14 replies to this topic

[Jeff B]

I'm a sucker for these "little" scopes, especially at their used prices, and even new prices (if they are still available).  So when gent I know here offered me first crack at his as he was gong to 150mm aperture class scope, I jumped at it. 

 

Thanks Brian!

 

This is my third sample.  I have reported on the other two samples here:

 

https://www.cloudyni...g/?hl=dpac test

 

Both are nice samples with the first, new, unit the better of the two visually (it's rather sharp actually).

 

Here are some pictures of the 3rd scope in preliminary DPAC, the collimation of the focuser with the laser (very good really) and collimation of the objective via my cheshire (again very good).  As this scope has been through the hands of about 3 to 4 previous owners, I found it a bit extraordinary that the collimation has stayed put after such extensive shipping and handling by so many owners.  But their typically robust build is one of the attractions of refractors for me.

 

However, like the other two samples, this unit also has a painfully small fully illuminated field of view when using a two inch diagonal.  It is essentially zero with the focuser draw tube inlet I.D. and it's internal baffling the choke points for the light cone.  And like my other units,  I'll be eliminating and/or moving some of those baffles aft to increase the fully illuminated FOV.

 

Next posting will be with some DPAC data.

 

Jeff

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Edited by Jeff B, 01 April 2022 - 10:13 PM.

[CHASLX200]

No clue what i am seeing in the last pics. I guess it is out of center a tiny bit. Maybe jig up the focuser some. At F/5 it would effect the image pretty bad.

Edited by CHASLX200, 02 April 2022 - 05:33 AM.

[Jeff B]

No clue what i am seeing in the last pics. I guess it is out of center a tiny bit. Maybe jig up the focuser some. At F/5 it would effect the image pretty bad.

Chas, the laser shot is less than 1/10 degree off axis so even at F5, any off-axis astigmatism due to focuser mis-alignment is visually invisible, even at high power.  The Cheshire view, which shows the reflections from the backs of the two elements as dots, shows a single bright dot indicating the lens elements are aligned with axis of the focuser as well.  With the focuser aligned, if I see distinct or overlapping dots through the cheshire, I will tweak the lens cell collimation so that the dots line up. I have found that achieving these alignments are good starting points to begin evaluation of an objective.  

 

Both are super easy tests to perform.  However, depending on the scope, the tweaking needed to line everything up can be a real challenge.  This sample didn't need any tweaking.  Now I found that about 1/4to 1/3rd of inexpensive achromats will visually show some degree of astigmatism at high power, even when "passing" these two alignment tests.  If I see just simple astigmatism (with no coma thrown in), this suggests that the best optical correction axis may be slightly tilted relative to the laser/cheshire axis.  I then tweak the lens cell collimation during the star test to dial out the astigmatism as best I can, and, in most cases, I can pretty much eliminate or greatly reduce it.  Rechecking the laser and Cheshire alignments after such tweaking almost always shows the laser still aligned with the center of the lens, but it then "fails" the cheshire test, showing two dots (or and oval or figure eight image).   To me, the lens is now well collimated and I lock the adjustments down as best I can and walk away.

 

Jeff

[Jeff B]

So here are the DPAC test results for this third sample.

 

Very good really.  As you can see in the at focus image in green, it rather smooth and has a bit of a compound correction with a mild center zone smoothly spanning a modest 50% of aperture.  But such a mild zone, even at that size, does the least damage being in the middle.  Overall, the lens behaves like it is about 1/5 to 1/6 wave overcorrected in green overall at full aperture with best correction in the yellow to yellow/red. 

 

But notice the dominance of the out of focus blue, which is way out.

 

But notice how well balanced the lens is from green to red in both color focus and spherochromatic content.  This is something I've seen in almost all of the China sourced lenses I've tested, and to me, helps to explain why many folks (including me) are surprised at how sharp some samples of these scope can be on the moon and planets despite the out of focus blue, especially with "minus violet" style filters (but some folks just cannot stand the resulting heavy yellow tint).

 

This unit performs very well in DPAC especially for the price (!!!).

 

I'll next show some testing at reduced aperture and comparison with my other two samples of this scope.

 

Jeff

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[AstroRed]

Hi Jeff, have you got any info as to what you did with the focuser tube/baffles? I have the same issue with my 150mm f5 when using 2 inch kit

[Exnihilo]

 

However, like the other two samples, this unit also has a painfully small fully illuminated field of view when using a two inch diagonal.  It is essentially zero with the focuser draw tube inlet I.D. and it's internal baffling the choke points for the light cone.  And like my other units,  I'll be eliminating and/or moving some of those baffles aft to increase the fully illuminated FOV.

