Scattering of light in the refractor optical tube assemblies

Discuss your refractor type scopes here.
Post Reply
User avatar
j.gardavsky Germany
Orion Spur Ambassador
Articles: 0
Offline
Posts: 711
Joined: Mon Sep 23, 2019 1:52 pm
4
Location: Germany
Status:
Offline

TSS Awards Badges

Scattering of light in the refractor optical tube assemblies

#1

Post by j.gardavsky »


Hello all,

the question why some refractors deliver higher contrast views than the others,
is not just an object of the speculative philosophical reflection.
It is in fact about the reflected and scattered light suppression in the tube and in its parts.

The following documenting pics show where the problem emerges in the low cost astronomy OTAs,
in comparison to a premium spotting scope refractor,

Scattering in OTA.jpg

1. The first source of scattered light is the unprotected rim of the achromat lenses, which shines really bright.
Its scattered light can be removed by a baffle in (or close to) the lens cell, at the cost of reducing the effective aperture.

2. The second source of scattered light are the unprotected surfaces in the OTA, in plain language not enough baffles.

3. The third problem emerges in the focusers.
Focusers with the outer larger diameter focusing tubes make sense. But even then, putting a baffle into the focuser may help. An alternative are filter wheels, which also trap some of the scattered light.

4. The last source of scattered light are the walls in the zenith diagonal mirror house.

And finally, what matters as well, is the quality of blackening of the surfaces.
The popular knife edge rills will replace the reflections of light with the diffraction,
diffraction on the focuser tube rills.jpg
diffraction on the focuser tube rills.jpg (14.27 KiB) Viewed 4924 times
and so, you have a choice between the scattered light and the diffracted light, reducing the contrast.


A counter example is the Leica APO Televid 82mm F/5.7, which shows just a very faint residual of the scattered light on the walls of the prism system. No wonder, why the Leica sports optics is regarded as delivering the highest contrast and the highest saturation of the colors.

So far I have tried out different solutions, the prisms seem to be less prone to scattering of light than the zenith diagonal mirrors.

And finally,
don't forget to mount an oversized lens hood on your refractor. Bigger is better.
I have the lens hoods for my astronomy binoculars as well.

Hoping this helps, and thank you for reading,
JG
6" F/5 Sky-Watcher achro, 2" BBHS Star Diagonal, 2" zenith prism, 1.25" Takahashi prism
Leica 82mm APO Televid
Eyepieces: Docter UWA; Leica B WW and WW Asph. Zoom; Leica HC Plan S and L, monocentric; Pentax SMC XW, O-, XO; Tak MC O, Carl Zeiss B WW, and Pl, E-Pl, S-Pl, W-Pl;
Swarovski SW; Baader Symmetric Diascope Edition; Nikon NAV SW, ; TMB supermonocentric; Rodenstock; Vixen HR; TV Delos
Filters: Astrodon, Astronomik, Baader, Balzers, Zeiss West and East, Lumicon
Binoculars (7x42 up to 15x85): Docter Nobilem, Leica Ultravid, Nikon Astroluxe, Swarovski EL Swarovision; BA8 (Kunming Optical)
User avatar
DEnc United States of America
Jupiter Ambassador
Articles: 0
Offline
Posts: 283
Joined: Tue Aug 13, 2019 11:52 pm
4
Location: Northern CA
Status:
Offline

TSS Photo of the Day

Re: Scattering of light in the refractor optical tube assemblies

#2

Post by DEnc »


This is very interesting, thanks for the post. It confirms my observations with a Stellarvue 110ED achromat.

Attached is the view through the focuser when the business end is illuminated with the light panel I use for flats. Oofta!

It’s been a great telescope to learn about astrophotography, astrometry and photometry, but I’ve found it impossible to get quality flat corrections with it. I’ve put a roll of flockboard in the dew shield, and I’ve tried a baffled light box, but to no avail. For photography I’m limited to relatively bright objects that don’t require as deep a stretch as dim targets. I can get to millimag photometry, but only if I limit the FOV to less than 0.5* of the 1.5* available.

