54 replies to this topic

[sergiomo]

Hi everyone,

 

I wanted to share with you all that I was able to attach an Olympus TCON-17X teleconverter to an S50, thanks to the Olympus CLA-13 teleconverter adapter. The whole setup fits together as well as you could hope... for equipment that wasn't designed to go together .

 

The teleconverter fits pretty snug and although it's front-heavy, it didn't detach even when the Seestar was pointed straight forward and I tapped on it semi-gently. It can definitely be dislodged without too much force so for extra safety I used an elastic tool tether to help add tension and keep it place. In theory, when using a full-size tripod, the tether would also prevent the teleconverter from hitting the ground if it ever falls off.

 

As far as I can tell, the adapter isn't touching the Seestar's front glass so I don't think there's much risk of damaging the telescope. As you can see, the lip of the adapter is thinner than the lip of the included solar filter.

 

I'm attaching a picture of Jupiter I took with this setup. It's not great, but it's a lot less pixelated than without the teleconverter. The photo is a stack of a ~6 minute video, taken with these settings: Focus = 1756, Exposure = 2 ms, and Gain = 17

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Edited by sergiomo, 14 February 2025 - 12:39 AM.

[sergiomo]

Adding a bit more info: here's a couple of reference photos. One was taken without the teleconverter, and one with it so you can see the magnification.

 

[Slimtannor]

i have that lens. its really heavy. i don't think you would want to dot a lot of slewing with it in place. Might mess up the gears, i don't know. You could slew and then place the lens on i suppose. Anyway adds some new dimension to digiscoping.

[Psion]

Great, thank you!

[CraigR]

The 1.7 increase in focal length (250mm to 425mm) changes the focal ratio to f/8.5. This will have a significant impact on how much light you'll be able to collect from DSOs. Planets and lunar might be your best bet.

 

But perhaps more interestingly, the bare Seestar is already at its Dawes limit. The pixel resolution is 2.39 arcseconds per pixel, and the Dawes limit is 2.32. With the teleconverter, you're at 1.41 arcseconds per pixel, so you're oversampling by quite a bit. What that basically means is that you're not seeing any more detail than you see without the teleconverter; you're seeing the same amount of detail but bigger. You'd get the same effect by loading an image from the bare S50 into your image processing software and up-scaling by 1.7.

 

So... interesting exercise and interesting that it works.

[Psion]

Maybe you've missed one important parameter in your reasoning. The diameter of the optics of the teleconverter is not 50 mm but I think 70 mm.

Edited by Psion, 14 February 2025 - 01:34 PM.

[CraigR]

The teleconverter doesn't change the aperture of the telescope; just the effective focal length. You could have a 1.7x teleconverter with a 200mm aperture, but the objective lens of the Seestar is still 50mm. The teleconverter increases the focal length, thus reducing the field of view and reducing the image scale from 2.39 arcseconds per pixel to 1.41 arcseconds per pixel. Since this is below the Dawes limit of the S50, you're not gaining detail (in principle), just making the image larger. And since you're increasing the focal length, you're increasing the focal ratio (from f/5 to f/8.5) and making the telescope slower.

 

I should also clarify that I'm assuming "stargazing" or "solar system" mode. For land-based scenery, you should gain magnification  as  you have noted. And the change from f/5 to f/8.5 doesn't matter as much in that situation because you're probably already reducing gain because you have so much light.

Edited by CraigR, 14 February 2025 - 04:37 PM.

[Psion]

I believe you might be mistaken. The way the teleconverter works on the Olympus camera is that it doesn't change the F ratio, so it has to change the lens aperture. It magnifies the image 1.7x on the sensor, and that would have to change the F ratio, but it doesn't. With the Seestar, it will be exactly the same.

[RedLionNJ]

While the larger 70mm aperture isn't going to buy very much in the way of improved resolution, it's important to remember the Dawes limit is not very relevant when it comes to stacking multiple images (or frames) together to produce a final image. This really matters when stacking hundreds or thousands of video frames together when imaging the planets, sun or moon. The final image resolution is going to be considerably finer than the Dawes limit expected for whatever size aperture - typically around 1/3 of that value when seeing permits.  So in the case of the 70mm teleconverter, we could potentially get down to around 0.55 arcsec in a planetary scenario.

[Peter Besenbruch]

The teleconverter doesn't change the aperture of the telescope; just the effective focal length.

This one does increase the aperture, as it is applied to the front of the telescope or camera. I used to own one for an Olympus point and shoot film camera. With it on or off the exposure remained the same for the camera. This was about the only tele-converter that I liked. I never had much use for the more common, behind the lens variety.

