9 replies to this topic

[Martin_1972]

First post and not sure if I post it in the right place...... I need some help or advice.

 

Ah while back I was able to buy this Celestron C90 500mm f/5.6 telescope/lens (see picture 1 and 2)

I bought a T-mount (picture 3) to be able to attach it to my Canon camera.

When the T-mount is attached and the O-ring screwed tight to the telescope there is a lot of play between the T-mount and the optics. almost as if the thread of the T-mount end the C90 telescope are different. Wen fully secured there is a gap of about 2 mm between the flange of the T-mount and the lens. What am I doing wrong????

Any advice would be appreciated a lot!

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[jkmccarthy]

Hi Martin !

 

Welcome to CN and congrats on your vintage C90 500mm f/5.6 telephoto.  Indeed you have come to the right place in the CN forums for help.

 

While I do not own a Celestron 90mm f/5.6 myself, the help I can provide you is a reference to this excellent on-line resource called "Astronomy Threads Explained" compiled by the folks at Agena Astro:

 

https://agenaastro.c...-explained.html

 

Searching on-line for "T-Adapter-MAK" (in double-quotes to return only exact matches) I found a SOLD eBay listing that included this photo of your unit alongside its original box:

 

 

... confirming that the Celestron part number is #93635-A.   To further educate myself, with google's help I found this on-line listing for the current Celestron #93635-A part:

 

https://www.highpoin...130-mak-93635-a

 

Selecting "SPECIFICATIONS" in the menu bar below the product photos, the adapter's telescope-side thread is identified only as "ETX 90-125/NX4" .. which in the "Astronomy Threads Explained" reference above is identified in Section 4.3.1 as having a [male] thread diameter of 1.375" and a thread pitch of 24 threads per inch. (In millimeters, these equate to 34.925mm diameter and 1.058mm pitch).

 

So the mystery is what are the specs on the male thread on the back-end of your C90 telephoto, onto which the 1.375" x 24t.p.i. female threads on the telescope-side of the Celestron MAK adapter are a poor fit.  (Apologies for just re-stating the obvious here ...).

 

If you loosen the radial thumbscrew, it would appear that the telescope-side "back-end piece" (i.e., the piece that has the mystery male threads) would slide out ... is that correct ?   [Answer:  NO ??]

 

In the CN forum archives, I found a discussion thread by CN member (jstrandberg) regarding his C90 f/5.6 telephoto, and from the photos included in his post here:

 

https://www.cloudyni...-f56/?p=9494997

 

... the telescope-side "back-end piece" onto which C90 f/5.6 telephoto owner jstrandberg attached his camera T-ring appears to be much different than the one that came with your telephoto. [Celestron #93635-A].

 

When you loosen the thumbscew and remove the telescope-side "back-end piece", can you describe it ?  Is it a slip-fit inside the back end of the telephoto (before being clamped in place by the thumbscrew) ?   If yes, what is the O.D. of the portion that fits inside the back end of the telephoto ?   Is the I.D. of the "back-end piece" smooth, or is it threaded (?) perhaps to defeat glancing-incidence reflections from the inside surface ?

 

And lastly, can you measure the O.D. of the male threads on the exit-side of your "back-end piece" ?   Is the O.D. of those male threads on your telephoto's "back-end piece" measurably larger or smaller than 1.375" = 34.9mm ?   If you measure across multiple crests of the thread (i.e., measure some number of thread pitch intervals, and then divide your measurement by the number of intervals), do you get a number near 1/24th of an inch (1.06mm) ?

 

Hopefully some of the above will prove helpful ... either to you (Martin) or other forum member(s) who may also respond.

 

Cheers,

 

       -- Jim

 

EDIT:   Note also this later post in the aforementioned CN dicussion thread from user Rolo:

 

https://www.cloudyni...-f56/?p=9518713

 

What caught my attention (besides his assessment of the optical performance of his best "2 out of 10" examples) is that in the photo Rolo included, the "back-end piece" on his C90 1000m f/11 appears very similar to the one that came with yours ... but in Rolo's photo, he shows what appears to be a 0.965" (24.5mm) prism diagonal inserted into the "back-end piece", rather than a T-adapter on the male threads on the rear of the "back-end piece".   So possibly the "back-end piece" you have is actually what we would call a 0.965" visual back ?  (Not obvious from the photo is how the 0.965" prism diagonal is held in place ... is there provision for a second thumbscrew, or did I misinterpret the purpose of the thumbscrew shown in your pictures --- perhaps its purpose is to secure a 0.965" O.D. barrel, and there is no separate "back-end piece" that slides out ?).  [Answer:  YES !?]

