Collimating lens and behavior within different focal ratio light cones?

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Collimating lens and behavior within different focal ratio light cones?

Post by MalVeauX »

Hi all,

Before I move onto a new project, since I know less about many optics, here's some maybe simple questions that I'm interested in confirming or getting more info on if anyone can indulge some time.

1) If a system is using collimating lenses and the lenses are designed for F7 and the system it is installed into is a faster F6 system, is it safe to assume that some of the OTA's resolution is lost, because it is effectively aperture masked by the masking of the light cone with the slower collimating lens?

2) If a system is using collimating lenses and the lenses are designed for F7 and the system it is installed into is F8, is it safe to assume that no resolution is lost since the light cone is not masked, and instead, it's simply losing some light/transmission and that's it basically?

Thanks!

Very best,


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Re: Collimating lens and behavior within different focal ratio light cones?

Post by marktownley »

Hi Marty,

I both cases, yes, and also the resultant beam will not be perfectly collimated, which may or may not be an issue.

Mark


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Re: Collimating lens and behavior within different focal ratio light cones?

Post by AndiesHandyHandies »

Hi

You can replace the two lenses with ones for focal lengths where available. For my F7.5 refractor I got a pair of 25mm +150mm and -150mm plano convex lenses so I can use the full aperture. Multiply the F No by 20. F10 x 20 = 200mm.

In between just put up with a bit of vignetting.

RodAstro made me a nosepiece for my F10 PST Mod to move the collimating lens 200mm inside focus, 80mm infront of the Etalon, use extension tubes as required, including the telescope back focus of 120mm to get to the majic -200mm. Used a couple of old barlow nosepieces, the front one was the collimating lens size. For the F7.5 I found a Astroboot barlow was 25mm so I can swap it for the -200mm nosepiece. They do two models the other has a different sized lens.

A warning about barlows. They are designed to work at 100mm inside of focus normally. If you put one 200mm inside infront of a PST collimating lens it doubles the magnification. RodAstro did some star drift tests to confirm this. I bodged a 2x Barlow in my F7.5, slid the nosepiece up the focuser, remember to catch it when you take the PST mod out!!!, and it worked nicely at F28, looked as good as Pedros images.

Cheers. Andrew.


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Re: Collimating lens and behavior within different focal ratio light cones?

Post by AndiesHandyHandies »

Hi

Also the collimating lenses fit in the PLASTIC Celestron/Sywatcher moon filters. Camarthen Cameras sells them as the picture!

I have the re-focus lens in one. So I have a CPL bodged in front of it, two filter rings slipped in the back PST adapter, to reduce reflections in the etalon phase space.

Cheers. Andrew.


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Re: Collimating lens and behavior within different focal ratio light cones?

Post by marktownley »

Thanks for the heads up on Camarthen Cameras Andrew, that's a proper camera shop! My wallet is twitching already :lol:


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Re: Collimating lens and behavior within different focal ratio light cones?

Post by christian viladrich »

Hello,
A quick note there, the optical beam is still collimated whatever the f-ratio of the collimating lens.
Collimation of the beam results from the fact that both the focus of the refractor and of the collimating lens coincide.


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Re: Collimating lens and behavior within different focal ratio light cones?

Post by AndiesHandyHandies »

Hi

But the collimating lens needs to be right to use the objectives full aperture.

Cheers. Andrew.


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Re: Collimating lens and behavior within different focal ratio light cones?

Post by MalVeauX »

Thanks all,

I have a successful PST mod as it is, and I have made it modular with no scope destruction by extending the collimating lens forward and it works well in that way. I'm just looking to now do something similar with another option, a bigger etalon, but it likely doesn't use F10 collimating lenses, it likely uses something else. So I'm just preparing for what may be ahead. That said, I at least have some experience with configuring this. I just am trying to get a concrete idea on what is happening so I don't just hack something together and get junk results because I'm ignorant.

I hope to soon have a new filter to test things on for modding. A Lunt pressure tuned internal filter with 35mm clear aperture and I assume collimating lenses but I'm not positive what focal-ratio they are designed around (I assume F7 because it's based on the internal 60mm Lunt modular scope and the native scope is 70mm F6 and the etalon makes it F7 so the aperture is being masked at the light cone either by a barrier or by the collimating lenses being used). As soon as I get it, I will share whatever is inside.