[CraigR]

I believe you might be mistaken. The way the teleconverter works on the Olympus camera is that it doesn't change the F ratio, so it has to change the lens aperture. It magnifies the image 1.7x on the sensor, and that would have to change the F ratio, but it doesn't. With the Seestar, it will be exactly the same.

A front-mounted teleconverter only magnifies the image and increases focal length; it does not collect more light than the objective lens allows. This is why the f-ratio increases from f/5 to f/8.5, and the aperture remains at 50mm. Cameras and telescopes/astrocams work differently. Unlike a camera lens with a variable aperture, the Seestar has no way to compensate for this optically or electronically. The teleconverter does not create a larger light cone, just a magnified one.

 

In the end, the proof of the pudding is in the eating. If you can shoot planets and get more detail, and if you can shoot dim emission nebulae and get the same results in less time than you were getting without the teleconverter, then I would say you found a nice trick and I would stick with it.

[Peter Besenbruch]

A front-mounted teleconverter only magnifies the image and increases focal length; it does not collect more light than the objective lens allows. This is why the f-ratio increases from f/5 to f/8.5, and the aperture remains at 50mm. Cameras and telescopes/astrocams work differently. Unlike a camera lens with a variable aperture, the Seestar has no way to compensate for this optically or electronically. The teleconverter does not create a larger light cone, just a magnified one.
 

In the end, the proof of the pudding is in the eating.

A front mounted tele-converter will increase focal length and aperture if the aperture of the front lens is greater than the original camera lens. The Olympus converters have a wider front lens, and when used on the cameras they were designed for, there is no change in exposure settings. How much larger than the Seestar's 50mm lens, this converter is I don't know. Olympus sold two sizes, once for each size of a point and shoot camera. I owned both cameras (the smaller and the larger), and both converters.

 

As for the pudding, the Seestar should be able to capture extended objects at the same rate of speed if the converter is properly matched. The wider opening should increase resolution and reduce the field of view.

[Psion]

This one does increase the aperture, as it is applied to the front of the telescope or camera. I used to own one for an Olympus point and shoot film camera. With it on or off the exposure remained the same for the camera. This was about the only tele-converter that I liked. I never had much use for the more common, behind the lens variety.

Exactly. If the teleconverter does not reduce the aperture ratio of the lens it is attached to, then the only possible explanation is that the diameter of the lens has been increased. Once the lens diameter is increased, new rules apply—resulting in a better Dawes limit and a larger surface area for gathering light. This design works on the principle that as the magnification of the teleconverter increases, the entrance pupil diameter also increases, thereby maintaining the same ratio between focal length and aperture (i.e., the f-number).

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Edited by Psion, 15 February 2025 - 01:02 AM.

[CraigR]

A front mounted tele-converter will increase focal length and aperture if the aperture of the front lens is greater than the original camera lens. The Olympus converters have a wider front lens, and when used on the cameras they were designed for, there is no change in exposure settings. How much larger than the Seestar's 50mm lens, this converter is I don't know. Olympus sold two sizes, once for each size of a point and shoot camera. I owned both cameras (the smaller and the larger), and both converters.

 

As for the pudding, the Seestar should be able to capture extended objects at the same rate of speed if the converter is properly matched. The wider opening should increase resolution and reduce the field of view.

The increased focal length is what reduces the field of view. Aperture doesn't affect field of view. FOV = (206.235 / SS) / FL, where SS is sensor size and FL is focal length.

 

When used on a camera, the camera makes adjustments to its iris aperture to accommodate the change in focal length. That's why you don't see a change in exposure times on a camera. The Seestar has no way of adjusting for the teleconverter. It has no way of changing its aperture at all.
 

The teleconverter does not collect more light; it spreads out the existing light (i.e. increases the focal length). If the aperture with the teleconverter is now 70mm, then the focal ratio is f/6.1. If not, then the focal ratio is f/8.5. The sample images you provided, in which the image without the teleconverter is significantly brighter (to the point of being overexposed) than the one with the teleconverter, suggests that the focal ratio changed significantly — i.e. from f/5 to f/8.5.

Since the teleconverter affects the focal length of the Seestar, you will get larger image on the sensor. That image will be dimmer — our discussion here is over how much dimmer. When photographing Jupiter, the larger image size will be evident. And since Jupiter is so bright, the reduced light at the sensor may not be as noticeable since you probably still have to reduce gain to get a good image, even at f/8.5.

Again, all of this is theory, and even the Dawes limit is an estimate and is affected by conditions. If you like the results, then it's working for you.

