanently installed is a bad idea.



#15 astrogeek64

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Posted 03 June 2023 - 12:22 PM

Would work most times, but when umidity is high, I put a fan behind the primary. But then, if I block the mirror cell, there will be no air flow... Yes, maybe a shower cap will work for me (m

The dust cover is cloth, it fits over the fan and allows airflow. In fact that's the whole point of it, to keep dust out while allowing heat to escape. A hair net might work if it's big (or small) enough and it would certainly be cheaper. I needed to cover a 14" diameter. I hope your ladies head isn't 14" in diameter.

 

Dave


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#16 fmendes

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Posted 03 June 2023 - 01:24 PM

The dust cover is cloth, it fits over the fan and allows airflow. In fact that's the whole point of it, to keep dust out while allowing heat to escape. A hair net might work if it's big (or small) enough and it would certainly be cheaper. I needed to cover a 14" diameter. I hope your ladies head isn't 14" in diameter.

 

Dave

 

Lol, definetly no!... My telescope is 8" so I used two hair bonnets. One, when stretched, became almost transparent, so I added a second. $4 for both.
 


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#17 fmendes

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Posted 03 June 2023 - 11:41 PM

Since it seems related to the subject, I believe I need darkening somewhere not identified yet. My images have this halo, and I tried with and without darks, so the issue is either with lights, bias (I take dark flats) or flats. I can share a few of each for inspection as I don't know how to inspect the flats. I'm using a Sky Watcher Quattro 200P with Starizona Nexus 0.75x and a camera Ogma AP26CC (IMX571).

 

I usually can eliminate these gradients with Dynamic Background Extraction, but it seems I shouldn't have them in the first place.

 

Gradient.jpg

 

Flats do no seem to have such gradient, but there's nothing to compare the flats with (that I know of).


Edited by fmendes, 03 June 2023 - 11:43 PM.


#18 Dan_I

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Posted 04 June 2023 - 07:20 AM

Since it seems related to the subject, I believe I need darkening somewhere not identified yet. My images have this halo, and I tried with and without darks, so the issue is either with lights, bias (I take dark flats) or flats. I can share a few of each for inspection as I don't know how to inspect the flats. I'm using a Sky Watcher Quattro 200P with Starizona Nexus 0.75x and a camera Ogma AP26CC (IMX571).

 

I usually can eliminate these gradients with Dynamic Background Extraction, but it seems I shouldn't have them in the first place.

 

attachicon.gif Gradient.jpg

 

Flats do no seem to have such gradient, but there's nothing to compare the flats with (that I know of).

Before reading your signature, your picture suggested me that you were using the GPU/quattro coma corrector, because I ran onto exactly the same problem. Similar ring of exactly the same size.

 

 

My Newtonian OTA is completely flocked as well as the edge of the secondary mirror, but I found that the culprit was the corrector itself. It seems that this corrector (at least my TS-branded version) has non-blackened lens edges leading to internal reflections.

 

The fix was to screw at the end of the corrector (telescope side) a short M48 extension lined with flocking material.


Edited by Dan_I, 04 June 2023 - 07:20 AM.

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#19 astrogeek64

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Posted 04 June 2023 - 08:22 AM

Before reading your signature, your picture suggested me that you were using the GPU/quattro coma corrector, because I ran onto exactly the same problem. Similar ring of exactly the same size.

 

 

My Newtonian OTA is completely flocked as well as the edge of the secondary mirror, but I found that the culprit was the corrector itself. It seems that this corrector (at least my TS-branded version) has non-blackened lens edges leading to internal reflections.

 

The fix was to screw at the end of the corrector (telescope side) a short M48 extension lined with flocking material.

I was just thinking the same thing. I had an extension tube with smooth walls that would do the same thing. Flocked it.....gone.


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#20 fmendes

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Posted 04 June 2023 - 11:11 AM

The fix was to screw at the end of the corrector (telescope side) a short M48 extension lined with flocking material.

I was looking into this right now. Telescope with both caps on, I looked into it to try to see anything reflexive. The walls of the drawtube are a bit reflective. That combined with the short reducer (Starizona Nexus 0.75) seem to be the cause of it. I liked the idea of the flocked ring, will save me from trying to flock inside the draw tube. The original SW coma corrector is longer than the draw tube so I don't expect the issue with it.



#21 MarMax

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Posted 04 June 2023 - 04:27 PM

Before reading your signature, your picture suggested me that you were using the GPU/quattro coma corrector, because I ran onto exactly the same problem. Similar ring of exactly the same size.

 

 

My Newtonian OTA is completely flocked as well as the edge of the secondary mirror, but I found that the culprit was the corrector itself. It seems that this corrector (at least my TS-branded version) has non-blackened lens edges leading to internal reflections.

 

The fix was to screw at the end of the corrector (telescope side) a short M48 extension lined with flocking material.

Very interesting. And as you are aware I'm getting flaring on the N208CF and R200SS. I don't have the room to put an M48 extension on the Nexus but I could use a 3D printed aperture mask that inserts into the end. With the TSGPU there is room for a flocked M48 extension tube. 


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#22 fmendes

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Posted 03 September 2023 - 09:44 PM

I wonder why manufacturers sometimes can't make it easy. I thought my telescope was all black and matte inside, and I recently saw this white rim around the focuser. Disassembled, painted and put back in place.

 

20230819-133924.jpg

 

20230819-135804.jpg

 

20230819-140337.jpg

20230819-141353.jpg

 

Then looking at the last picture above, I realized: if I can see the inside of the draw tube, the flat panel can see it too. The spacer with flocking inside may help a little, but there will still be light from the flat panel reaching the coma corrector directly:

Untitled.png

 

So looks like I'll need to figure out a way to take flats with the panel at a distance such that no direct light hits the coma corrector.



#23 Dan_I

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Posted 04 September 2023 - 08:49 AM

Put the panel on top of the  dewshield  ?



#24 fmendes

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Posted 04 September 2023 - 09:32 AM

Put the panel on top of the  dewshield  ?

 

I retired the dewshield, but that would be my first option... I'm thinking of putting the flat panel on a tripod and point the telescope to the side (park position). This is the same thing I'd do if I had a dome, except that the panel would be on the wall.



#25 Starman1

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Posted 04 September 2023 - 09:39 AM

If the focuser is so close to the top of the tube that light can directly hit the coma corrector's lens, the scope would benefit from an extended light shield on top.

Why the manufacturers don't extend their tubes a bit farther up is beyond me.  I guess they're not astronomers worried about contrast.

Here is an example of a light shield:

https://astrozap.com...ts/light-shield


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