I’m overdue for an OTA better suited to these applications, but we’re moving to coastal Humboldt County in California, under largely overcast skies. I’ll wait to make a purchase once I see how far inland I need to travel for clear skies.

SV110.jpg
User avatar
Lady Fraktor Slovakia
Co-Administrator
Co-Administrator
Articles: 0
Offline
Posts: 9860
Joined: Mon Apr 29, 2019 9:14 pm
4
Location: Slovakia
Status:
Offline

Re: Scattering of light in the refractor optical tube assemblies

#3

Post by Lady Fraktor »


Good advice JG, I re-baffled my 150mm f/8 quite a while ago as well as blackening the edges of the lens set.
I also blackened the interior of the black anodized drawtube as well as the front of the drawtube in the telescope tube.
A very good improvement of light management from stock.
See Far Sticks: Antares Elita 103/1575, AOM FLT 105/1000, Bresser BV 127/1200, Nočný stopár 152/1200, Vyrobené doma 70/700, Stellarvue NHNG DX 80/552, TAL RS100/1000, Vixen SD115s/885
EQ: TAL MT-1, Vixen SXP, AXJ, AXD
Az/Alt: AYO Digi II/ Argo Navis, Stellarvue M2C/ Argo Navis
Tripods: Berlebach Planet (2), Uni 28 Astro, Report 372, TAL factory maple, Vixen ASG-CB90, Vixen AXD-TR102
Diagonals: Astro-Physics, Baader Amici, Baader Herschel, iStar Blue, Stellarvue DX, Takahashi prism, TAL, Vixen flip mirror
Eyepieces: Antares to Zeiss
The only culture I have is from yogurt
My day was going well until... people
Image
User avatar
MistrBadgr United States of America
Orion Spur Ambassador
Articles: 0
Offline
Posts: 748
Joined: Sat May 11, 2019 9:27 pm
4
Location: Broken Arrow, Okla, USA
Status:
Offline

TSS Awards Badges

Re: Scattering of light in the refractor optical tube assemblies

#4

Post by MistrBadgr »


I have gotten to the point that every entry level refractor I get my hands on ends up with the main tube flocked, baffles pulled and left out. I found that every point in the image reaching my eye in a 60mm refractor these days only sees a 40mm diameter circle of the lens area. I also flock the draw tube and leave out the baffles, blacken the lens edges, flock the dew shield, and make sure all smooth plastic surfaces, that cannot be flocked but are still exposed, are sanded concentric to the tube circumference with 60 grit sand paper and painted with a thin coat (not thick) of the flattest, blackest paint I can find.

For the 60mm scopes, I make a dew shield extension out of a 40 oz peanut butter plastic jar, paint the inside with black paint, then flock over the top of that. I found it helpful to block out all possible area where light can come from that is not part of the actual image light cone.

With all that done, I have seen some things with a long 60mm refractor that some of the local astronomers simply do not believe. I can tell that by the expressions on their faces, even though they don't say anything.
Bill Steen
Many small scopes, plus a Lightbridge 12, LX 70-8R,6R,6M
Many eyepieces, just not really expensive ones.
User avatar
j.gardavsky Germany
Orion Spur Ambassador
Articles: 0
Offline
Posts: 711
Joined: Mon Sep 23, 2019 1:52 pm
4
Location: Germany
Status:
Offline

TSS Awards Badges

Re: Scattering of light in the refractor optical tube assemblies

#5

Post by j.gardavsky »


A very good experience shared, my friends!

One of the substantial improvements on my 6" F/5 achro has been the oversized lens hood, made from a very black and very rough door mat.

Ask me please,
if I have ever seen the Wolf Rayet WR 134 Dolphin Nebula in Cygnus.
Yes, I have seen it, and here is the entry in my observing logbook:
6" F/5: 12,5mm DOCTER, Leica HC Plan 20mm: OIII 10nm and 5nm showing the bowshock, H-Beta 12nm showing the LBN 182
making the WR nebula appearance like a nearly eaten lollypop


