8 replies to this topic

[Rick-T137]

Hi everyone,

 

I've got a Meade 8x50mm finder that I cut the back off of, and stuck a 2.00" to 1.25" adapter into it. That way I can stick a diagonal in the back and use various eyepieces. I realize the optics aren't going to be top shelf, but I'm hoping it will do for kind of an "extended monocular" type of thing.

 

My issue is that when I did the cut (probably about 6 years ago), I didn't calculate it very well. The scope will only focus up close. I vaguely remember realizing this way back in the day, and that's why I put it in storage. Well, I'm hoping to fix that mistake this week.

 

What I've done is remove the objective from one end and the 2-1.25" adapter from the other end. I've replaced the optical tube (current length: 5.5") with two rolled up and taped pieces of thick paper (each about 3.5" long) that slide into one another. So I can have an OTA that is about 4" long up to about 6.5" long. The goal is to figure out the correct length of tube that I need. My guess is I'll have to take another 1/2" to 1" off the metal OTA so this thing will focus at infinity. But I'm using the paper tubes to double check. I'll be using my TV Plössls with this, and they are all parfocal to each other.

 

The diagonal also has a helical focuser to give me a bit of play, and the object lens housing also is on threads so it has about 1/2" of movement available to it.

 

So my plan is to wait for clear skies (maybe Wednesday?) and using a 20mm Plössl (giving about 10x with this lens) I'll extend/retract the paper body to figure out what the length of the tube should be to focus on the Moon (or Jupiter or Mars - anything up there). I'll have my helical focuser on the diagonal set to the middle of its travel so I'll have a bit of adjustment on either side. Right now the paper is right up against the objective lens, so it's not taking into account the additional 1/2" of movement available due to the threads.

 

So my theory is, I focus on the Moon (for example) and record the length of the paper tube from the bottom of the objective housing back to the 2-1.25" adapter. I try this with all of my Plössl eyepieces - in theory they should all need the same tube length. Suppose that comes out to 4.65". I then add 1/4" to put the objective roughly in the middle of it's thread travel. And that's what I cut the metal OTA to - 4.90" (or whatever it comes out to).

 

What do you think? Am I missing something critical here? I want to measure 10 times and cut only once if I can help it.

 

Some pictures to help illustrate my ramblings:

 

The 50mm finder as it is currently - the tube is too long as when I try to focus, it won't, and if I pull the eyepiece out a bit or try to unscrew the objective a bit, it gets worse instead of better indicating to me that proper focus is "inward":

 

Here's the rig with the paper tubes retracted:

 

Here's the rig with the paper tubes extended:

 

Any advice on this would be appreciated. Otherwise, I'll let you know how I make out when the clouds finally clear.

 

Thanks!
 

Rick

 

[Rick-T137]

Well, I got lucky! The Sun popped out for a little bit between the clouds, and I got some measurements. I measured with just the diagonal (1x) and with a Barlow (2x) both on a light post about 60 feet away and the Sun (93,000,000 miles away). I'm actually amazed at how similar the focus was...

 

Eyepiece              1xNear  2xNear  1xFar  2xFar
Tele Vue 32mm Plössl  11.6    10.3    11.5   10.2
Celestron 25mm Plössl 10.9    10.1    11.0   10.2
Tele Vue 20mm Plössl  11.7    10.3    11.7   10.2
Tele Vue 15mm Plössl  11.7    10.3    11.7   10.2
Tele Vue 11mm Plössl  11.7    10.3    11.7   10.2
Tele Vue 8mm Plössl   11.7    10.3    11.7   10.2

 

So my OTA needs to be able to position the objective lens from 10.1 cm to 11.7 cm away from the 2-1.25" adapter. The current OTA is 13.0 cm long to the middle of the objective lens threads.

 

Based on looking at the range of adjustment, I might have to decide on it either working with the Barlow (10.2 cm) or without the Barlow (11.0-11.7 cm). It doesn't look like there's enough adjustment to cover both.

 

I'm inclined to rely on the Barlow, as it provides a useful power range of 13x to 50x whereas without it I get 6x to 25x.

