Collimating a catadioptric lens?

[AstroViking]

Hi all,

I picked up a catadioptric lens the other week that appears to need collimation. I tried some astro with it and all the stars came out a sort of 'V' shape, mostly seeming to point into the centre of the frame. (The vignetting is also terrible, and uneven, leading me to suspect it's been dropped or abused over it's life.)

The lens itself has zero information that I can find on the Internet. The closest to a brand or maker's name is the text "Reflex BirdScope 1000" and "Made in Japan" on the body.

It has a standard M42*0.75 thread on the back, so I can fit my astro cam to it.

Useful information:
* Maker: Unknown. Body stamped with "Reflex Birdscope 1000"
* Year of manufacture: Unknown
* Country of manufacture: Japan
* Focal length: 1000mm
* Aperture diameter: 104mm
* Aperture: F11 (fixed)
* Focus: Manual
* Weight: 1516g
* Length: 195mm (dew shield retracted), 245mm (dew shield extended)

If you know anything about this beat, I'd love to hear it.

Looking at the sample output, aside from the awful noise and poor focus (both operator induced), is there hope for this thing as an astro-lens? Or will it be relegated to terrestrial imaging?

If it can be salvaged, where can I get it collimated?

Many thanks,
V.

[By.Jove]

That is coma, and "collimating" wont fix it. Don't waste your money on this, move it on and buy a better lens.

The lack of a decent name from the film era (ie Pentax/Takumar, Nixon, Canon) should be a clear warning it's a "yum-cha" cheapie with poor performance.

Another thing to bear in mind is that the resolution of most 35mm films was around 20 microns. Lens performance (spot sizes) aimed to match that. But 30 years later in this age, the pixel spacings of modern sensors are 2-5 microns - and a lens with a 20 micron spot size will appear very soft on a modern camera.

[JA]

Hi Steve,

The triangular shapes in the stars, whilst possibly due to some coma, are also indicative of astigmatism and other aberrations related to "pinched" optics, where the optics, the mirror in particular, is too tightly held or clamped in position to the extent that it slightly distorts the mirror's shape and results in the aberration. Relaxing the retention of the mirror should improve things.

One of our members here, Bojan (Hi Bojan) has done this to one of his Russian Mirror lenses. As I recall there is a thread on the process, which should steer you in the right direction

Best
JA

[bojan]

Hi JA,

Yep, you are right, actually I did two of my Cats (also tried the third one (Rubinar500/8), but no improvement after process), with success (MTO 1000A and MC Rubinar 10/1000).
The thread is here (https://www.iceinspace.com.au/forum/showthread.php?t=108772).
In the first post there is a link which inspired the whole process (https://www.strickling.net/russentonne.htm).


Steve,
Your image definitely looks like a bad case of "pinched optics" (almost always main mirror is affected).
I found that the mounting of lens body (well.. this is a scope now) has effect too.

Too tight bracket around the back side of my MTO resulted in images like on this link (https://www.iceinspace.com.au/forum/showpost.php?p=358341&postcount=11) .


All in all, the main mirror should NOT be tightened at all.. just lightly touched by retaining mechanism, just enough not to rattle inside.

I also added 3 kapton tape seats (procedure described in one of links on Dr Strickland's webpage).

[AstroViking]

Jove, JA and Bojan,

Thanks very much for your replies! (Being a relative newcomer to this hobby, and only having a refractor 'scope, my apologies for using all the wrong terminology. These reflecting telescopes are all dark magic to me at the moment..)

This is what I love about this forum - so many talented people happy to share their knowledge and experience. :thumbsup:

Jove - it was a crazy impulse buy that may cause me to regret it. We'll see. If I can't get the optics de-coma'd then it will be relegated to terrestrial imaging when I need a super-long focal length. (My partner loves whale watching, so I can see it being used for that.)

Bojan - thanks for the links to your thread about your Russian lenses. I'm off to read them now and see how well they translate to my "Gyoza Cannon" lens. (Well, it's Japanese, and Yum-Cha is Chinese, so I guess a Japanese dumpling is close enough for government work....)

I don't think the tube is being pinched because it uses a tripod foot which originally had a single 1/4-20 thread in it. I drilled and tapped a second 1/4-20 thread in it and then used two bolts to hold it to a dovetail. I didn't want it to suddenly pivot on that single bolt!