Best,
JG
6" F/5 Sky-Watcher achro, 2" BBHS Star Diagonal, 2" zenith prism, 1.25" Takahashi prism
Leica 82mm APO Televid
Eyepieces: Docter UWA; Leica B WW and WW Asph. Zoom; Leica HC Plan S and L, monocentric; Pentax SMC XW, O-, XO; Tak MC O, Carl Zeiss B WW, and Pl, E-Pl, S-Pl, W-Pl;
Swarovski SW; Baader Symmetric Diascope Edition; Nikon NAV SW, ; TMB supermonocentric; Rodenstock; Vixen HR; TV Delos
Filters: Astrodon, Astronomik, Baader, Balzers, Zeiss West and East, Lumicon
Binoculars (7x42 up to 15x85): Docter Nobilem, Leica Ultravid, Nikon Astroluxe, Swarovski EL Swarovision; BA8 (Kunming Optical)
User avatar
MistrBadgr United States of America
Orion Spur Ambassador
Articles: 0
Offline
Posts: 748
Joined: Sat May 11, 2019 9:27 pm
4
Location: Broken Arrow, Okla, USA
Status:
Offline

TSS Awards Badges

Re: Scattering of light in the refractor optical tube assemblies

#6

Post by MistrBadgr »


That is great, JG! Thanks for sharing that!
Bill Steen
Many small scopes, plus a Lightbridge 12, LX 70-8R,6R,6M
Many eyepieces, just not really expensive ones.
User avatar
Lady Fraktor Slovakia
Co-Administrator
Co-Administrator
Articles: 0
Offline
Posts: 9860
Joined: Mon Apr 29, 2019 9:14 pm
4
Location: Slovakia
Status:
Offline

Re: Scattering of light in the refractor optical tube assemblies

#7

Post by Lady Fraktor »


I find most refractor come with a lens hood that is always to short.
I make mine from 2.5x to 3x the lens diameter.
See Far Sticks: Antares Elita 103/1575, AOM FLT 105/1000, Bresser BV 127/1200, Nočný stopár 152/1200, Vyrobené doma 70/700, Stellarvue NHNG DX 80/552, TAL RS100/1000, Vixen SD115s/885
EQ: TAL MT-1, Vixen SXP, AXJ, AXD
Az/Alt: AYO Digi II/ Argo Navis, Stellarvue M2C/ Argo Navis
Tripods: Berlebach Planet (2), Uni 28 Astro, Report 372, TAL factory maple, Vixen ASG-CB90, Vixen AXD-TR102
Diagonals: Astro-Physics, Baader Amici, Baader Herschel, iStar Blue, Stellarvue DX, Takahashi prism, TAL, Vixen flip mirror
Eyepieces: Antares to Zeiss
The only culture I have is from yogurt
My day was going well until... people
Image
User avatar
j.gardavsky Germany
Orion Spur Ambassador
Articles: 0
Offline
Posts: 711
Joined: Mon Sep 23, 2019 1:52 pm
4
Location: Germany
Status:
Offline

TSS Awards Badges

Re: Scattering of light in the refractor optical tube assemblies

#8

Post by j.gardavsky »


j.gardavsky wrote: Fri Aug 27, 2021 8:29 am A very good experience shared, my friends!

One of the substantial improvements on my 6" F/5 achro has been the oversized lens hood, made from a very black and very rough door mat.

Ask me please,
if I have ever seen the Wolf Rayet WR 134 Dolphin Nebula in Cygnus.
Yes, I have seen it, and here is the entry in my observing logbook:
6" F/5: 12,5mm DOCTER, Leica HC Plan 20mm: OIII 10nm and 5nm showing the bowshock, H-Beta 12nm showing the LBN 182
making the WR nebula appearance like a nearly eaten lollypop


Best,
JG
On a side line, the WR 134

This Wolf-Rayet nebula makes an amazing pair with the touching LBN 182, for blinking the OIII filter against the H-Beta:
OIII: The WR 134 becomes faintly visible, the LBN 182 disappears
H-Beta: The WR 134 disappears, the LBN 182 becomes faintly visble.

