12 replies to this topic

[ww321q]

Do you blacken the edges or sides of the secondary mirror? If not, why? Thanks JW

[wrvond]

I do not. Primarily because a highly respected mirror maker advised against it.

Before that, though, I did actually take a Sharpie to the edges of my (since replaced) secondary. I couldn't tell any difference so when I replaced it, I did not blacken the edges of the new one.

Edited by wrvond, 12 July 2019 - 10:15 PM.

[Jim Waters]

If you blacken the edge of the secondary it makes it real hard to recoat the mirror.  Also when you clean the mirror the black marker is likely to run getting on the front.

 

The reflections, if any coming off the exposed glass are too small to notice.

Edited by Jim Waters, 13 July 2019 - 12:14 AM.

[Oregon-raybender]

I would not suggest it, not needed IMHO. But others may think not. If you are concerned about light bouncing about, add a small disk on the secondary mount, slightly larger than the mirror. Or perhaps black tape, easy to remove when ready for coating.

 

Starry Nights

[Pinbout]

if its a sharpie, they sux at blocking light.

 

you could tape it.

 

black 2.0? nah.

[wells_c]

Careful with a sharpie on lenses too— I’ve been messing around with DIY eyepieces a la Martin Pond et al, was darkening a doublet, and the ink began leeching between the cemented lenses. Given they were cheap bino EP lenses and it made a barely noticeable darker area but I stopped right away.

Just sayin...

[Bob4BVM]

Careful with a sharpie on lenses too— I’ve been messing around with DIY eyepieces a la Martin Pond et al, was darkening a doublet, and the ink began leeching between the cemented lenses. Given they were cheap bino EP lenses and it made a barely noticeable darker area but I stopped right away.

Just sayin...

WELCOME to CN Wells !   (from another EP DIYer who has learned a lot from Martin P )

 

You are right. Sharpie can be used on lenses but gotta be careful around lens seams.  When I get serious about edge blacking I have a can of thinned flat black sign enamel that works real well and seem impervious to most water based lens cleaners.

 

I see no reason to blacken sec mirror edges tho.

 

CS

Bob

[stargazer193857]

Optical experts have written here and there to not pain it black.
I agree that sharpy does not do much. Flat black is better for reflections. But then there is diffraction.
Oh well, Black 2.0 for me anyway. I like seeing more blackness when I look in the focused hole.

Oh, and sharpy comes off easily with isopropyl.

Edited by stargazer193857, 16 July 2019 - 12:36 AM.

[wells_c]

WELCOME to CN Wells ! (from another EP DIYer who has learned a lot from Martin P )

You are right. Sharpie can be used on lenses but gotta be careful around lens seams. When I get serious about edge blacking I have a can of thinned flat black sign enamel that works real well and seem impervious to most water based lens cleaners.

I see no reason to blacken sec mirror edges tho.

CS
Bob

[wells_c]

cold busted...

Thanks for the welcome bob, you caught me red-handed trying to sneak out of the lurker area unnoticed

I’ll have to do a proper intro post now, but basically I’m a photographer and carpenter by day and a science geek/tinkerer by night. My first telescope was born from the ashes of a holography hobby; I sold off most of my expensive equipment but kept the 8” f/5 parabolic mirror I’d been using to collimate the beam. It’s now in a plywood and sonotube dob, has had its first light a few weeks ago, and is a ton of fun. As for EPs, I’ve never used a commercial one; the threads with you, Martin and the others combined with a junk box full of lenses was all I needed

I’ll do a proper post-build thread on my dob soon.

Cheers, Wells

[Bob4BVM]

cold busted...

Thanks for the welcome bob, you caught me red-handed trying to sneak out of the lurker area unnoticed

I’ll have to do a proper intro post now, but basically I’m a photographer and carpenter by day and a science geek/tinkerer by night. My first telescope was born from the ashes of a holography hobby; I sold off most of my expensive equipment but kept the 8” f/5 parabolic mirror I’d been using to collimate the beam. It’s now in a plywood and sonotube dob, has had its first light a few weeks ago, and is a ton of fun. As for EPs, I’ve never used a commercial one; the threads with you, Martin and the others combined with a junk box full of lenses was all I needed

I’ll do a proper post-build thread on my dob soon.

Cheers, Wells

You're most welcome !

 

Awesome stuff, seems we have a lot in common.

CN is a great place , keeps you moving in this hobby

 

CS
Bob

[Starman1]

If you want the edge of the mirror to be black, try flocking it with flocking paper.

It can be easily removed and the residuum of adhesive removed with alcohol or acetone before sending it off to be coated.

But that reflection from the edge is why I use an Astrosystems holder.

I found it visible in a couple scopes I owned with cemented secondaries.

Edited by Starman1, 17 July 2019 - 12:23 PM.

[howardcano]

+1 for flock paper!