Looking for a CS telelens for ZWO ASI290MC

[inFINNity Deck]

Dear stargazers,

one of the cameras I use in my observatory is a ZWO ASI290MC, which I bought together with ZWO's New CS lens 2.8mm-12mm F1.4. Although this lens is not really suitable for astronomic imaging it gives me plenty pleasure for other purposes for which I want to use a small astronomical camera, like the odd meteor shower or other wide-field images that do not require the highest quality. Last week I started using this lens/camera combination for a bath-inteferometer and for foucault testing and found out that it does work reasonable well, but the image scale is on the low side:

These two images are true to scale and show the 130mm f/5 tested mirror as a circle with a diameter of about 275px, while the ZWO ASI290MC has a resolution of 1936 x 1096 pixels:

So now I am looking for a way to increase the image scale, so that the full mirror takes at least half the image height, but preferably 75%, and I was hoping that some of the forum members have experimented with other low-budget C or CS lenses or a barlow to increase magnification.

Thanks!

Nicolàs

 

 

Edited December 1, 2020 by inFINNity Deck

[vlaiv]

I don't think you really need tele lens - just regular CS lens with a bit more focal length. This is because sensor is already very small and it has small FOV.

Look for both CS and C-mount lenses. You need 5mm extension for C-mount lens to be mounted instead of CS mount lens (since you have t2/c mount thread adapter - just add 5mm of optical path in T2). Most CCTV equipment uses those kind of lenses. E-bay is good place to hunt down these.

This one does not look bad:

https://www.ebay.nl/itm/Fujinon-TV-Z-LENS-Zoom-14-70mm-f-2-H5X14-mit-C-Mount-Gewinde-gebraucht/363074977444?hash=item5488f47ea4:g:dMUAAOSwWplfNQPL

Or perhaps this one as really cheap option:

https://www.ebay.nl/itm/Tokina-CCTV-LENS-25mm-f2-0-C-mount-Lens-For-Various-Formats-Including-16MM-film/174517944950?hash=item28a2148276:g:TdMAAOSwnwlfrywB

Of course, you can browse whole lot and select one that you like the best:

https://www.ebay.nl/sch/i.html?_from=R40&_nkw=c-mount+lens&_sacat=0&_sop=15&_pgn=1

I searched for c-mount only - but you can also search for CS-mount as well.

 

[inFINNity Deck]

Hi Vladimir,

great finds, thanks! I just contacted the seller. That lens should well double the image scale.

cheers,

Nicolàs

 

[inFINNity Deck]

Finally I ended up in China, where I found the following two lenses of respectively 25mm and 35mm focal length:

- Fujian 35mm F1.7 CCTV TV Movie lens
- Non-branded 25mm f/1.4 CCTV C 1/2 Lens

I ordered them on 2 December and arrived yesterday, so 10 days later. Both came with four adapters to go from C to CS mount and two to go to Nikon mount. Despite their large focal lengths they are still quite small:

At the left are the ZWO 2.1mm fish-eye and the ZWO 2.8 - 12mm zoom-lens, at the right are the 25mm (left) and 35mm Fujian lenses. The first impression is very good, they seem to be well made, run smoothly and fit the ZWO camera well. Most important is of course the image scale that I was looking for. Being slightly larger than the ZWO zoom-lens, I first had to modify the brackets for the Foucault-contraption. Once finished I took the following images:

As can be seen there is quite some improvement, even more than expected. I anticipated that the 25mm lens would provide an interferogram twice the size of the 2.8 - 12mm zoom-lens, but in fact it became well over 3 times as much (327% to be more exact). The 35mm enlarged it even by 442% (1.35 when compared to the 25mm lens). As a result the 35mm lens produced an image too large to fit the ZWO ASI290MC chip (sensor height 1096px), so I am glad I ordered both and now use the 25mm for the tests.

Next step is to see how they perform under the sky... 🙂

Nicolàs

 

[vlaiv]

2 minutes ago, inFINNity Deck said:

Next step is to see how they perform under the sky...

Could you do one more internal test?

I wonder how sharp are these small CS/C mount lenses. I'm not holding my breath, but I do have a plan of trying one for EEVA myself and wonder what is their resolution.

Could you do FWHM measurement on artificial star on those lenses? Does not need to be very long distance - just enough that star is not resolved, I don't care about possible spherical (so 5-10m should be good distance).

[inFINNity Deck]

Hi Vladimir,

yes, can do that, but what do you want me to do exactly? Obviously these 20 euro lenses are not Takahashi-quality, so expect a lot of coma and other issues. This morning I already took a bunch of pictures of my 9 micron fibre-optic artificial star at 5 metres distance that I can send you. Those were taken in red and green (my artificial star can do RGB). At the end I took two images with the 35mm lens, one with the diaphragm fully open (left) and one with it nearly closed (right):

The exposure times differed between them to compensate for the stop. I focused as well as I could on the blue label (the artificial star is in front of it, you may notice the green light from it).

Obviously there is a lot of difference in sharpness and blur, especially noticeable around the fibre-optic cable.

So questions for the test:

- artificial star collimated or not?;
- diaphragm open or closed?;
- which colour(s);
- through-the-centre shots only or also corner-shots?;
- how to focus (corner shots same focus as centre shot, or re-focused)?

Here is an enhanced image of one of the corner-shots in green and focused at the centre of the FOV (upper-right corner, magnified 400%):

As you can see there is a D-shaped artefact around the light source.

Nicolàs

Edited December 14, 2020 by inFINNity Deck
Added explanation which half of the image was taken with open and closed diaphragm.