Clear skies,
JG
6" F/5 Sky-Watcher achro, 2" BBHS Star Diagonal, 2" zenith prism, 1.25" Takahashi prism
Leica 82mm APO Televid
Eyepieces: Docter UWA; Leica B WW and WW Asph. Zoom; Leica HC Plan S and L, monocentric; Pentax SMC XW, O-, XO; Tak MC O, Carl Zeiss B WW, and Pl, E-Pl, S-Pl, W-Pl;
Swarovski SW; Baader Symmetric Diascope Edition; Nikon NAV SW, ; TMB supermonocentric; Rodenstock; Vixen HR; TV Delos
Filters: Astrodon, Astronomik, Baader, Balzers, Zeiss West and East, Lumicon
Binoculars (7x42 up to 15x85): Docter Nobilem, Leica Ultravid, Nikon Astroluxe, Swarovski EL Swarovision; BA8 (Kunming Optical)
User avatar
John Baars Netherlands
Co-Administrator
Co-Administrator
Articles: 5
Online
Posts: 2723
Joined: Sat May 11, 2019 9:00 am
4
Location: Schiedam, Netherlands
Status:
Online

TSS Awards Badges

TSS Photo of the Day

Re: Scattering of light in the refractor optical tube assemblies

#9

Post by John Baars »


You are quite right JG!
Thats why I examined all my telescopes, prisms and diagonals on light scattering and made adjustments when needed.

A nice practical example:
Images through my 102 mm Maksutov. The first with the factory settings. The second one fully flocked.
DSC02795 (640x426).jpg
DSC02794 (640x426).jpg
Refractors in frequency of use : *SW Evostar 120ED F/7.5 (all round ), * Vixen 102ED F/9 (vintage), both on Vixen GPDX.
GrabnGo on Alt/AZ : *SW Startravel 102 F/5 refractor( widefield, Sun, push-to), *OMC140 Maksutov F/14.3 ( planets).
Most used Eyepieces: *Panoptic 24, *Morpheus 14, *Leica ASPH zoom, *Zeiss barlow, *Pentax XO5.
Commonly used bino's : *Jena 10X50 , * Canon 10X30 IS, *Swarovski Habicht 7X42, * Celestron 15X70, *Kasai 2.3X40
Rijswijk Public Observatory: * Astro-Physics Starfire 130 f/8, * 6 inch Newton, * C9.25, * Meade 14 inch LX600 ACF, *Lunt.
Amateur astronomer since 1970.
User avatar
j.gardavsky Germany
Orion Spur Ambassador
Articles: 0
Offline
Posts: 711
Joined: Mon Sep 23, 2019 1:52 pm
4
Location: Germany
Status:
Offline

TSS Awards Badges

Re: Scattering of light in the refractor optical tube assemblies

#10

Post by j.gardavsky »


John Baars wrote: Sat Aug 28, 2021 10:56 pm You are quite right JG!
Thats why I examined all my telescopes, prisms and diagonals on light scattering and made adjustments when needed.

A nice practical example:
Images through my 102 mm Maksutov. The first with the factory settings. The second one fully flocked.
Image Image
Hello John,

this is a fantastic docu!
Manufacturing the telescopes should not be left to the plumbers.

Thank you for sharing the pics,
JG
6" F/5 Sky-Watcher achro, 2" BBHS Star Diagonal, 2" zenith prism, 1.25" Takahashi prism
Leica 82mm APO Televid
Eyepieces: Docter UWA; Leica B WW and WW Asph. Zoom; Leica HC Plan S and L, monocentric; Pentax SMC XW, O-, XO; Tak MC O, Carl Zeiss B WW, and Pl, E-Pl, S-Pl, W-Pl;
Swarovski SW; Baader Symmetric Diascope Edition; Nikon NAV SW, ; TMB supermonocentric; Rodenstock; Vixen HR; TV Delos
Filters: Astrodon, Astronomik, Baader, Balzers, Zeiss West and East, Lumicon
Binoculars (7x42 up to 15x85): Docter Nobilem, Leica Ultravid, Nikon Astroluxe, Swarovski EL Swarovision; BA8 (Kunming Optical)
Post Reply

Create an account or sign in to join the discussion

You need to be a member in order to post a reply

Create an account

Not a member? register to join our community
Members can start their own topics & subscribe to topics
It’s free and only takes a minute

Register

Sign in

Return to “Refractor type Telescopes”