 

Yeah, the only way to practically use these scopes is to cut back the focuser tube, or go with a different focuser.

 

Thanks for running these tests, this is really interesting!

[Orion68]

Very interesting thread. I have a question.

 

My ST120 requires the focuser to be racked most of the way out for the eyepieces that will get heavy use. Given that, does the focuser tube still need to be cut back?

[Exnihilo]

It depends on what diagonal and eyepieces you use, but I would at least cut it as far as the rack, see this thread:

 

https://www.cloudyni...105-to-a-st120/

 

I replaced my focuser years ago with a GSO linear bearing focuser. If I had to do it again though, I’d use the regular GSO crayford focuser

Edited by Exnihilo, 04 April 2022 - 02:23 AM.

[Orion68]

It depends on what diagonal and eyepieces you use, but I would at least cut it as far as the rack, see this thread:

 

https://www.cloudyni...105-to-a-st120/

 

I replaced my focuser years ago with a GSO linear bearing focuser. If I had to do it again though, I’d use the regular GSO crayford focuser

ok, thanks.

[Letterman]

I don’t have a laser collimation device myself, so I can’t test this potential use of it. However, I was wondering if it’s possible to determine the size of the fully illuminated field of a refractor by using a laser collimation device set facing square on and beamed at the front, outside edge of the objective lens. The idea being that the laser beam could serve as a ray trace, so to speak.

 

Mark a cross on a piece of tissue paper and center it on top of the diagonal racked out to the typical infinity focus of the refractor being tested. Measure the distance from the centered cross mark to where the refracted laser beam strikes the tissue paper. This distance would be one half of the diameter of the fully illuminated field (am I right?). Then rack the focuser tube in and out to see if and when it cuts off the narrow beam of light at some point. This might help to determine if it is worth replacing the standard issue focuser with another brand to better suit the needs at hand.

 

This might also be used to determine the size of the fully illuminated field when using a 2” diagonal vs. a 1-1/4” diagonal as well.

[Jeff B]

I suppose you could do that Letterman.  Choose a green laser.  The trick would be to get it precisely parallel to the axis of the lens.

 

Honestly, I have removed the focuser draw tube baffles to make sure I got the full aperture with a two inch diagonal.  For the DPAC images, I also swapped out the stock focuser for an identical one but with the draw tube about an inch or so shorter.  

 

I'm working on the design for making it bino-friendly and the preliminary numbers are very interesting. The short answer is I can do it but I have to be very specific in choosing my viewer, diagonal and, especially, the focuser/backplate.

 

Jeff

[Rick Runcie]

I would be curious to see if it is even noticeable the difference between two
120s set up next to each other, light gathering wise.

One with the shorter focuser and removed baffle etc. and one without any modifications. Just to see if it is really that much of a benefit cutting the tube etc.

I understand the advantages of a two speed focuser, but my Jaegers 5" f-5 refractor with single speed focuser does very well without it. I only use lower magnifications on it so I really dont see it being that much of an advantage at lower powers.

Of course my larger refractors all have 2speed focusers, but I mainly use them at much higher magnifications.

Best regards, Richard

[Bomber Bob]

I keep it simple:  Draw the light cone on a piece of graph paper, and take my measurements from that.  On a pre-built scope, I determine how wide the focus tube needs to be at its typical distance (with diagonal), and remove any internal baffles as needed.  Chopping off the focus tube?  Yes, if it is vignetting the field -- especially on an RFT.

[Jeff B]

I keep it simple:  Draw the light cone on a piece of graph paper, and take my measurements from that.  On a pre-built scope, I determine how wide the focus tube needs to be at its typical distance (with diagonal), and remove any internal baffles as needed.  Chopping off the focus tube?  Yes, if it is vignetting the field -- especially on an RFT.

Which is exactly what I did today BB.  The focal lengths of the two samples I have on hand were identical at 583mm from the back surface of the aft element (R4).  The length of the stock 2" draw tube is 156mm and the optical path length of my AP 2" diagonal is 105mm.  Just doing the Y=MX+ B calculating thing to a point focus, shows the inlet ID of the stock focuser tube does indeed clip the aperture to about 114mm, something I confirmed visually using my newtonian sight cap.

 

Jeff

[Orion68]

Interesting thread.

 

So, is the consensus this?

 

- the ST120 light cone is only affected if the eyepiece or "eyepiece+barlow" being used comes to focus within the first 1" of focuser travel.

 

I don't think I have any eyepieces that fit this situation. Need to check on my "eyepiece+barlow" results.