 

I'm going to sit on it (Potsie!) for a bit and think it over....

[Rick-T137]

...and that's a wrap! I cut off just over an inch of the OTA so that the shoulder of the 2-1.25" adapter would sit 10.2 cm from the bottom edge of the objective housing when it's in the middle of it's threads. The result is that with a 2x Barlow (I tried my Parks 3-element Barlow and a Celestron Omni 2x) all of my eyepieces easily come to focus - even my 6mm Ortho! All of my Tele Vue eyepieces, plus my Celestron Omnis all worked.

 

The views weren't too bad either! I look forward to surfing the Moon with this bad boy. I realize the CA will be pretty apparent, but having some kind of telescope is better than no telescope.

 

If you read this thread, thank you for stopping by!

 

 

Clear skies!

Rick

Edited by Rick-T137, 24 March 2025 - 04:39 PM.

[152ED]

Wow, I'm doing this exact same experiment right now with an 80mm ~f/3.8 finder I just made, from an old bino lens.  The skinny is this:  focusing on near objects requires a longer tube.  Focusing on stars is just slightly shorter than focusing on a hillside tree or tower that is a few miles away. 

 

I like the look of your mini-scope!

[Rick-T137]

Wow, I'm doing this exact same experiment right now with an 80mm ~f/3.8 finder I just made, from an old bino lens.  The skinny is this:  focusing on near objects requires a longer tube.  Focusing on stars is just slightly shorter than focusing on a hillside tree or tower that is a few miles away. 

 

I like the look of your mini-scope!

Thanks! I was really worried after I finished the cut - I was thinking "Oh man! You took too much off! It looks too short!" but it turned out perfectly fine.

 

Good luck with yours!

[The Cloud Gazer]

Really cool thread, enjoy the easy wide views   

[vtornado]

Thanks Rick.  I never thought of trying to use a 2 to 1.25 inch adapter in one of these finders.  I had an Antares finder with a replaceable eyepiece and using a 32mm plossl was field curvature city, also it's amici prism was not full aperture so the field was heavily vignetted.    I've got a bunch of spare parts around so this is a no risk endeavor.

[Rick-T137]

Thanks Rick.  I never thought of trying to use a 2 to 1.25 inch adapter in one of these finders.  I had an Antares finder with a replaceable eyepiece and using a 32mm plossl was field curvature city, also it's amici prism was not full aperture so the field was heavily vignetted.    I've got a bunch of spare parts around so this is a no risk endeavor.

The adapter wasn't quite big enough to fit into the back of the OTA snugly, so I wrapped some electrical tape around it to the point where I had to really shove to get it to go in. My theory being that would keep the adapter straight with the OTA even if my cut was a bit off. I'll know if it worked for sure when I get a clear sky and can have a look at some stars with it. But my views of nearby streetlights are nice and sharp - and since the most I'll use this at is 50x, it likely isn't a big deal either way. Cheers!

[Rick-T137]

Just to close out this thread, I had some clear but cold skies last night (-8° C) and I went out in the backyard with my little contraption to check out what it looks like.

 

I was quickly reminded of why this little finder was in storage in the first place. The images weren't good at all. With a 32mm at 13x they were okay, but a little spikey. Likely when this was running at it's native 8x the stars looked like points.

 

But using the 20mm eyepiece at 20x the stars definitely didn't look like points - there's got to be some astigmatism in this lens or something. With the 8mm eyepiece (50x) things just looked terrible. I tried to look at Jupiter with both eyepieces, and although I could kind of make out the disk of the planet, there were no details and there were spikes and smears all over the place. I had great difficulty picking out the moons.

 

So to summarize, this little guy isn't going to cut it. I'll start a different thread on tiny travel scopes. My 70mm Sky-Watcher should be back up and running in a few weeks, and although it will make a great travel scope (assuming the new lens is good), it still requires a substantial mount which makes it a hassle for airline travel. I also liked the idea of the 50mm finder because if it got lost or damaged, it would be inexpensive to replace.

 

C'est la vie! It makes a really nice desktop decoration. It was a fun experiment no matter how it turned out.

 

Clear skies!
 

Rick