Explore Scientific 3x telecentric - not enough field lens?

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Florin Andrei
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Explore Scientific 3x telecentric - not enough field lens?

Post by Florin Andrei »

This image shows the TeleVue 2.5x Powermate telecentric (on the left), and the Explore Scientific 3x telecentric (on the right). Both are 1.25".
post-145735-0-63273000-1612229389.jpg
post-145735-0-63273000-1612229389.jpg (90.59 KiB) Viewed 450 times
The system I have in mind is based on the Orion ED80 (D=80mm, F=600mm, f/7.5) and a Quark Combo. I'm trying to figure out a good alternative to the Baader 3x telecentric, which is very large. This is more of an exploratory thing - I don't need to change anything, I'm just thinking of alternatives. I know that performance-wise the Baader 3x can't be beaten for hydrogen alpha.

With a 3x telecentric and the aperture stopped down at D=60mm, I would get the f/30 that the Quark wants. Just plug the Quark into the telecentric and go.

Let's say I want to observe the full disk of the Sun, all at once - I plug a 40mm TeleVue Plossl into the Quark and observe. Or say I want to image the full disk: a 0.79x reducer and an APS-C sensor would do it. I've tested these configurations with the Baader 3x telecentric and the geometry checks out.

But look at the field lens on the ES 3x. It's only about 11mm. Isn't it going to clip out a lot of light? I could probably try to do the geometry proof myself, pencil on paper, but I just want to ask what everyone thinks of it first.

The field lens on the Baader 3x is pretty large, and I'm sure that one does not clip anything.

I was told the 2" version of the ES 3x telecentric has a field lens of about 18mm, but that version is out of stock.

There's also the question of whether the ES telecentrics really do provide the parallel rays that the Quark needs. I'm sure the Baader 3x is essentially perfect that way.

Thoughts?

P.S.: I can't test anything right now, bad weather.


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Re: Explore Scientific 3x telecentric - not enough field lens?

Post by marktownley »

Tee hee hee! You're working out why the Explore is cheap and the Baader is not.

You pay for what you get i'm afraid!

Do you already have the Baaader?


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Re: Explore Scientific 3x telecentric - not enough field lens?

Post by marktownley »

Sorry to double post...

I'd prefer a 32mm plossl for visual anytime over the 40mm, for me the 40mm is like looking down a drinking straw...
astronomy_tools_fov (14).png
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Re: Explore Scientific 3x telecentric - not enough field lens?

Post by Florin Andrei »

Haha, okay, I get it.

Yeah, I have the Baader 3x already. I'm still working to troubleshoot the Quark (just the bad weather getting in the way - but Daystar gave me a scenario to try that looks reasonable, and I'm gonna do that the first sunny week-end), and meanwhile I'm exploring other options.

I don't doubt the Baader is the best option performance-wise. But I thought it would be nice to have a good-enough solution that's more compact. Like, say, at some point in the future we'll be able do outreach again, right? And imagine a bunch of kids at the park milling around the scope to take a peek at the Sun. The leverage in the system with the Baader installed is huge, just waiting for a little one to use it as a hand rail. Whereas, with a more conventional focal extender, I could plug everything in the diagonal and it would be much more compact.

I wish TeleVue made a Powermate that did 3x exactly. With the 2.5x I would have to reduce aperture even more to reach f/30 - like, around D=50mm. And yeah, their magnification depends on the back focus, but for the 2.5x it actually decreases when the back focus increases, and 2.5x is the highest value. :(

Re: the 32mm Plossl - I will probably get one of those (I was thinking about it) but right now the priority is troubleshooting the etalon. Once that pipe is unclogged I'll try other things.


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Re: Explore Scientific 3x telecentric - not enough field lens?

Post by Florin Andrei »

Well, that's interesting. Siebert Optics is a name not unknown to me.

My question is this: how well do the Siebert telecentrics satisfy the telecentricity criterion? My understanding is that the Powermates are not too bad (and they're definitely great for visual astronomy), and I already have a 2.5x.

If the Siebert glass performs very well in conjunction with a hydrogen alpha etalon, then it would make sense to add a compact 3x telecentric to my tool box.


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Re: Explore Scientific 3x telecentric - not enough field lens?

Post by MalVeauX »

Heya,

True telecentricism is about parrallel off-axis rays, the Powermate nor the ExpScientific do this completely.

Siebert or Baader is a great way to go if you want a real telecentric amp. Keep in mind Siebert is back logged with work. They will respond to orders and questions, but keep in mind it may not be a fast process currently (but thank goodness they're still in business and operating through this terrible time!).

Also, F30 is the minimum goal for effective focal-ratio through the Quark's etalon, instead, if you want to get the most out of it, target F40+ instead.

In general, doing full disc viewing or imaging with a Quark is the least flexible way to use it as it requires such a long focal-ratio to operate at its best, which means you either use a really small aperture (like 40mm to 50mm) on a short scope, or you give up performance on your etalon, or give up the full disc.

Very best,


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Re: Explore Scientific 3x telecentric - not enough field lens?

Post by Florin Andrei »

MalVeauX wrote: Fri Feb 05, 2021 3:57 pm Also, F30 is the minimum goal for effective focal-ratio through the Quark's etalon, instead, if you want to get the most out of it, target F40+ instead.
Marty,

That is something I did not know. Okay, so for top performance I should aim for f/40 or above. Got it. Thank you.

Siebert Optics makes a 5.5x telecentric that would put me at f/41.25

I'm aware of the performance trade-offs due to full disk viewing. I just want to have this capability simply for the pleasure of seeing the whole disk at once. But yes, for top performance I know I have to give up the full disk view, and I will. So likely the system will have at least a couple different configurations: say, one for full disk view, and another for zooming into details.


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Re: Explore Scientific 3x telecentric - not enough field lens?

Post by MalVeauX »

Florin Andrei wrote: Sat Feb 06, 2021 3:51 am
MalVeauX wrote: Fri Feb 05, 2021 3:57 pm Also, F30 is the minimum goal for effective focal-ratio through the Quark's etalon, instead, if you want to get the most out of it, target F40+ instead.
Marty,

That is something I did not know. Okay, so for top performance I should aim for f/40 or above. Got it. Thank you.

Siebert Optics makes a 5.5x telecentric that would put me at f/41.25

I'm aware of the performance trade-offs due to full disk viewing. I just want to have this capability simply for the pleasure of seeing the whole disk at once. But yes, for top performance I know I have to give up the full disk view, and I will. So likely the system will have at least a couple different configurations: say, one for full disk view, and another for zooming into details.
Courtesy of Christian V:

Use the 0.6A line to represent something closer to the Quark.
Telecentric_Quark_FWHM_Bandpass.JPG
Telecentric_Quark_FWHM_Bandpass.JPG (85.47 KiB) Viewed 343 times
Very best,


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