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Struggling with upper/lower/SIPS/focuser distance

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#1 kb58

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Posted 19 September 2020 - 10:32 AM

I built a 16" f/4.3 Newt hexapod and have been chasing my tail regarding primary-to-focuser distance. I know that focus is achieved at 16 x 4.3" from the primary, and that's been set, but then I ran into unexpected difficulties because I have a SIPS coma corrector. I know that by itself, it has a focal effect of 15% (don't recall whether it's 'shortening" or "stretching" the focus). I wondered if it's shortening or stretching the entire focal distance, or just the portion between the SIPS and the eyepiece, but realized that it doesn't matter, it just has to be wherever it has to be.

 

The SIPS installation instructions say to rack the focuser all the way in, remove the eyepiece, and put tape across the top of the tube. Then, you point the scope at a bright star and adjust the threaded focuser assembly in and out to bring the image on the tape into focus. At that point you're done, supposedly, but I must be doing something wrong. After the SIPS image is in focus and I add an eyepiece, I can't achieve focus - the focuser hits the end of travel. If I screw the focuser assembly in/out to allow focus to be achieved via the eyepiece, when I recheck the SIPS placement, it's now wrong.

 

Is this even possible? Since I've not seen anyone else post about having this issue, I assume that I'm doing something boneheaded. Once the SIPS is focused per their instructions, does that implicitly mean that any eyepiece will then be able to be focused?


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#2 Mike Lockwood

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Posted 19 September 2020 - 11:07 AM

If you follow the procedure, most TeleVue eyepieces (YMMV on other brands of EP) will come to focus within the 1.5" range of the focuser on the SIPS.  There is at least one (55mm Plossl) that may need some extra out-travel which is easily arranged.  Don't worry about the 15%, it doesn't matter for the setup, just follow the instructions.

 

Just for clarity, you should put tape across the top of the plastic focusing/setup aid that comes with the SIPS, not the top of the focuser.

 

If you have a strong prescription for your glasses, a couple of very wide-field eyepieces may not quite come to focus due to limited in-travel, but I believe this has been partly addressed in newer SIPS, and adjusting the unit slightly inward to fix this is not a big deal.  Seems like only a millimeter or two is required.

 

I don't wear glasses, so for me everything works, and I simply put a parfocalizing ring on my 55mm Plossl to push its position out.  (Now that it's converted to 67mm for nightvision, it requires an extension tube, but again this is easy.

 

So yes, once it is positioned properly, everything should focus.  Be mindful with 1.25" eyepieces that need adapters.  Some adapters have different heights than others.

 

What eyepiece are you having issues with?



#3 MitchAlsup

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Posted 19 September 2020 - 01:54 PM

You need to understand the optical relationship the Paracorr 2 adds to the system.

 

Secondary2.jpg

 

The primary mirror would form a focal plane as illustrated in the figure (Newtonian Focus).

 

The Paracorr adds negative coma and forms a focal plane 57mm farther out (Paracorr Focus) from the 1.15× multiplication factor..

 

SIPS was designed so that when fully racked in the top of the draw tube is at that 57mm position (Paracorr focus) so scotch tape and a star can be used to set the position of the P2.

Alternately one can built a 2" barrel with scotch tape and set the tube lengths so that primary focus occurs at 3" + 9mm (+ margin) away from the central column of the primary. 



#4 kb58

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Posted 19 September 2020 - 02:56 PM

Thanks for the help. For clarification, yes, the tape was placed across the plastic insert included with the SIPS, and yes the focuser tube was fully racked in. At the moment, I run out of outward focuser assembly thread to reach SIPS focus (with the focuser tube fully racked in as recommended), but eyepieces focus is fine. What's unclear to me is how detrimental a slighly out of focus SIPS is to overall scope operation when the eyepieces focus fine. Anyway, since it's a hexapod, the rod-ends can be backed out a bit to reach SIPS focus, then I'll see where eyepiece focus ends up.

 

This whole focus range issue has sure gotten the best of me, with several iterations of tube inserts ending up too long or too short—more than once. Grrr.

 

FWIW, I looked at both Mars and the Orion nebula last night and had the opposite than expected experience. Mars was something of a blurry blob with just vague detail of a darker center area (guess I've looked at too many Hubble pics). OTOH, Orion was freakin awesome; there was a lot more dust cloud detail than expected. Keep in mind that I've looked through very few scopes in my life so I don't have a good frame of reference on what to expect, especially for Mars. Guess I should set up the Takahashi and compare.


