Author Topic: Focal Ratio of Older Meade Lightbridge 16"?  (Read 185 times)

David Knoll

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Focal Ratio of Older Meade Lightbridge 16"?
« on: December 30, 2017, 03:26:32 AM »
I am moving an old primary and secondary from a 16" Lightbridge into a DobStuff kit. I think the primary is at least 10 years old and could be much older.

The current 16" Lightbridge is listed on their website as having a focal ratio of 4.5 and focal length of 1829mm.

But in my partial build test I am getting something more like a focal ratio of 4.3. (I will have to double check my measurements tonight).

Anyone have experience in older Meade primaries? Could they have been faster than their current product?



bijstentetal

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Re: Focal Ratio of Older Meade Lightbridge 16"?
« Reply #1 on: January 07, 2018, 10:11:33 PM »
It was f/4.5, but there was always a deviation of +/- a couple inches from that (f/4.38-f/4.63).
They were packed with a focuser extension tube in case it ran long.
But the design was always f/4.5 and they haven't changed.

Sam Citadelle

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Re: Focal Ratio of Older Meade Lightbridge 16"?
« Reply #2 on: January 10, 2018, 01:34:32 AM »
Thanks for the prompt response Don.

My focus test last night was using 3 wooden 2 x 2s as surrogates for the aluminum tubing. I ended up cutting the 2x2s shorter by ~2" than I had expected in order to reach focus with a range of EPs.

I am just trying to think this through before I cut the tubing to its final length.

Jeff Weiss

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Re: Focal Ratio of Older Meade Lightbridge 16"?
« Reply #3 on: January 10, 2018, 01:48:05 AM »
Remember to leave adequate secondary-to-focus dimensions when setting up the design.
Secondary-to-focus is the sum of:
mirror radius
tube clearance
tube thickness (or focuser board thickness)
focuser height from tube or plate
1" more to allow for in-focus required by coma corrector (varies by brand) and eyepieces needing in travel
On a 16", that would usually be
8" + 0.75" + 0.125" + 2.5" + 1" = 12.375"
I think the LB UTA might have a bit more UTA clearance (i.e. be larger than 17.5" I.D.) than that, and the focuser might be a tad taller counting the mounting plate.
At least you can figure it out based on where the focal plane should be so you can make the struts/poles the right length.

Ghassan Pham

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Re: Focal Ratio of Older Meade Lightbridge 16"?
« Reply #4 on: January 11, 2018, 11:47:29 PM »
I have a solid tube Meade 16". Back from the '90s I believe. Weighs a ton. It is f/4.5. I want to test the mirror with a Bath interferometer to get an idea if it is decent or not.

If I ever reconstruct it, I would try one of those single arm designs. Very portable!

Mark Deutsch

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Re: Focal Ratio of Older Meade Lightbridge 16"?
« Reply #5 on: January 12, 2018, 07:03:16 PM »
Quote
I have a solid tube Meade 16". Back from the '90s I believe. Weighs a ton. It is f/4.5. I want to test the mirror with a Bath interferometer to get an idea if it is decent or not.

If I ever reconstruct it, I would try one of those single arm designs. Very portable!


A couple of thoughts:

- The 16 inch Light Bridges were introduced in 2007 so it couldn't be more than 10 years old..

- I rebuilt my 16 inch Starfinder with a Dobstuff kit.  It's portable enough. I would be be reluctant to go the single arm route, stiffness and supporting the shrouds etc gets tricky.

Jon

breakagalkit

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Re: Focal Ratio of Older Meade Lightbridge 16"?
« Reply #6 on: January 16, 2018, 07:57:14 AM »
Yes, my scope is the Starfinder also.

I doubt I'll do anything with it anyway. I had to replace my sewer line yesterday because it was made of orangeburg - a material not unlike tar paper that caves in.

$5000+ and not one red cent from the homeowners.

So I'll be lucky to afford a daily $1.50 coffee .

bauradoubpay

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Re: Focal Ratio of Older Meade Lightbridge 16"?
« Reply #7 on: January 17, 2018, 12:51:52 AM »
Two nights ago I used the surrogate poles to determine their appropriate length in order for my EPs to come to focus. That includes, Panoptics, Delos, Pentax XW, ES100 series; all with and without a Parracor II. I also installed my Earthwin Binos with 24 Pans.