 

EDIT #2:   Assuming this is true, I've now greyed-out my erroneous comments in the above.

Edited by jkmccarthy, 07 January 2025 - 09:42 AM.

[jkmccarthy]

FYI, here is CN member jstrandberg's post from 2019 that mentions where he got his C90 f/5.6 telephoto T-adapter "back-end piece" pictured in the forum post cited above --

 

https://www.cloudyni...aphy/?p=9481621

 

... and here is a post from 2021 containing a lead that might help you get in touch with Mr. Rothman --

 

https://www.cloudyni...ler/?p=10889744

 

Hope this helps -- even if it doesn't get any closer to explaining why the Celestron #93635-A adapter doesn't work as intended on the C90 500mm f/5.6 telephoto ! - (

 

Cheers,

 

        -- Jim

 

EDIT:   The final conjecture added to my post above would then imply that the T-adapter jstrandberg obtained from Don Rothman actually threads onto (and covers, so as to hide : - ) the same male threads at the back-end of your C90 which didn't seem well-matched to the female ETX 90-125/NX4 -threaded ring on the telescope-side your Celestron #93635-A adapter.  Hmmm.

Edited by jkmccarthy, 06 January 2025 - 07:41 PM.

[jkmccarthy]

Among the Customer Reviews of the #93635-A on Celestron's product page here --

 

https://www.celestro...ter-nexstar-4se

 

... is this one from customer Albert B. whose experience using this adapter with his C90 matches yours exactly.   His solution (June 2019) was to make a thick washer --- that I infer goes behind the lip (i.e., the front flange) of the C93635-A's front tube, with the threaded collar then behind the thick washer --- to occupy the gap that both of you find once the threaded collar of the #93635-A T-adapter is tightened as much as it can be ....

 

 

5+ years later now, 3D-printing such a thick washer would probably be the way to go ... but whether or not this would give you the optimum back-focal-distance for the camera's imaging sensor is not necessarily a valid assumption, as clearly the #93635-A adapter was designed for different (later?) generations of small Celestron Maks than the C90 500mm f/5.6 telephoto that you have.   From jstrandberg's photo in the first of his posts cited above, my (visual only) impression is that the vintage adapter he acquired looks shorter than the the full #93635-A assembly ... but I certainly could be mistaken .....

 

Best wishes,

 

        -- Jim

Edited by jkmccarthy, 06 January 2025 - 07:44 PM.

[Orion928]

If all else fails and as a last resort, check out the lapping solutions at:

https://www.cloudyni...ifferent-sizes/

 

Greg

[Chuck2]

Threads may have changed over the years as production shifted from US to China?

My T-Adaptor-C90 from 1984 threads on smooth and snugs tight with 0 zero wobble or shift on my 1984 US made C90. No o-ring necessary.

 

Is your T-Adaptor-Mak a newer Chinese product being threaded on an older US made C90 telephoto? Appears the ‘slop’ on your adaptor is due to a different thread length or milled face, not allowing it to ‘seat’ on your older C90 lens. Possibly it was designed for the newer Chinese made C90s?

 

 

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[RichA]

First post and not sure if I post it in the right place...... I need some help or advice.

 

Ah while back I was able to buy this Celestron C90 500mm f/5.6 telescope/lens (see picture 1 and 2)

I bought a T-mount (picture 3) to be able to attach it to my Canon camera.

When the T-mount is attached and the O-ring screwed tight to the telescope there is a lot of play between the T-mount and the optics. almost as if the thread of the T-mount end the C90 telescope are different. Wen fully secured there is a gap of about 2 mm between the flange of the T-mount and the lens. What am I doing wrong????

Any advice would be appreciated a lot!

T-mounts have internal rings (held in place by three small set screws) that allow the mount to be rotated and locked in place.  Check the screws.

[Martin_1972]

Wow! That is a lot of replies and from what I see from quickly scrolling through its very helpfull!. I wil try to go trough all replies tomorrow evening and come back here anyway to let you know if things worked out or if I have any further questions. So far: Thank you very much for the help!

[jkmccarthy]

Hello again Martin --

 

See also this CN discussion forum post from 2022, and the discussion that precedes it ....

 

https://www.cloudyni...c90/?p=12069355

 

... for another option (ScopeStuff ADP9 .. for 34.5mm threads) in lieu of the longer Celestron #93635-A adapter.  The ADP9 has a 1.25" I.D. and 42mm x 0.75mm male T-ring threads under a removable compression ring for clamping the 1.25" eyepiece or star diagonal in place.

 

Cheers,

 

        -- Jim

Edited by jkmccarthy, 07 January 2025 - 10:36 AM.

[Martin_1972]

Threads may have changed over the years as production shifted from US to China?

