47 replies to this topic

[Princess Leah]

Has anyone ever managed to source an adapter (26.5mm? - 1.25 inch) for the 50/600mm Pentax?

Lovely optics on this scope and a focuser that works!

 

Thanks Leah.

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[Princess Leah]

A friction fit with a sacrificial Celestron prism worked well with a similar Greenkat, but is more difficult here, as the rack extends to almost the end of the draw-tube.

Also it seems a waste to destroy a thread opportunity.

Will see if someone can make me one.

 

Otherwise the only option would be to saw through part of the rack , but then you would be minus a screw - superglue might hold?

 

Note: The optics in the Greenkat are very good, perhaps as good as the Pentax, but the focuser, although pretty, is poor. The draw-tube tapers, and is felted on one side only. It's was fun and games replacing the worn felt. Despite my efforts to feed teflon into the other side, it's still a bumpy ride!

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[Princess Leah]

Edit:

 

The Greenkat draw-tube doesn't taper, but the feeder for the draw-tube does.!

Edited by Princess Leah, 27 March 2025 - 12:50 PM.

[CharlieB]

The way the Pentax diagonal fits on the tube with grub screws makes it tough to adapt any other diagonal, plus I'd be willing to bet there would not be enough focuser in-travel to reach focus.  I have a couple of these scopes and have never found a way of using 1.25" eyepieces.  If you do, please post the results.

 

Chatlie

[Princess Leah]

The way the Pentax diagonal fits on the tube with grub screws makes it tough to adapt any other diagonal, plus I'd be willing to bet there would not be enough focuser in-travel to reach focus.  I have a couple of these scopes and have never found a way of using 1.25" eyepieces.  If you do, please post the results.

 

Chatlie

That grub screw - along with the other one on the lens cell is the smallest I've seen.

 

Your right about in-travel Charlie. An adapter would have to thread directly onto the 1.25 inch diagonal - minus the nosepiece.

[Princess Leah]

I tried the telescope without the nosepiece of the Celestron diagonal. There is enough in-focus. Maybe not for a mirror.

 

There is a company online, who is a member here that may be able to make the part.

The more people that demand this part the cheaper it may be.

 

I need to be sure of the measurements. Can anyone help.

Are all 1.25 inch nosepieces the same?

 

I measure 26mm width (female) for the Pentax. Unsure of thread width.

For attachment to 1.25 inch prism I measure 30mm width (Male). Unsure of thread width.

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Edited by Princess Leah, 28 March 2025 - 04:52 AM.

[Princess Leah]

Is there a tool on the market for measuring thread width.

Is a ruler accurate enough?

[deSitter]

Is there a tool on the market for measuring thread width.

Is a ruler accurate enough?

Yes, a set of thread gauge blades - looks like a fan of thin pieces of metal.

 

To measure diameter, you measure the outside edge of the thread and then reference an accurate table of standard threads. There are several standards - ISO, JIS etc.

 

Thread gauge blades usually come with tap and die sets. Here's a standalone.

 

https://www.amazon.c...t/dp/B07J9V9JTK

 

-drl

[deSitter]

Is there a tool on the market for measuring thread width.

Is a ruler accurate enough?

BTW I have counted threads against a ruler using an eyepiece with the barrel removed as a magnifier. You count the number of threads needed to get to an integral multiple of whatever standard the thread is. Of course you don't count the first one placed against 0 on the scale.

 

Example - a 0.7mm pitch (modern M4 screw) you will count 10 threads in 7mm.

 

-drl

[Weisswurst Josef]

Is there a tool on the market for measuring thread width.

Is a ruler accurate enough?

If you have a fine ruler and enough threads you can.

By not measuring one pitch but multiple and dividing the measurement by the number of threads.

Easier when taking a picture of ruler and thread.

[Princess Leah]

BTW I have counted threads against a ruler using an eyepiece with the barrel removed as a magnifier. You count the number of threads needed to get to an integral multiple of whatever standard the thread is. Of course you don't count the first one placed against 0 on the scale.

 

Example - a 0.7mm pitch (modern M4 screw) you will count 10 threads in 7mm.

 

-drl

Mr DeSitter your comments are extremely helpful. Hopefully get it done soon.

[Princess Leah]

8 threads for 4mm on Pentax.

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Edited by Princess Leah, 01 April 2025 - 06:23 AM.

[Princess Leah]

I thought a better way, might be to use a Baader T2 prism for the other side?

You can buy the nose pieces cheap and then it would just be a matter of screwing the diagonal in. You would also be able to use a Baader micro focuser.

The nose piece of the Baader seems to useca 0.75mm thread.

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[Princess Leah]

Silly me you wouldn't want a nosepiece on the diagonal as there would be to little in focus.

The adapter would screw directly into the diagonal.

 

So final measurements.

An adapter with 26mm 0.5mm thread female,

Other side

31.7 , 0.75 thread male.

[deSitter]

Silly me you wouldn't want a nosepiece on the diagonal as there would be to little in focus.

The adapter would screw directly into the diagonal.

 

So final measurements.

An adapter with 26mm 0.5mm thread female,

Other side

31.7 , 0.75 thread male.

Is that 26mm crest to crest? If so the actual thread size would likely be 25mm.

 

-drl

[Princess Leah]

Will check thanks 👍 

[Princess Leah]

Measured again and seems to be 27mm!? Trough to trough.

 

Or 26.5mm

Edited by Princess Leah, 01 April 2025 - 11:15 AM.

[Princess Leah]

Definitely not 25mm....

[Princess Leah]

Strange when I measure the Baader nosepiece it's 40.5 to 41mm Max. Yet Baader lists it as 42mm on their website?

[deSitter]

Strange when I measure the Baader nosepiece it's 40.5 to 41mm Max. Yet Baader lists it as 42mm on their website?

Measuring threads with a physical ruler is nearly impossible - parallax, hard to see, etc.

 

You can get a digital caliper for nothing. Indispensable in this hobby.

 

-drl

[Princess Leah]

I was just thinking the same thing!

[Cavs56]

I use my digital caliper more than any tool I have.

[Cavs56]

I also have two thread gauges.

[Princess Leah]

In the end I made my own visual back for this and achieved infinity!

 

And yet only some of my eyepieces come to focus. One would have to shorten the tube to get more in-focus.

I'm not what Pentax was thinking - there is enough outward travel, to focus on my shoelace.

 

Crucially I was able to compare the Pentax with the Towa (Greenkat).

 

Both are excellent, really surprisingly good - however the Towa is actually quite a bit better. Sharper, more contrast - seemingly less scatter. There is quite a bit of scatter with the Pentax.

The views of flowers/birds etc through the Towa are stunning.

Perhaps the Pentax might be better at night.

 

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Edited by Princess Leah, 02 April 2025 - 01:31 PM.

[deSitter]

In the end I made my own visual back for this and achieved infinity!

 

And yet only some of my eyepieces come to focus. One would have to shorten the tube to get more in-focus.

I'm not what Pentax was thinking - there is enough outward travel, to focus on my shoelace.

 

Crucially I was able to compare the Pentax with the Towa (Greenkat).

 

Both are excellent, really surprisingly good - however the Towa is actually quite a bit better. Sharper, more contrast - seemingly less scatter. There is quite a bit of scatter with the Pentax.

The views of flowers/birds etc through the Towa are stunning.

Perhaps the Pentax might be better at night.

What an ordeal

 

I would just cut an inch off the end of the tube, drill some new holes for the focuser and sight and there you go! I have done this - it was very straightforward work. Cutting the tube square is done using blue painter's tape to make a precisely square channel to guide the hacksaw.

 

-drl