55 replies to this topic

[rik ter horst]

A short introduction:

BlackGEM/MeerLicht is a Dutch project specifically designed to detect the optical counterparts from gravitational wave sources detected with Virgo and LIGO and involves an array of at least three 60 cm F/5.5 astrographs with a 10Kx10K detector (~10x10cm!). Based on the optical design I’ve made for the BlackGEM/MeerLicht project together with Harrie Rutten, I’ve been looking into alternative ways to deal with Atmospheric Dispersion as we didn’t have the possibility and space to use rotating prisms in the system. The 60 cm F/5.5 instrument is based on a Harmer-Wynne with three field correcting lenses placed in the main baffle in front of the Primary mirror. After some trials it seemed possible that by shifting one of these corrector lenses sideways you‘re able to correct for AD without the need of adding extra optical components. At the end, I managed to optimize the optical system without sacrificing on image quality for 0 degree Zenit Distance as well as for 70 degrees Zenit distance (and everything in between of course). This year the MeerLicht prototype has been built and tested and the ADC function work excellent.

So far for BlackGEM.

My interest in observing and imaging moon and planets made me decide to further investigate the possibility designing an instrument where this new type of ADC could be implemented, especially knowing that from my country (the Netherlands) the planets will generally stay very low above the horizon for the next couple of years. After spending hours and hours with Zemax I found a good solution for a 400 mm F/12.5 Modified Dall Kirkham Cassegrain Telescope with integrated ADC (see figure 1). The last lens of the optical train is moved laterally to compensate for dispersion of the atmosphere. This 3-lens field corrector is a well-balanced integral part of the telescope and is optimized specifically for this instrument, in other words, it cannot be placed in another system just like that. That being said, for almost every system a solution can be found, as long as you have at least three (field-)corrector lenses.

Edited by rik ter horst, 30 November 2016 - 06:35 AM.

[rik ter horst]

Below the (theoretical!) performance of this new system showing the minimal loss in image quality at 70 degrees Zenit distance compared to 0 degrees Zenit distance (see figures 2 and 3, my apologies for the mixed order in which the fields are displayed). A field diameter of 35 mm is still diffraction limited in both situations. For my new ASI178MC with 7.4 x 5 mm sensor this is more than sufficient and also the F/ratio matches very well the 2.4 micron pixels of this camera.


image quality at Zenit


Image quality at 20 degrees altitude


Regarding the optics, a slightly better design with custom lenses is possible but I’ve tried very hard to use existing stock-lenses to reduce the costs of building the telescope. At the moment I’ve started grinding the 400 mm meniscus shaped 18 mm thin primary mirror, almost identical to the 400 mm F/3 parabolic mirror I’ve made last year for my F/3 Dobson. In between the spherical secondary mirror will be made.

Edited by rik ter horst, 30 November 2016 - 06:38 AM.

[rik ter horst]

The focuser and corrector lenses will be mounted on top of the telescope tube as I wanted a kind of Nasmyth configuration for comfortable viewing (see figure 4 below). The telescope will be mounted like a Dobsonian riding on an Equatorial Platform. In this way the ADC lens (the last lens of the optical train) is only shifted in one direction, making life a bit easier ;-)
A couple of years ago I wanted to build a 400 mm Schmidt Cassegrain and I had started building the tube and altitude bearings of the mount but recently I decided to use the tube for this new design. Fans will be added for improved thermal behavior.
I hope to have first light of this instrument somewhere the second half of 2017.

[Chriske]

Hi Rik,

 

If I'm not mistaken another telescope dealing with AD is a Schupmann, DaveG can tell you more about it I think.

Nice project btw...

[rik ter horst]

That's interesting Chris, I didn't know of it. I just googled it and found a brief description. If I'm right they adjust the field mirror (which is a kind of Mangin mirror), in order to create opposite dispersion. Nice trick! Thanks!

[bertb]

Mmmmm,  a large obstruction for a planetary telescope? 

[DAVIDG]

That's interesting Chris, I didn't know of it. I just googled it and found a brief description. If I'm right they adjust the field mirror (which is a kind of Mangin mirror), in order to create opposite dispersion. Nice trick! Thanks!

Partially correct. The field mirror in a Schupmann is not the mangin corrector  but you do adjust the field mirror  while observing to cancel atmospheric dispersion.  It works perfectly when viewing the planets or the Moon to totally cancel out atmospheric dispersion.  You can go the other way and   turn all the stars into a spectra and study the spectra. This is also  a nice feature when hunting down small planetary nebula since they emit mostly in one wavelength so they will stay round while all the other stars will smear out into a spectra.

 

               - Dave 

Edited by DAVIDG, 30 November 2016 - 04:04 PM.

[rik ter horst]

Mmmmm,  a large obstruction for a planetary telescope?

