Homemade reflector has 3 images

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Phoenix22
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Homemade reflector has 3 images

#1

Post by Phoenix22 »


I am new to telescopes. I made a simple reflector at home with a 15cm primary and a small circular mirror (available at your general store). Just to give an idea, my telescopes body is made out of a Pringles can.

I did all the calculations precisely and even collimated the scope. The result is that am getting a clear image but there are ghost images appearing to the left and right of the main image.

It may help to mention that the primary mirror is not parabolic.

Can you please help with what went wrong?
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Re: Homemade reflector has 3 images

#2

Post by PapaFox »


saw this on the internet, is the inside of your pringles can black?

Some telescopes suffer from internal reflections inside their tubes, because the inside of the tube needs to be black.
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Re: Homemade reflector has 3 images

#3

Post by goldstar »


That small circular store bought mirror is certainly aluminum coated only on the back surface.
This causes multiple reflections from the uncoated front surface and anyway either surface will not be flat enough for astronomical purposes.
Secondary mirrors are coated only on the front surface which has been manufactured to high flatness and polish hence the higher cost.
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KathyNS Canada
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Re: Homemade reflector has 3 images

#4

Post by KathyNS »


Yes, any rear-coated mirror will produce multiple reflections. All mirrors made for household use are rear-coated for durability. All astronomical or optical mirrors are front-coated for clarity.
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Re: Homemade reflector has 3 images

#5

Post by Voyageur »


How did you fit a 15 cm primary mirror into a Pringles can? Must be some big Pringles out there that I don't know about!
Scopes: Vixen VMC200L, D=200mm, F=1950, f/9.75; Televue 2" Everbright diagonal. Coronado PST; AstroTech EDT 80mm, F=480, f/6.
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Re: Homemade reflector has 3 images

#6

Post by Lady Fraktor »


Congratulations on the DIY project, as has been said already the issue is with the second surface mirrors.
Other than the image ghosting can you get any decent views from it?
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Re: Homemade reflector has 3 images

#7

Post by Ylem »


Homemade stuff is definitely fun :) years ago some Boyscouts had a homemade Dob with a sonatube, must have been 12"
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Re: Homemade reflector has 3 images

#8

Post by Phoenix22 »


Voyageur wrote: Sun Sep 29, 2019 2:54 pm How did you fit a 15 cm primary mirror into a Pringles can? Must be some big Pringles out there that I don't know about!
My mistake. Its focal length is 15 cm. It's of a radius of 20 mm. Hope this clears up your confusion on the Pringles can size :)
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Re: Homemade reflector has 3 images

#9

Post by Phoenix22 »


PapaFox wrote: Sun Sep 29, 2019 9:53 am saw this on the internet, is the inside of your pringles can black?

Some telescopes suffer from internal reflections inside their tubes, because the inside of the tube needs to be black.
I had spray painted the insides matt black thankfully. :)
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Re: Homemade reflector has 3 images

#10

Post by Phoenix22 »


Lady Fraktor wrote: Sun Sep 29, 2019 4:28 pm Congratulations on the DIY project, as has been said already the issue is with the second surface mirrors.
Other than the image ghosting can you get any decent views from it?
I have been having cloudy skies here for the past week (since the completion of the tube). Will definitely report the results once I get to see some clear night skies. :)
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Re: Homemade reflector has 3 images

#11

Post by Phoenix22 »


Thank you so much @KathyNS and @goldstar. This must be the cause of the abberation. Let me see if I can find any optical M2s of the same size.

Also, is it true that parabolic M2s are better or do circular ones work equally well?
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Re: Homemade reflector has 3 images

#12

Post by Voyageur »


Phoenix22 wrote: Mon Sep 30, 2019 5:59 am
Voyageur wrote: Sun Sep 29, 2019 2:54 pm How did you fit a 15 cm primary mirror into a Pringles can? Must be some big Pringles out there that I don't know about!
My mistake. Its focal length is 15 cm. It's of a radius of 20 mm. Hope this clears up your confusion on the Pringles can size :)
Well, darn, I was hoping for some giant Pringles!

Seriously, it's pretty cool that you made a telescope. Lots of ideas here on how to perfect it. Good luck!
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Mounts: Vixen SXW/Starbook (original); Stellarvue M2C alt-az.
Eyepieces: Televue: 55mm Plossl, 22mm Panoptic, 17.3mm Delos, 13mm Nagler, c. 1980, 11mm Plossl, 7mm Nagler, 5mm Radian; Meade 15mm Super Plossl; VERNONSCOPE 2.4X BARLOW
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Re: Homemade reflector has 3 images

#13

Post by KathyNS »


Phoenix22 wrote: Mon Sep 30, 2019 6:14 am Also, is it true that parabolic M2s are better or do circular ones work equally well?
Your focal ratio is f/3.75. That is pretty fast, so the difference between parabolic and spherical is significant. You can expect some pretty bad spherical aberration.
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DSO AP: Orion 200mm f/4 Newtonian Astrograph; ATIK 383L+; EFW2 filter wheel; Astrodon Ha,Oiii,LRGB filters; KWIQ/QHY5 guide scope; Planetary AP: Celestron C-11; ZWO ASI120MC; Portable: Celestron C-8 on HEQ5 pro; C-90 on wedge; 20x80 binos; Etc: Canon 350D; Various EPs, etc. Obs: 8' Exploradome; iOptron CEM60 (pier); Helena Observatory (H2O) Astrobin
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