15 replies to this topic

[saemark30]

Just wondering, or is the Tele Vue unit better?

[chemisted]

Just wondering, or is the Tele Vue unit better?

Collins went out of business ~2009.  Have you found a used I³ that you are considering? And, if so, is it a thin film model?

[saemark30]

Yes and I don't know but I'll ask. Is thin film better?

[chemisted]

Yes there was a big improvement in SNR and Photocathode Sensitivity with the move to thin film.  You can tell by the serial number on the Collins device.  If it is thin film it will have TF following the G3 in the serial number.

 

I have to head out now but will check back this afternoon.  The Collins tube I have that dates from 2008 is thin film and is an excellent performer.  Good used Collins devices were sold by jdbastro a few years ago for around $1750.

[saemark30]

I just read this review:

https://www.cloudyni...old-and-new-r33

 

The reviewer said you can't see the Horsehead, Veil Nebula with it?

Edited by saemark30, 16 August 2024 - 07:48 AM.

[sixela]

"I have not tried a real narrow H-Alpha filter." And obviously, it also depends on the effective f/ratio at the photocathode that was tried.

Edited by sixela, 16 August 2024 - 08:36 AM.

[Souldrop]

Agreed with sixela. Effective f/ratio can make or break one’s subjective experience with an nvd and scope.

Just a heads up the biggest question I would encourage you to consider is price. Even a great unit from pre ~2018 is going to be more in line with many commonly available gen 3 units being built up today. So I would hope you would be purchasing an I3 unit at a significant discount otherwise a televue unit should be a better purchase imo. (Unless the I3 unit has very excellent-unicorn specs then that could make the decision less black and white)

[bobhen]

I just read this review:

https://www.cloudyni...old-and-new-r33

 

The reviewer said you can't see the Horsehead, Veil Nebula with it?

It sounds like he wasn't using a strong enough filter. From that review...

 

I have not tried a real narrow H-Alpha filter. That is my next test. I purchased a Lumicon H-Alpha filter which is very wideband, which also works in a dark sky situation right after sunset.

 

You need a narrow band Ha filter. 

 

Bob

[sixela]

Yeah, the Lumicon H-alpha filter is really a misnomer, it's a 645 nm Red/IR-pass filter. That's not what you need for the Horse Head or Veil nebulae.

[saemark30]

Does it work at F/6 or F/7.5. I got a Baader Ha filter 7nm.

[chemisted]

I just read this review:

https://www.cloudyni...old-and-new-r33

 

The reviewer said you can't see the Horsehead, Veil Nebula with it?

Hello again; I just made it home.  You got the early Lindy Williams review before he started the narrow band revolution.  This is the one you want to read: https://www.cloudyni...i3-system-r1602

 

 

Does it work at F/6 or F/7.5. I got a Baader Ha filter 7nm.

I have an original Collins device that I used for globular cluster observations at f/9.38 and I published an article about them.  It is:  Seeing Through the Dust by Ed Mihelich, Sky & Telescope 2017, July issue pages 57-59.

 

What are the details on the unit you are considering?  There is an important issue to consider.  My unit has the 25mm eye lens and that is the only way I would recommend it.  Some people purchased the Collins I³ with a 15mm eye lens and it was a disaster and to be avoided at all costs.

Edited by chemisted, 16 August 2024 - 12:54 PM.

[sixela]

Does it work at F/6 or F/7.5. I got a Baader Ha filter 7nm.

You're not going to like narrowband H-alpha with even a TF Collins I3 at these f/ratios. Come to think of it, that's awfully slow even for a modern Gen 3 TF, and the reason why people who do narrowband H-alpha often end up using a afocal stack (to get the effective f/ratio at the photocathode a lot faster). f/6 is likely too slow to use most "classic" single-device focal reducer setups (like the Nexus or a StarSweeper), unless you can find a 0.5x reducer setup that really works well (and allows you to still get focus) without adding tons of aberrations.

That is fine for broadband filter usage (e.g. Red/IR-pass 645nm filter, or possibly even a UHC filter with red passband) or filterless setups for globulars and galaxies, though.

Edited by sixela, 16 August 2024 - 01:44 PM.

[saemark30]

So through PM I find out this unit is of first generation, not TF and has the dreaded 15mm eyepiece.

Among these things I was advised to pass on the unit.

Thanks to all for your info.

[saemark30]

Why is  the 15mm eyepiece a disaster?

Does the I3 unit take regular 1.25" or 2" eyepieces?

[chemisted]

It is not an eyepiece and the device does not use normal eyepieces. It was offered as an alternative to a 25 mm eye lens. Why I'm not sure. When I spoke with Bill Collins he recommended the 25 mm for use with my RC-10 and it worked out very well for me.

I tried to help another CNer who had the 15 mm but it was so bad he ended up getting a PVS-14. He had purchased his Collins unit over 20 years ago but never really used it.

Edited by chemisted, 18 August 2024 - 05:25 PM.

[sixela]

The focal length of the eyepiece after the phosphor screen determines the magnification of the system in a given scope.

Usually a shorter eyepiece is chosen to give a higher magnification with a large apparent field of view (although that is a bit of a misnomer, since the system doesn't really have an exit pupil°). In other words, for an 18 mm photocathode², instead of acting like a 25mm 40° AFoV eyepiece when used in prime mode on a telescope, it acts like a 15mm 66° AFoV eyepiece. More magnification, but at the expense of surface brightness and resolution (expressed in arcminutes in the field at the output, while the lp/mm metric is the resolution measured on the phosphor screen).

It's an alternative to using a barlow on the NVD equipped with the 25mm output eyepiece (which would give you a 15mm 40° AFoV eyepiece and the same surface brightness, but with better resolution).

It seems "better" than the barlowed NVD but, since the 25mm is usually selected primarily to match the resolution of the image on the phosphor screen to that of the eye, that may be penny wise but pound foolish. It's also harder to make an eyepiece with little distortion, less aberrations and easier eye placement if you make it shorter focal length (remember, there is no exit pupil forcing you to centre the eye accurately).

On PVS-14 there are "50° AFoV" objective/eyepiece combinations with shorter focal length than the usual 27mm. Still "1x" if you use both, but a larger AFoV at the expense of surface brightness and resolution. Most people don't like them that much (at least the current iterations for them).

If you have a set of these and change the eyepiece but keep the original objective you have a 1.25x system with an AFoV of 50°. That increases the image scale, but at the expense of surface brightness and resolution. I think Gavster experimented with that as well and didn't like it that much either.

--
°Here it's the angle between bundles through the eye for objects opposing sides of the edge of the phosphor screen, with a centred eye pupil, and with the diopter setting set neutrally.
²atan(9/25)*2 = 39.6°. In reality on e.g. a PVS-14 the paraxial focal length is closer to 27 mm, but with distortion that yields a similar AFoV

Edited by sixela, 20 August 2024 - 02:01 AM.