Bert, the obstruction of this telescope will be smaller than the obstruction of a regular Schmidt-Cassegrain, and probably you know what can be done with them ;-) I'm not worried about that. Thermal issues are way more important to control.

[nicknacknock]

Rik,

 

It's been a month and a half without any updates. Any progress you'd like to share with us?

[rik ter horst]

Nick, an update ;-)

 

Yesterday I finished the 18 mm 'thick' meniscus primary mirror, it went smoother than expected so I'm happy! Compared to the first 400 mm meniscus F/3 mirror that I made last year for my rich-field telescope this mirror has a better shape. Below a Foucaultgram of the mirror. In order to avoid possible astigmatism (due to bending of the thin mirror) testing had been done on a 10 degree tilted test stand, so the mirror could rest on a six-point support. The approach of continuously rotating the mirror during polishing to avoid astigmatism seemed to have worked once more. Soon I'll start with the spherical secondary, but prior to that I need to make a testglass.   

 

[nicknacknock]

Quite smooth surface coming out of the Foucault test! 

[rik ter horst]

Update:

 

The primary mirror is aluminized now and slowly the mechanical parts get ready. The image below shows the back plate of the telescope that will hold the 400 mm primary mirror. The plate has been watercut and I'm still surprised at what accuracy! Soon the 18 point mirror cell will be finished, I'll post an image when it's done.

The mirror will laterally be hold by a central tube to avoid an edge support which would be tricky with an 18 mm thick mirror. See also the Sketchup drawing...

 

 

[rik ter horst]

I've been thinking about a curved spider since a long time, and decided to give it a try. If it works for me, great, if not I'll make a more 'conservative' spider ;-)
Thickness of the spiderarms is 1.5 mm and they are made of stainless steel



Somewhere in May/June I hope to be able to point the telescope upwards to perform some star tests which will guide me through the final phase of making the optics. The last corrections will be done on the spherical secondary.

Edited by rik ter horst, 23 March 2017 - 04:12 PM.

[rik ter horst]

Just finished the mirror support for the thin meniscus primary mirror. The central tube, which will also hold the tertiairy mirror and baffle, keeps the mirror well centered, while the 18 support points will prevent the mirror from deforming due to gravity. The triangles have been watercut, a technique that proved to be very accurate with a high repeatability and fairly cheap. Next thing will be the ADC lens adjustment. I think the watercutter will be visited once more ;-)

 

 

Edited by rik ter horst, 30 March 2017 - 01:18 PM.

[rik ter horst]

Fit-test of the primary mirror....

 

[rik ter horst]

New update: It works!

 

Although not finished by far, I had preliminary first light with an un-corrected and un-aluminized secondary. Having the light equivalent of an 90 mm telescope I could test the system quite well, and I'm happy with the results. There is about 1/4 wave that I have to correct on the secondary which will be done using star tests. But the ADC function has made my day completely. Because of the azimuthal set up I only have to adjust the third ADC lens in one direction. Observing with the telescope is very comfortable, sitting on a chair behind the instrument.

 

Here's the lens cell with two of the three Corrector lenses:

 

[rik ter horst]

Here's the instrument on the Equatorial Platform I use for guiding

 

[rik ter horst]

The open back with mirror support:

 

[rik ter horst]

OK, this is not a very good image, but I just wanted to see if the ADC does its job, and obviously it does. So this image was taken with an un-aluminized and unfinished secondary giving me only about 5% of the light I'll have when the instrument is finished. The gain was set at max and I used 1000 frames for the stack @ 10 frames per second. Camera: ASI178MC, seeing lousy.... Altitude 25 degrees.

Edited by rik ter horst, 15 June 2017 - 08:40 AM.

[clivemilne]

Love your work... this is very impressive.

[rik ter horst]

Thanks a lot!

 

From yesterday night, an image of Jupiter and one of Saturn. The secondary mirror got a small correction (but I'm not done yet!) so these images were made with an un-aluminized secondary. In any case, the heigth of Saturn was challenging enough 14 degrees, so an ultimate test for the integrated ADC, which seems to work very fine. At full Gain I could capture 8 images per second with the ASI178MC (noisy). In total about 1500 frames were used to make these images. The altitude of Jupiter during imaging was 25 degrees, less challenging compared to Saturn. Plenty room for improvement but considering that I use just 5% of the potential light I'm very happy with these results. 

 

 

 

[star drop]

An excellent job, Rik!

[tommm]

Beautiful, clever work!

[ukyochen]

Nice image

[rik ter horst]

Thank you, maybe it's time time for a new update:

This is a very recent image of Jupiter, from last weekend. The telescope is finished and I got a couple of quite good night's to test the ADC. The camera I used for this image is an ASI178MC. For this stack I used about 1500 frames. I'm very happy with this result and will post some more lunar images later today :-)