Edited by kb58, 19 September 2020 - 03:16 PM.


#5 a__l

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Posted 20 September 2020 - 05:17 PM

You didn’t answer what eyepieces you have problems with?
If you have severe myopia, there may be problems. Nagler 31, Ethos 21 and Ethos 17. C ES 20 100 is a problem even without myopia. The problem is not solved, except to look with glasses. Which is usually not applicable to Ethos.  For paracorr-2 this  position "A".
There is another problem in the other direction. For SIPS, this is not enough travel length for a 1.5" focuser. This TV 55 or TV67. In paracorr 2 it is solved by adding an expansion ring 60 mm long (for TV 67 position "H").
Use paracorr-2 as a result, it will be more convenient. Although at the beginning it seems the opposite. Even if you don't have myopia, someone will come up to see from friends.


Edited by a__l, 20 September 2020 - 05:20 PM.


#6 Kunama

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Posted 20 September 2020 - 05:35 PM

Kurt, it sounds like you need to extend the poles to bring the SIPS optics to the correct distance from the primary.  I found that 1mm either side was still ok for coma correction but any more than that wasn't ideal.  I assume your system doesn't allow for you to lower the primary a bit to achieve the same result....

 

Once the SIPS is at the correct distance all my Delos and Panoptic come to focus with a bit to spare.  I guess Televue build the Paracorr to suit their own eyepiece lineup, seems people have issues when using some other brands of eyepieces.  SIPS doesn't suit people with severe myopic vision.


Edited by Kunama, 20 September 2020 - 05:48 PM.


#7 kb58

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Posted 02 October 2020 - 01:46 PM

A last post for closure. I pretty much figured out what was going on, and that was not keeping track of which eyepieces I was testing with. The eyepieces are a mixture of different Tele Vue models. The last number is the field stop, "F", from the Tele Vue data table (https://www.televue....page.asp?id=214):

 

31mm Nagler (T5), -0.38

21mm Ethos, -0.38

14mm Delos, -0.23

10mm Delos, 0.25

6mm Ethos, 0.7

 

All the eyepieces focus within the focuser's range, except for the Delos 14mm. Its focus is so far in that it hits the mechanical stop before it gets there, and but based on the numbers, this shouldn't happen. I checked about three times to make sure I wasn't mixing myself up, and got the same result every time. The only thing I can think of is that I was sold a defective (used) eyepiece, because it should be mid-back and is anything but. It really threw me off because I wasn't keeping track of what eyepiece focused where.

 

It's a shame because the eyepiece is in perfect condition, no dents, dings, and doesn't rattle.

 

[Edit] see below.


Edited by kb58, 02 October 2020 - 04:14 PM.


#8 MitchAlsup

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Posted 02 October 2020 - 03:07 PM

A last post for closure. I pretty much figured out what was going on, and that was not keeping track of which eyepieces I was testing with. The eyepieces are a mixture of different Tele Vue models. The last number is the field stop, "F", from the Tele Vue data table (https://www.televue....page.asp?id=214):

 

31mm Nagler (T5), -0.38

21mm Ethos, -0.38

14mm Delos, -0.23

10mm Delos, 0.25

6mm Ethos, 0.7

 

All the eyepieces focus within the focuser's range, except for the Delos 14mm. Its focus is so far in that it hits the mechanical stop before it gets there, and but based on the numbers, this shouldn't happen. I checked about three times to make sure I wasn't mixing myself up, and got the same result every time. The only thing I can think of is that I was sold a defective (used) eyepiece, because it should be mid-back and is anything but. It really threw me off because I wasn't keeping track of what eyepiece focused where.

A 2× Barlow and a 31Nt5 pretty much obviate the need for the 14D



#9 Mike Lockwood

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Posted 02 October 2020 - 03:16 PM

Is there a tall 1.25" adapter being used with the 14mm Delos? 

 

Or is there something on the eyepiece that prevents it from going into the focuser all the way?

 

That's all that I can think of.  It should focus.



#10 ad701xx

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Posted 02 October 2020 - 03:57 PM

Is there a tall 1.25" adapter being used with the 14mm Delos? 