My T-Adaptor-C90 from 1984 threads on smooth and snugs tight with 0 zero wobble or shift on my 1984 US made C90. No o-ring necessary.

 

Is your T-Adaptor-Mak a newer Chinese product being threaded on an older US made C90 telephoto? Appears the ‘slop’ on your adaptor is due to a different thread length or milled face, not allowing it to ‘seat’ on your older C90 lens. Possibly it was designed for the newer Chinese made C90s?

 

Hi Martin !

 

Welcome to CN and congrats on your vintage C90 500mm f/5.6 telephoto.  Indeed you have come to the right place in the CN forums for help.

 

While I do not own a Celestron 90mm f/5.6 myself, the help I can provide you is a reference to this excellent on-line resource called "Astronomy Threads Explained" compiled by the folks at Agena Astro:

 

https://agenaastro.c...-explained.html

 

Searching on-line for "T-Adapter-MAK" (in double-quotes to return only exact matches) I found a SOLD eBay listing that included this photo of your unit alongside its original box:

 

s-l1600.jpg

 

... confirming that the Celestron part number is #93635-A.   To further educate myself, with google's help I found this on-line listing for the current Celestron #93635-A part:

 

https://www.highpoin...130-mak-93635-a

 

Selecting "SPECIFICATIONS" in the menu bar below the product photos, the adapter's telescope-side thread is identified only as "ETX 90-125/NX4" .. which in the "Astronomy Threads Explained" reference above is identified in Section 4.3.1 as having a [male] thread diameter of 1.375" and a thread pitch of 24 threads per inch. (In millimeters, these equate to 34.925mm diameter and 1.058mm pitch).

 

So the mystery is what are the specs on the male thread on the back-end of your C90 telephoto, onto which the 1.375" x 24t.p.i. female threads on the telescope-side of the Celestron MAK adapter are a poor fit.  (Apologies for just re-stating the obvious here ...).

 

If you loosen the radial thumbscrew, it would appear that the telescope-side "back-end piece" (i.e., the piece that has the mystery male threads) would slide out ... is that correct ?   [Answer:  NO ??]

 

In the CN forum archives, I found a discussion thread by CN member (jstrandberg) regarding his C90 f/5.6 telephoto, and from the photos included in his post here:

 

https://www.cloudyni...-f56/?p=9494997

 

... the telescope-side "back-end piece" onto which C90 f/5.6 telephoto owner jstrandberg attached his camera T-ring appears to be much different than the one that came with your telephoto. [Celestron #93635-A].

 

When you loosen the thumbscew and remove the telescope-side "back-end piece", can you describe it ?  Is it a slip-fit inside the back end of the telephoto (before being clamped in place by the thumbscrew) ?   If yes, what is the O.D. of the portion that fits inside the back end of the telephoto ?   Is the I.D. of the "back-end piece" smooth, or is it threaded (?) perhaps to defeat glancing-incidence reflections from the inside surface ?

 

And lastly, can you measure the O.D. of the male threads on the exit-side of your "back-end piece" ?   Is the O.D. of those male threads on your telephoto's "back-end piece" measurably larger or smaller than 1.375" = 34.9mm ?   If you measure across multiple crests of the thread (i.e., measure some number of thread pitch intervals, and then divide your measurement by the number of intervals), do you get a number near 1/24th of an inch (1.06mm) ?

 

Hopefully some of the above will prove helpful ... either to you (Martin) or other forum member(s) who may also respond.

 

Cheers,

 

       -- Jim

 

EDIT:   Note also this later post in the aforementioned CN dicussion thread from user Rolo:

 

https://www.cloudyni...-f56/?p=9518713

 

What caught my attention (besides his assessment of the optical performance of his best "2 out of 10" examples) is that in the photo Rolo included, the "back-end piece" on his C90 1000m f/11 appears very similar to the one that came with yours ... but in Rolo's photo, he shows what appears to be a 0.965" (24.5mm) prism diagonal inserted into the "back-end piece", rather than a T-adapter on the male threads on the rear of the "back-end piece".   So possibly the "back-end piece" you have is actually what we would call a 0.965" visual back ?  (Not obvious from the photo is how the 0.965" prism diagonal is held in place ... is there provision for a second thumbscrew, or did I misinterpret the purpose of the thumbscrew shown in your pictures --- perhaps its purpose is to secure a 0.965" O.D. barrel, and there is no separate "back-end piece" that slides out ?).  [Answer:  YES !?]

 

EDIT #2:   Assuming this is true, I've now greyed-out my erroneous comments in the above.

 

The thread size is the problem I have for sure. Will go look into a different adapter with the correct thread to fix the issue. Thanks again for the help!