Today I checked the measurements of the DobStuff scope and my surrogate poles. The focal length is actually 71" indicating the the primary is F/4.43. This is within the manufacturing range that Don indicated in Post #2.

acbacema

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Re: Focal Ratio of Older Meade Lightbridge 16"?
« Reply #8 on: January 18, 2018, 08:51:32 AM »
Quote
Two nights ago I used the surrogate poles to determine their appropriate length in order for my EPs to come to focus. That includes, Panoptics, Delos, Pentax XW, ES100 series; all with and without a Parracor II. I also installed my Earthwin Binos with 24 Pans.

Today I checked the measurements of the DobStuff scope and my surrogate poles. The focal length is actually 71" indicating the the primary is F/4.43. This is within the manufacturing range that Don indicated in Post #2.


Did you account for shift in the focuser location due to the presence of the Paracorr?

Jon

Tim Massey

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Re: Focal Ratio of Older Meade Lightbridge 16"?
« Reply #9 on: January 23, 2018, 06:22:04 AM »
Quote
Quote

Two nights ago I used the surrogate poles to determine their appropriate length in order for my EPs to come to focus. That includes, Panoptics, Delos, Pentax XW, ES100 series; all with and without a Parracor II. I also installed my Earthwin Binos with 24 Pans.

Today I checked the measurements of the DobStuff scope and my surrogate poles. The focal length is actually 71" indicating the the primary is F/4.43. This is within the manufacturing range that Don indicated in Post #2.


Did you account for shift in the focuser location due to the presence of the Paracorr?

Jon
Do you mean the new focal plane due to the Paracorr?

Theodore Inlaw

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Re: Focal Ratio of Older Meade Lightbridge 16"?
« Reply #10 on: January 30, 2018, 12:11:18 AM »
When selecting pole length, this is done without the Paracorr in place.

Tyler Fonseca

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Re: Focal Ratio of Older Meade Lightbridge 16"?
« Reply #11 on: January 31, 2018, 06:44:50 AM »
Don & Jon:

My exercise was to confirm for myself that all of my current EP combinations would work at what appears to be my final pole length.

Jim Snyder

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Re: Focal Ratio of Older Meade Lightbridge 16"?
« Reply #12 on: February 02, 2018, 01:31:55 PM »
Quote
Don & Jon:

My exercise was to confirm for myself that all of my current EP combinations would work at what appears to be my final pole length.

<p class="citation">QuoteDo you mean the new focal plane due to the Paracorr

[/quote]

I was thinking of your measurement of the actual focal length of the mirror. The Paracorr eats up about 0.5 inches of inward focuser travel. If you are measuring focuser height with the Paracorr in place, the actual focal length would be about 1/2 inch longer.

In terms of determining the pole length, there are a number of ways to do it. The last time I did it, I found the focuser height with a known eyepiece, the scope focused with 41 mm Panoptic without cutting the poles.

Then another telescope, I measured the relative focuser heights for various combinations of the Paracorr, my Eyepieces and a Barlow and using the 41 mm Panoptic as a reference height, decided how much to cut off based on the inward focuser travel required.

Jon

Mark Rivera

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Re: Focal Ratio of Older Meade Lightbridge 16"?
« Reply #13 on: February 03, 2018, 01:24:38 AM »
Quote
Quote

Quote

Don &amp; Jon:

My exercise was to confirm for myself that all of my current EP combinations would work at what appears to be my final pole length.

<p class="citation">QuoteDo you mean the new focal plane due to the Paracorr


I was thinking of your measurement of the actual focal length of the mirror. The Paracorr eats up about 0.5 inches of inward focuser travel. If you are measuring focuser height with the Paracorr in place, the actual focal length would be about 1/2 inch longer.

In terms of determining the pole length, there are a number of ways to do it. The last time I did it, I found the focuser height with a known eyepiece, the scope focused with 41 mm Panoptic without cutting the poles.

Then another telescope, I measured the relative focuser heights for various combinations of the Paracorr, my Eyepieces and a Barlow and using the 41 mm Panoptic as a reference height, decided how much to cut off based on the inward focuser travel required.

Jon
Jon:

the above was was my process as well. I cut my wooden poles for what I think was the appropriate length for f4.5. But I needed to remove another 1" to allow all of my EPs to come to focus. Either my measurements are sloppy or the primary is f4.43.