 

Or is there something on the eyepiece that prevents it from going into the focuser all the way?

 

That's all that I can think of.  It should focus.

This is probably what is going on. I had the same issue with my 17.3mm Delos which has it's focal plane in the same spot as the 14mm.

 

I ended up getting a Baader 1 1/4" adapter for it and my UO Orthos which also came up just short of reaching focus. Now they all work fine.

 

https://www.baader-p...-part-15a).html



#11 kb58

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Posted 02 October 2020 - 04:16 PM

Hmm, very good point, and yes there is a Tele Vue 1.25" adapter used with it that is about 1/2" tall. The 10mm Delos uses the same adapter, but because its field stop is about 1/2" in the opposite direction, it works perfectly.

 

That's a little surprising about the adapter, because it could have easily been made lower profile by maybe half. Oh well, I learned something today, thank you!



#12 ad701xx

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Posted 02 October 2020 - 04:30 PM

Televue even sells an in-travel adapter for the 14 and 17.3mm Delos so they can be parfocal with the other eyepieces in that line. I went with the Baader, though, because I exclusively use 2" filters for visual and NV.

 

https://www.televue....id=61&Tab=_parf


Edited by ad701xx, 02 October 2020 - 04:31 PM.


#13 Mike Lockwood

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Posted 02 October 2020 - 07:56 PM

Hmm, very good point, and yes there is a Tele Vue 1.25" adapter used with it that is about 1/2" tall. The 10mm Delos uses the same adapter, but because its field stop is about 1/2" in the opposite direction, it works perfectly.

 

That's a little surprising about the adapter, because it could have easily been made lower profile by maybe half. Oh well, I learned something today, thank you!

The mystery is solved!  Excellent.

 

The moral of the story is:  Always pay attention to eyepiece adapters for use with the SIPS (for focusing) and also the drop-in Paracorr (for best performance).

 

FYI, I normally use the Feathertouch 1.25" adapter with my SIPS, both the "Easy Grip" or the "Flush" adapter puts 1.25" eyepieces at almost the same height as 2" eyepieces.



#14 kb58

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Posted 04 October 2020 - 10:37 AM

After learning that the issue was the eyepiece adapter, I thought that was the end of this... not yet.

 

The strut rod ends were backed out to make them longer, and the focuser/SIPS was unscrewed outward as far as possible, and now the SIPS just barely achieves focus on its fully-seated SIPS adapter. When using the 31mm Nagler eyepiece, however, the focuser had to be racked all the way in until it hit its stop, just short of focus. This is part of the problem I found earlier but thought it was the 1.25" adapter.

 

So right now things seem stuck. I'd like to achieve eyepiece focus with the focuser housing not being unscrewed so far out (it's hanging on by only a few threads). To do so means making the struts longer, but if I do, the Nagler focus will only get worse. The dilemma is that the SIPS needs a longer focus path, and the Nagler needs a shorter focus path. I'm very puzzled why no one else is running into this and assume it's something I'm doing, but I can't see what it is.



#15 Mike Lockwood

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Posted 04 October 2020 - 12:10 PM

If you make the truss poles longer, you can screw the SIPS assembly back in where it should be and you'll still have the same relative focus position for each eyepiece.

 

If you set the SIPS so the 31mm and 21mm just focus with nearly max in-travel of the focuser, you will be close enough.



#16 ad701xx

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Posted 04 October 2020 - 07:20 PM

It sounds like you are pretty close. Rather than lengthening the struts, how about making a shim for underneath the mounting plate?


Edited by ad701xx, 04 October 2020 - 07:20 PM.


#17 kb58

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Posted 04 October 2020 - 08:45 PM

Thanks, Mike, I was hoping that you'd reply. Since the focuser tube has about 1" of thread, and with it hanging way out at the end of travel now, it'll be moved inboard about 0.75". This moves the CG in and also allows unscrewing the SIPS lens from the inside of the tube without removing the entire focuser tube assembly from the outside (the upper assembly is thick around the focuser).



#18 kb58

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Posted 04 October 2020 - 08:47 PM

It sounds like you are pretty close. Rather than lengthening the struts, how about making a shim for underneath the mounting plate?

Understood, but I'm going to use the opportunity to also lengthen the threaded inserts on one end of all the struts, which also allows screwing all the rod ends in further. Right now they're a bit overextended as part of debugging the issue.


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