[inFINNity Deck]

Here is a centre-shot in green, magnified 400%:

With a 2.9 micron pixel-size this is not too bad, the light only covers maximum of 3 pixels.

Nicolàs

[vlaiv]

1 hour ago, inFINNity Deck said:

yes, can do that, but what do you want me to do exactly?

Thank you very much for these recordings.

I was interested in general performance of the lens. I'll describe my idea so we can figure out if these lenses will be suitable for it, although I believe it will ultimately come down to testing it in real conditions.

Idea is to do EEVA with Mak102. That is of course not the best telescope being F/13 but I wanted to try out this approach that will effectively make it F/4-F/5 telescope. In order to accomplish this, I thought of putting together following setup:

Telescope - 32mm Plossl - eyepiece projection adapter - 11-12mm CS lens - ASI178mm camera.

Here is rough ray diagram of what I want to achieve:

Here is my reasoning for this:

- 32mm Plossl has field stop of about 26-27mm and above telescope should illuminate that much field. It will be somewhat vignetted at extreme edges.

- Exit pupil of such system is 32 / 13 = ~2.47mm

- I use 12mm CS lens. If I'm not mistaken that will give me reduction factor of 32/12 = ~2.667. I will "compress" 27mm illuminated circle into ~10.125mm. ASI178mm has diagonal of 8.93 so it won't capture whole field - but it will be vignetted anyway.

- Lens will operate stopped down at about 12mm / 2.47 = F/4.85 - so it should be fairly sharpish at that setting.

- Compound system will operate at F/4.875 (13/2.66666) with focal length of about 487.5mm and sampling rate of 2"/px once super pixel debayering is used - rather nice resolution for EEVA.

In any case - I just wanted to know if stars will look round or not with such lens. You test suggests that center stars will look nice but corner ones might be distorted quite a bit.

[inFINNity Deck]

Hi Vladimir,

I have tried that kind of set-up with my 60mm Galilean Type Telescope (GTT60). I did that with the ZWO ASI290MC in combination with the ZWO 2.8-12mm zoom-lens behind a plano-concave eyepiece:

As you can see the result was a bit disappointing, although it is exactly capturing how the FOV of the GTT60 is experienced.

Using the 25mm or 35mm lens I would expect magnifications like in the Bath-interferometer, but would need to test that once the skies clear again (if they ever...).

Nicolàs

[vlaiv]

12 minutes ago, inFINNity Deck said:

As you can see the result was a bit disappointing, although it is exactly capturing how the FOV of the GTT60 is experienced.

I would not call that disappointing at all!

That looks rather good for 30mm of aperture.

[inFINNity Deck]

Glad you like it! 🙂

Indeed the image was taken with the telescope being stopped down to 30mm. The thing I found disappointing about the image is the pixel scale. I was hoping to get a larger image of the planets (I imaged Jupiter as well), so this 23px disc was smaller than anticipated. If the 35mm objective works as above, that would give me a disc of about 100px, which would be much better.

This is the contraption I made to do the afocal imaging:

The bracket is fastened to a 2" to 1.25" adapter that slides into the focuser. The aluminium plate at the right has a central hole in which a 1/4" screw holds the camera. The spring-loaded screws in that plate press the camera gently against the eyepiece.

Nicolàs

[vlaiv]

3 hours ago, inFINNity Deck said:

The spring-loaded screws in that plate press the camera gently against the eyepiece.

Have you tried with Barlow lens? It is also negative lens element and they usually have quite longer FL - like 100mm or so.

[inFINNity Deck]

No, I did not as I wanted to capture the view of the GTT60 as an early 17th century observer would have seen, so without additional glass between objective and eyepiece. Now what is not visible in that image is how small the FOV is, but that becomes clear from this daytime image of a nearby (3km) church tower:

The grey halo is the inside of the tube, so basically that already can be chopped off using a longer focal-length lens in the afocal imaging. Of course I could use a Barlow, but I expect to get a different view (perhaps I would need to try that first 🤔).

Nicolàs

 

[vlaiv]

10 minutes ago, inFINNity Deck said:

No, I did not as I wanted to capture the view of the GTT60 as an early 17th century observer would have seen

Ok, get it - this was more experiment to that effect rather than optical principles.

[inFINNity Deck]

This evening we had a clear period which allowed me to test the two lenses under the sky. To start with I took two images per lens, one with the diaphragm fully opened, and one with it almost closed.

First the 35mm lens with fully open diaphragm (exposure time 1.5s):

Ouch, that does not look too well, quite a bit of halo around all objects!

Then with its diaphragm closed (30s exposure):

That looks much better, halos are gone! According to Astronomy.net the image has the following properties:

Size:    9.05 x 5.12 deg
Radius:    5.198 deg
Pixel scale:    16.8 arcsec/pixel

http://nova.astrometry.net/user_images/4230332#annotated

 

Then the 25mm fully opened (1.5s exposure):

Not bad at all, appears to be a better lens.

Then the 25mm with its diaphragm closed (40s exposure):

Also not bad. From these simple tests I would rate the 25mm the better of the two. The Astrometry.net results:

Size:    12.1 x 6.85 deg
Radius:    6.950 deg
Pixel scale:    22.5 arcsec/pixel

http://nova.astrometry.net/user_images/4230428#annotated

 

I also used the 35mm with my GTT60 to see if afocal imaging increased the pixel scale:

Saturn is now shown at about 72px diameter over it rings, so indeed quite an improvement (some 300%). Please note that this image is taken through my GTT60, a 60mm singlet (plano convex) objective, stopped down to 30mm, and a singlet (plano concave) eyepiece and that both were made to mimic 17th century standards.

Nicolàs

 

Edited December 16, 2020 by inFINNity Deck