Details of a telecentric lens for C8 Edge and Daystar filter

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Details of a telecentric lens for C8 Edge and Daystar filter

Post by p_zetner » Thu Jul 05, 2018 7:35 pm

Greetings Everyone.

I thought I would add some information about the latest telecentric lens I've designed for use with a C8 Edge. The telecentric magnification is about 4.4 with a (design) field angle of 0.012 degrees. My design goals were to keep the lens train as short as possible and use off the shelf lenses, with a preference for achromatic doublets. I also wanted to use as high a telecentric magnification as possible to fully optimize the operation of the Daystar filter. I have analyzed designs up to 8x magnification. The calculations of Christian Vladrich ( ... lar/FP.htm) show that it is not unreasonable to aim for focal ratios of around 50 (implying a magnification of 50/10.27= 4.9x for the C8). Focal reduction after the H alpha filter would then be used. I have found that, after focal reduction to give my design goal of f25 to f35 at the camera, the final image becomes progressively more subject to vignetting (by the filter), the higher the telecentric magnification. Consequently, I used 4.4x for an acceptable degree of vignetting which can be removed by flat-fielding. For a focal reducer, I use a simple system of two closely-spaced achromatic doublets of 300mm focal length each to achieve focal ratios at the camera of f24 to f36 (corresponding focal reductions of 0.43x to 0.29x). The focal reducer's operating demagnification is simply determined by fixing the image distance to the camera to the required value.

My first images taken with this system can be found here on the forum:



The first figure below shows a WinLens analysis of the telecentric lens. It is a 4-element design comprising 40mm diameter achromatic doublets from Edmund Optics (-80mmf and 120mmf) and (eBay) (-120mmf). The optical prescription for the C8 Edge scope comes from the website of Vladimir Sacek, "Notes on Amateur Telescope Optics" ( The Daystar filter is modelled as a BK7 window of clear aperture 31.75mm and thickness 23mm, as specified in the Daystar QPE white paper. The spot pattern shows the performance of the lens (and C8 system) to be diffraction limited. My studies of the properties of this lens show that it maintains the design focal ratio and a high degree of telecentricity (field angle ~0.01) as well as diffraction limited performance over a very wide range of image distances.
C8 Edge Scope_200mmf10 - Telecentric EO - m80-m120-m120-120-f46.png
C8 Edge Scope_200mmf10 - Telecentric EO - m80-m120-m120-120-f46.png (97 KiB) Viewed 147 times
The second figure shows the lens assembly. The lens holders are from Edmund Optics, eBay and Surplus Shed. The first three (negative) lenses are held inside a 2 inch extender for the CHL focuser, rigidly threaded onto the focuser. The mechanical design is quite rigid with no flexure that I can determine. In this picture, F'o is the design location of the C8 focal plane required by the lens. BF is the physical (optimal back focus) location of the C8 focal plane. By focusing the C8 mirror with the mirror control and fine focusing with the CHL, the design focal point and the physical back focus are brought into alignment near the optimal C8 back focus.
C8 Edge Scope_200mmf10 - Telecentric EO - m80-m120-m120-120-f46 assy view2.png
C8 Edge Scope_200mmf10 - Telecentric EO - m80-m120-m120-120-f46 assy view2.png (139.04 KiB) Viewed 147 times
Finally, a photograph of the system is shown below. The distance from A (back surface of the telescope) to B (front flange of the Daystar) is 25cm. Beyond the Daystar is a 50mm T-thread extension with 2 inch adapter. The two lenses comprising the focal reducer (, 300mmf) are held inside a 2 inch OD tube which smoothly slides into the 2 inch adapter (allowing easy rotation for image framing). The assembly beyond the Daystar is quite light in this configuration. The 50mm T-thread extension is not strictly necessary. It's meant as a "place holder" for a Lunt LS50Ha filter which I have tried (so far unsuccessfully!!) to stack with the Daystar.
IMG_2734_edit.png (1.34 MiB) Viewed 147 times
The system in the field:

One thing that bothers me is my inability to come up with a way to bench test the telecentricity of the system. Has anyone come up with a procedure for this?

Hope this was informative and useful.
Cheers, Peter.

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Re: Details of a telecentric lens for C8 Edge and Daystar filter

Post by DJD » Fri Jul 06, 2018 9:13 am

thanks for your reply on the other thread. Together with this posting it has been very educational ( and useful ).
Best wishes,
Making the possible difficult and the difficult impossible.

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