I had a look around for tube rings, but didn't find anything that was the right size. Also, there's no easy way to use two rings on it - the entire lens body (aside from the part with the lens collar on it) moves forward / backward as the focus changes.

Anyway, I'll have a poke around and see what I can find.

Thanks again for the information!

Cheers,
V

[bojan]

Steve,
At the end, you may find your lens is not good (enough).. but I would definitely try to see if mirror is pinched.

I have similar (not so good) experience with this one (Samyang, Opteka, Whatever) (https://www.iceinspace.com.au/forum/showthread.php?t=130976&highlight=500mm+f6.3).. It has coma in the corners. I know there are good specimens out there (from searching Astrobin), but mine is like that and I will be using it for guiding..
Also I intend to use it for Sun eclipse in 2028 (provided I'll still be alive and kicking).


EDIT:
One more thing - I am using Canon cameras with APS sensor... so if your Nikon is full frame.. it will have worse results in corners.

[Stefan Buda]

Here is a 21 minute video worth watching for anyone contemplating the use catadioptric lenses for astrophotography: https://www.youtube.com/watch?v=x2BiM7BGQMU&ab_channel=HuygensOptics

[bojan]

Very instructive video(s), Stefan. Thank you for sharing :thumbsup:

[AstroViking]

Bojan - my Nikon has an APS-C sensor, so I expect some sort of distortion in the corners. I'll have a look at the lens and see if I can take it apart.

Stefan - thanks for the link. I'll watch it later today.

Cheers,
V

[AstroViking]

Hi all,

Well, I think I found the source of the coma in my lens, and it is NOT repairable.

The primary mirror is not pinched - I was able to remove the back cover and rotate the mirror in it's mounting with my fingers. There is some eccentricity in the lens body, as the mirror would rotate a few degrees and then 'stick', and could be moved laterally a tiny amount when not in the 'stuck' position. I left it in the 'stuck' position because I didn't want the mirror moving around.

I was not able to remove the mirror because the alloy locking ring appeared to be glued in place. Nor do I have a suitable tool to rotate the ring, either.

Somewhere in it's life the lens has been dropped. I saw that the tripod foot is bent slightly. When looking at the lens yesterday I saw that the front of the body (just outside of the front glass) is deformed by a millimeter or so. There is no way I can straighten that deformation. (See attached image.)

So it looks like the lens is going to be either sold, or relegated to terrestrial imaging (and even then I have some doubts about the quality of the images I'll get from it. :()

I'm disappointed, obviously. So let's see what good I can get out of this situation...

Cheers,
V

[bojan]

It is possible with a proper tool, similar to the one on the on the image below.
I made it specifically for a specific job.
As for mirror.. do you have picture showing some details?
Try to soften the glue with a drop of isopropil or acetone. But, if it moves.. that is that, no room for improvement here.

[JA]

Hi Bojan,

That's a very nice special tool for the job.:thumbsup:

I have another design that I was going to use to repair a similar dent on a larger lens. (see diagram). Basically it's a circular disc with a segment removed (Shown in RED). To work well it would need to be sized to just under the diameter of the internal filter thread of the lens and be radiused at the "corners" (intersection of the chord and the circumference of the circle.) in order to ease the dent back in to shape. It might be best if the disc radii/corners were transitioned over a much longer length to ease the dent back in to shape. It is rotated using a spanner or such from some feature either cut in or welded to the repair disc - it should gradually render the dent back to smooth with some care and some lubricant on the dent.

The downside on anything like this is that it would need to be custom made for the size, but may be worth it to effect a repair on a damaged lens. There also might be a concern with some damage to the crest of the thread so some experimentation/care/rework may be required.

Another possibility if it's a thin lens rim that's damaged, might be a clamp with some appropriately cut radiused blocks: one on the inside of the filter thread the other on the smooth outer surface of the lens.

Best
JA

[bojan]

Hi JA,

yes, the idea should work.. it should be OK for smaller dents.. just as mine.
Now you gave me a prompt to make a tool suitable for more serious damage (deeper dents.. I have Canon 300 mm lens with similar damage), I will make my tool to be adjustable (one of the bearings should be mounted on movable plate, to adjust for diameter of the damaged tube). Also, instead of just middle bearing, I think I should have a ferrule with grooves to accommodate and repair the existing thread on the lens.