[Psion]

Unfortunately, you're wrong. The Seestar lens in this assembly is another optical member of the optic that is no longer the input lens. Nowadays they commonly make teleconverters that change the f-ratio, this one is different.

[DevilJack]

I do like the Chase pictures. The close-up using the teleconverter is rather eye popping. Now maybe there’s four approaches to getting better  planetary images => native SS, SS+upscaling in post, SS+teleconverter, and SS+teleconverter+upscaling in post. I did find myself checking out Internet prices of both new and used Olympus Teleconverters.

As a wide field guy, I’d like to see similar add-on lenses to get wide field DSO/Milky Way pictures with the full power of Seestar (stacking, mosaics, plan mode). Or see the S30 take full advantage of the wide field lens for stacked Milky Way.

 

BTW, I’ve got a a box of add-on iPhone lenses collecting dust!

 

[Psion]

We just have to take it as a curiosity. For planets you need a focal length of at least 2 meters, better 4 meters. When the teleconverter I ordered arrives, I'll do some tests.

 

I will add note that the t-ratio changes with the Seestar because the Seestar has a 50mm lens and the Telekonvetor would have to have a lens diameter of 85mm and it doesn't.

Edited by Psion, 15 February 2025 - 09:40 AM.

[Peter Besenbruch]

When used on a camera, the camera makes adjustments to its iris aperture to accommodate the change in focal length. That's why you don't see a change in exposure times on a camera. The Seestar has no way of adjusting for the teleconverter. It has no way of changing its aperture at all.

Not so, as one can see both the f-stop chosen and the shutter speed chosen, and neither changed. You are fundamentally wrong on this one.

[Wildetelescope]

That is for sure going to shorten the life of your scope.  

 

JMD

[Peter Besenbruch]

The teleconverter does not collect more light; it spreads out the existing light (i.e. increases the focal length). I

 

You keep confusing front mounted tele-converters with rear mounted ones, and they work differently. The 50mm lens of the Seestar is not the light gathering lens, but a relay member of a larger optical system. The light gathering is determined by the front lens, which now belongs to the tele-converter.

[Peter Besenbruch]

That is for sure going to shorten the life of your scope.

Well, the converters were pretty solidly built, so you might be right.

[CraigR]

You keep confusing front mounted tele-converters with rear mounted ones, and they work differently. The 50mm lens of the Seestar is not the light gathering lens, but a relay member of a larger optical system. The light gathering is determined by the front lens, which now belongs to the tele-converter.

If, in fact, adding the teleconverter turns the Seestar into a 70mm aperture, 425mm focal length OTA, at f/6.1 we expect a slightly darker image. If it only affects focal length, then it will be f/8.5 and darker still. The S50 will make up for it in either case by adjusting gain, I assume.

 

As far as the Dawes limit thing, I'm happy to let that argument go. OP will get the results he gets in the seeing conditions he has. Arguing about pixel scale and whatnot is an academic exercise and not a hill I care to even climb, let alone die on.

 

The way to test this would be to switch to manual exposure control and shoot some images at the same gain and exposure settings with and without the teleconverter and compare the average brightness of the two images. Scenery mode would work best for this I think, since you actually have some bright things to look at, as opposed to either stargazing or solar system modes. Regardless of who's right and who's wrong, the image will be darker with the teleconverter. It won't be enough darker that OP will have anything to complain about. Again, we're talking about theory here — "spherical chickens in a vacuum" so to speak. But we live in a real world and if this combination of lenses generates images we like, then that's all that matters.

[Psion]

The resolution is mainly dependent on seeing, so we usually can't reach the Dawes limit. In my area, it is usually 2-3". Anyway, the resolution of the details in the craters on the moon must be fundamentally improved with a teleconverter, it certainly can't replace an upscale image. I'll test how much the exposure needs to change to get the same clarity. The 70mm lens has twice the surface area of the 50mm lens. However, the focal length is extended 1.7x (the ratio will be F/6), and therefore, fewer photons per pixel per unit of time will be incident. My guess based on the calculation is that a 1.5x longer exposure will be needed.

[Digital Don]

It's always interesting to try out new ideas and I think this one is quite creative

 

I also think that if you want good planetary images, start with something other than a Seestar.

 

Don  

[Peter Besenbruch]

A little more information on the tele-converter. I think they were produced by Olympus for the IS series cameras. I owned a smaller IS-1 and a larger IS-3. I purchased a tele-converter for each of them. They were wonderful cameras to work with, especially because my samples were very sharp (the ED lenses didn't hurt), and they remained sharp under low light conditions, in part because you did not have to sacrifice shutter speed at full zoom with the converter attached. None of this was cheap back in the day, but they were very much worth it.