I'm wondering if it will be possible to mount it in an F8 light cone scope. Or if it must be F7 without bad things happening. I'm hoping to try to configure it to work mounted on my 150mm F8 lens system and then be able to simply mask the aperture as needed (which will increase the focal-ratio, drop resolution potential, but shouldn't cause problems other than light loss right?). So if I operate it at 120mm F10 for example, it lowers the resolution and loses light, but doesn't harm the operation of the etalon or collimation correct?

I'm curious how it works with the F8 light cone though. If I put the F7 collimating lenses in an F8 light cone, what happens to the resolution since it's not being masked? Is it altered? Or is it just loss of light/transmission and that's it? Collimation issues?

Very best,


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Re: Collimating lens and behavior within different focal ratio light cones?

Post by Bob Yoesle »

I am by no means an optical expert, but I am not sure that the longer f ratio of the undersized collimator acts to reduce resolution. As Christian points out, as long as the focal points of the objective and collimator are coincident, the output will be a collimated bundle. Depending on whether the collimator is a positive or negative lens, this is in effect a simple astronomical or the Galilean telescope. Using a positive lens for clarity:
Image1.jpg
Image1.jpg (207.54 KiB) Viewed 5026 times
The collimator as an eyepiece, same FL, different diameter & F ratios.

Does the design of an eyepiece affect a telescope's resolution? If we consider the collimator lens an eyepiece, would a smaller diameter longer f ratio eyepiece reduce the objective's resolution? I have not heard of this. It appears it may vignette the image or otherwise reduce image brightness. Conversely, would a larger shorter f ratio eyepiece with the identical focal length increase the resolution in comparison? Again I have not heard of this variation causing a change in resolution, but it might result in a waste of available light which is unable to enter the eye.

As I'm not an expert, I hope someone with more knowledge might shed some additional light on the subject ;-)
Last edited by Bob Yoesle on Sun May 23, 2021 2:17 am, edited 3 times in total.


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Re: Collimating lens and behavior within different focal ratio light cones?

Post by MalVeauX »

Thanks,

Yea, that's my thought as well, it seems to make sense, but I'm not knowledgeable enough to be confident on that hunch which is why I'm asking. Even if resolution isn't altered, I'm curious what other issues there may be in the configuration that I'm not aware of. Or, is it safe to say for example that any of these collimator lens based systems are pretty much ok to use in slower focal-ratio systems without worrying about anything other than transmission/light loss basically. If that's the case, that's a beautifully simple way forward. But, like all things, I expect no free lunch so figured I'd ask!

Very best,


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Re: Collimating lens and behavior within different focal ratio light cones?

Post by MapleRidge »

Marty...

The Lunt DSII Etalon I use in my mods is straight out of the box...I haven't changed any of the lenses in it, so what you see for my images from the mod is what it produces on a f8 OTA (DSII built for the 80mm, f7 optics of the LS80T).

The sweet spot is well short of getting full disk, but produces a FOV good enough for my use on AR's and smaller scale features.

Brian
Last edited by MapleRidge on Sun May 23, 2021 12:49 pm, edited 1 time in total.


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Re: Collimating lens and behavior within different focal ratio light cones?

Post by AndiesHandyHandies »

Hi

If you put in a collimating lens designed for a longer F No PST mod it vignettes the objective but the image plane is fully illuminated and the image scale is the same. So the image is dimmer than the right collimating lens.
As the collimating lens at its F L inside the objectives focus is smaller than the light cone where its positioned.

If you put in a collimating lens designed for a shorter F No PST mod you have a fully illuminated image and the image scale is smaller. So the image is brighter than the right collimating lens.
As the collimating lens at its F L inside the objectives focus is larger than the light cone.

Times the F No of the objective by the size of the collimating lenses effective aperture to get the right F L for the collimating lens.

F 10 x 20mm = 200mm, the majic distance. F 7.5 x 20mm = 150mm. You are limited by available 25mm plano-convex standard lenses in practice unless you have a custom one made.

Cheers. Andrew.
Last edited by AndiesHandyHandies on Tue May 25, 2021 6:51 pm, edited 1 time in total.


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Re: Collimating lens and behavior within different focal ratio light cones?

Post by MalVeauX »

MapleRidge wrote: Sun May 23, 2021 1:11 am Marty...

The Lunt DSII Etalon I use in my mods is straight out of the box...I haven't changed any of the lenses in it, so what you see for my images from the mod is what it produces on a f8 OTA (DSII built for the 80mm, f7 optics of the LS80T).

The sweet spot is well short of getting full disk, but produces a FOV good enough for my use on AR's and smaller scale features.

Brian
Thanks Brian,

That's the overall idea; something completely modular that can be dropped into any scope basically that is suitable. I'd love to not have to do any destructive mods and simply have some machined adapters or 3D printed things made to make this work.