[JA]

Sounds like a great idea Bojan. It just depends on the sort of forces you need to generate, for instance if it is a reasonably thick lens edge it would need to be quite robust and remain anchored to the lens without potential slip, which might result in other damage.


Best
JA

[bojan]

Yes,
Something like this below...
I think that ferrule with groves might help to keep the tool straight. Also additional plate with screws (not shown)

[bojan]

Closer to final design... radius/thickness of the tube adjusted by screw, the lower bearing is movable.

[AstroViking]

Bojan and JA, what a great discussion, and such cool ideas for fixing damaged lens bodies.

I'm undecided where to go from here. As Jove said, the optics of the lens are going to be pretty poor when compared to modern glass, and with the damaged body (which I doubt I could repair) there will always be optical distortions.

Just thinking about it, if you need a 'crash test dummy' for your magic tool, Bojan, I don't mind offering this lens for you to test it on.

Cheers,
V

[bojan]

Hi Steve,
As to making the tool.. it will take some time :-)
I will let you know when I am ready, after I test it on my lenses.

[bojan]

BTW.. See here (https://www.pentaxforums.com/forums/189-repairs-warranty-service/391531-how-straighten-dented-front-rings-lenses.html) (video at 2/3 down the page, very similar to JA's idea., but adjustable).
It was available on Amazon (https://www.amazon.com/DSLRKIT-Repair-Filter-Ajustment-Steel/dp/B00SYBYTZO).
So, my tool may look like this on attached pic.

[AstroViking]

Wow. I never knew such tools existed. I watched the video, and it's impressive that he was able to recover the lens as well as he did.

[bojan]

Steve, can you advise the filter thread pitch, I would like to try to make the collar for it.

[AstroViking]

Hi Bojan,

The thread pitch is 1mm. Internal diameter of the lens is 104mm, if that makes any difference.

Cheers,
V.

[bojan]

Yep, it seems 1 mm is standard.

I think I need to make the collar from something harder, aluminium is a bit soft.. but it may work.
We can try it over next week (https://www.iceinspace.com.au/forum/showthread.php?p=1597309#post159730 9).. if you have the courage :-)

[AstroViking]

Sounds like a plan to me.

One thing I just realised - the collar with the grooves will need to taller than the screw-head, to fully reach all the filter threads in the lens. There is only about 5mm or so of space in front of the lens. I hope that makes sense...

Cheers,
V

[bojan]

It does, very much so.
Maybe I will have to trim the screw head that holds the roller/collar a bit to prevent a damage to glass.
Can you make a rough cross-section drawing (or better image) of the front glass assembly with some measurements?

[AstroViking]

Hi Bojan,

Attached are some images I just took. If you need any more, or I've missed something then don't hesitate to ask.

First off, the front of the lens to show the diameter of the central obstruction and how much glass there is around it. (It doesn't look that dirty usually! I presume it's the light highlighting all the dust.)

Second, the depth of the primary lens from the front of the body. It's actually about 10mm, not the 5mm I mentioned before.

Lastly, the outside of the body, showing the 5mm gap between the end of the lens body and the fully retracted dew shield / light guard.

I had to scale them down a long way to fit under the file size restriction here. Let me know if you need bigger images and I'll get them to you.

Cheers,
V

[bojan]

Steve,
It seems the damaged tube is pretty thick, 2 mm ?
Damage is visible as marked?

[AstroViking]

It's a very hefty contraption, that's for certain. I never thought to measure how thick it was, so I'll agree that it looks about 2mm thick from the photos.

As for the damage, you can see it in my post in page 1 of this thread. The top-down view I gave doesn't really show it clearly.

Regards,
Steve

[AstroViking]

Just a quick update - Bojan's had a look at the lens and his magic tool isn't able to straighten the thread. When the lens got dropped, the impact both bent the threaded area and squashed the threads, so rolling the dent out will not restore the thread.

My option now is to perform surgery on the lens to remove all the internals and then have the lens body machined down a bit to remove the damaged section.

This will also give me the opportunity to see if any of the internals are out of position and hopefully restore the optical qualities of the glass and mirrors.

This is another job on the 'To Do' list...

Cheers,
V.