For sale, 2 inch dielectric coating star diagonal

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denebman
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For sale, 2 inch dielectric coating star diagonal

Post by denebman »

Several factors are important for star diagonal,

1. Surface smoothness and accuracy
2. Reflectivity of the reflective layer
In general there are two categories: reflective metal coating and dielectric coating. Metals such as Al, Ag, Au are evaporated and deposited on the optical flat to give a reflective coating, always a secondary or multiple layers are over coated to provide protection and to enhance the reflectivity. However, reflectivity drops as a function of time, corrosion sometimes starts from the periphery and develops inward for silver enhanced coatings. As a much more superior alternative, dielectric coating provides the best durability and reflectivity, they never age and their reflectivity are simply based on the intrinsic nature of interference of light wave. But still, dielectric coatings vary. They vary in terms of the number of coated layers and coating materials, this is why some manufacturer gives a warning not to clean their mirror with water-based liquids.
3. Rigidity and 45-degree alignment
Only a few reputable manufactures make a one-piece mirror cell from a solid metal block. If the mirror is placed correctly, the alignment is deadly kept even if a heavier camera and eyepiece are inserted.
Now, I come back to my homemade 2-inch dielectric star diagonal according to the above-mentioned notes. (Pls visit: http://www.singastro.org/viewtopic.php?t=2558)
1. I use 10 mm thick fine annealed BK7 glass optical flat, before dielectric coating, the surface is close to 1/20 lambda ptv and 1/100 lambda rms. The surface drops to 1/10 lambda after dielectric coating according to my contractor’s statement.
2. Test results show that from 400 to 700 nm spectrum my diagonal gives >99% reflectivity of incoming light. Due to the use of a novel water-resistant refractory material as dielectric layer, the coating is very durable and withstands nitric acid.
3. The mirror cell is CNC machined from one piece aluminium alloy block, the adapter side is still designed as a screw-on tube however I changed the material to more rigid stainless steel, which I think betters many other brands on the market.

I did a side-by-side test finally. I used to test eyepieces and diagonals on an achromat scope, the results are frustrating, secondary color and poorly figured lens flood any differences, it is hard to make any reliable verdict. Finally I got a TMB115 apochromatic scope, a very good performer with excellent color correction and near unnoticeable spherical error. This scope enables me to do a hair-split comparison over eyepieces and diagonals. Here it goes:
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Last edited by denebman on Wed Jan 11, 2006 12:44 pm, edited 5 times in total.
10" Telekit with Zambuto optics~TMB 152/1200 (construction)~Takashi Mewlon 210~TMB 80/600

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denebman
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Post by denebman »

1. Day time visual test
Diagonals include my homemade dielectric one, a famous brand dielectric one and a very popular brand 97% enhanced mirror diagonal. I tested five eyepieces (fig.1) on these diagonals:

* University Optics MK70, 40 mm fl, clear aperture: 45 mm (almost maximum for 2 inch ep)
* Televue Nagler T5/31, clear aperture: 42 mm
* Televue Plossl, 32mm fl, clear aperture: 27mm (maximum for 1.25 inch ep)
* Televue Nagler Zoom 3-6 mm
* TMB supermonocentric 4mm
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Last edited by denebman on Wed Jan 11, 2006 12:30 pm, edited 2 times in total.
10" Telekit with Zambuto optics~TMB 152/1200 (construction)~Takashi Mewlon 210~TMB 80/600

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denebman
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Post by denebman »

At low power (with PL 32 and longer fl ep) I tried my best and failed to tell any difference between the three diagonals. At highest power of 266 X, both dielectric coated diagonals seem to have an edge in terms of contrast and sharpness.
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Last edited by denebman on Wed Jan 11, 2006 12:46 pm, edited 6 times in total.
10" Telekit with Zambuto optics~TMB 152/1200 (construction)~Takashi Mewlon 210~TMB 80/600

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denebman
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Post by denebman »

3. Sharpness and contrast (fig.3 (97% enhanced), fig.4 (my homemade dielectric), fig. 5 (a famous brand dielectric diagonal))
The following results are based on a daytime object, a ventilation pole on top of a HDB flat. The scope was set at focal ratio of 14.0 (barlowed, equivalent focal length: 1610 mm), several tens of prime focus shots were taken by Phillips ToUcam 840K for each diagonal, and then hand selected the best single frame as the representative. My verdict is: Both my homemade dielectric and a branded dielectric diagonal outperform the 97% enhanced diagonal in terms of image quality.

Fig. 3 A popular brand 97% enhanced mirror diagonal
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Last edited by denebman on Wed Jan 11, 2006 12:36 pm, edited 5 times in total.
10" Telekit with Zambuto optics~TMB 152/1200 (construction)~Takashi Mewlon 210~TMB 80/600

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denebman
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Post by denebman »

Fig.4 My homemade dielectric diagonal
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Last edited by denebman on Wed Jan 11, 2006 12:34 pm, edited 1 time in total.
10" Telekit with Zambuto optics~TMB 152/1200 (construction)~Takashi Mewlon 210~TMB 80/600

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denebman
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Post by denebman »

Fig. 5 A famous brand dielectric coated diagonal
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Last edited by denebman on Wed Jan 11, 2006 12:35 pm, edited 2 times in total.
10" Telekit with Zambuto optics~TMB 152/1200 (construction)~Takashi Mewlon 210~TMB 80/600

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denebman
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Post by denebman »

Price (Singapore dollar)

199.00 each
Last edited by denebman on Tue Aug 17, 2010 5:50 pm, edited 1 time in total.
10" Telekit with Zambuto optics~TMB 152/1200 (construction)~Takashi Mewlon 210~TMB 80/600

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wucheeyiun
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Post by wucheeyiun »

luiming,
Can you get a sct version for the di-electric diagonal....?

chee yiun

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VinSnr
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Post by VinSnr »

Liu Ming,

You still selling these?

Would appreciate if you can upload those pictures again

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denebman
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Post by denebman »

Yes, still have one refractor type in stock and 3 SCT version on the way.
10" Telekit with Zambuto optics~TMB 152/1200 (construction)~Takashi Mewlon 210~TMB 80/600

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