Do you mind sharing some info how you adapted your DSII etalon to mount in your F8 scope?

Very best,


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Re: Collimating lens and behavior within different focal ratio light cones?

Post by MapleRidge »

Hi Marty...

Like you, I was trying to keep the system 'modular' as best I could. My Celestron 6"f8 OTA works with an adapter to match the DSII module and the scope didn't need to be shortened (but its at the limit for in focus now that I have had to add a few extra adapter to made the double stack work.

I'll try to post some pics...Its been on my list of things to do after several requests for others here. Pardon my tardiness ;)

Brian


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Re: Collimating lens and behavior within different focal ratio light cones?

Post by MalVeauX »

MapleRidge wrote: Sun May 23, 2021 3:59 pm Hi Marty...

Like you, I was trying to keep the system 'modular' as best I could. My Celestron 6"f8 OTA works with an adapter to match the DSII module and the scope didn't need to be shortened (but its at the limit for in focus now that I have had to add a few extra adapter to made the double stack work.

I'll try to post some pics...Its been on my list of things to do after several requests for others here. Pardon my tardiness ;)

Brian
Thanks,

Any help on that adapter or specs or schematic for it would be awesome!

Very best,


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Re: Collimating lens and behavior within different focal ratio light cones?

Post by torsinadoc »

Timely post. I have been considering a similar set up on my Celestron 150/1200. I will follow with interest


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Re: Collimating lens and behavior within different focal ratio light cones?

Post by Bob Yoesle »

If you put in a collimating lens designed for a longer F No PST mod it vignettes the objective but the image plane is fully illuminated and the image scale is the same. So the image is dimmer than the right collimating lens.

As the collimating lens at its F L inside the objectives focus is smaller than the light cone where its positioned.

If you put in a collimating lens designed for a shorter F No PST mod you have a fully illuminated image and the image scale is smaller. So the image is brighter than the right collimating lens.

As the collimating lens at its F L inside the objectives focus is larger than the light cone.

Times the F No of the objective by the size of the collimating lenses effective aperture to get the right F L for the collimating lens.

F 10 x 20mm = 200mm, the majic distance. F 7.5 x 20mm = 150mm. You are limited by available 25mm plano-convex standard lenses in practice unless you have a custom one made.

Bobs diagram is not relevant here at all.
Hi Andy,

How exactly is the diagram not relevant?

First, is shows exactly what you have described. Second, I'm not describing how to size and locate a collimating lens, which is not the issue raised by the OP. Moreover, for two collimators of differing f ratios but identical focal lengths you would have the exact same "image scale" (field angle magnification) for a given objective diameter, and therefore equal brightness, although vignetting can obviously occur as shown and described.

The relevant issue I'm referring to the OP's question regarding loss of resolution r/t the diameter of the collimator doing exactly what I showed and you describe. The issue appears to be mostly dealing with pupils and stops.

Obviously you can choose COTS lenses or have lenses made to a specification. However, in the OP's case, and many if not most others, the size and FL of the collimator is dictated by the collimator/refocusing lens assembly - in this case a Lunt PT etalon module - and why the OP asked his original question r/t resolution.

The other issue that generally takes place for these mods is the significant (and frequently overlooked) reduction of the Jacquinot spot diameter due to field angle magnification, which may or may not be relevant for a specific purpose - but that's another issue ;-)


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Re: Collimating lens and behavior within different focal ratio light cones?

Post by Rusted »

Did I miss the point where somebody mentioned using the Baader 1.125 GPC ahead of the etalon in a 6" f/8? :)


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Re: Collimating lens and behavior within different focal ratio light cones?

Post by MalVeauX »

Rusted wrote: Sat Jun 05, 2021 12:25 pm Did I miss the point where somebody mentioned using the Baader 1.125 GPC ahead of the etalon in a 6" f/8? :)
I've done this; I still have it. But ultimately I'm not sure how its performing or what the actual magnification is, depending on placement. Conventional wisdom seems to be to place it right before the collimating lens, practically on top of it. More important for someone wanting to take F8 to F10(ish) for a PST collimating lens (F10). But it's not needed for things that are slower than the collimating lenses (like F8 scope and F7 collimating lenses).

I kind of want to image with and without the 1.25x GPC and pixel measure to see the true image scale change of a static feature and determine what actual magnification is occurring at what placement. But ultimately I'd rather not use the GPC at all if I can help it. I much prefer simply optical trains (much easier to trouble shoot).

Very best,


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