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New (pre-owned) Teeter: questions about secondary

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#1 tannehill

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Posted 16 June 2013 - 02:07 PM

I acquired a beautiful 18" Teeter recently. A smart looking Obsession-on-steroids scope. Zambuto mirror.

This is first dob I've owned with 4 screws for secondary alignment.

I've long struggled with secondary adjustments. If the screw spring tension is loose, the laser beam on mirror surface (or AC image at focuser) moves obediently but I always worry the tension is too loose and it will wiggle with altitude changes. Too tight, and when I tighten a screw, it's hard, the image moves due to my force on the secondary assembly, but recoils after release and often it hasn't moved at all, or worse yet seems to move aberrantly.

This issue seems more pronounced with this 4-screw secondary. Each screw has an opposing screw, rather than three offset adjustments with a tripod like force balance.

I'm sure I'm not the only one who fights this. I asked this once years ago on a different forum, about my struggles with tightness on my three-screw secondry, and the replies included a few kilobyte-wasting suggestions of "the tension needs to be just right". Ah. Okay, s'all clear now...

This Teeter is a high end scope. All the secondary elements, spring, etc seem in appropriate near new condition. The scope holds collimation well during use. I use a glatter laser then Catseye tools and the two-hole auto collimation.

What are your opinions and experience on tension and technique regarding the secondary adjustment screws? Specifically, what screw tension is too loose, or too tight? How would you communicate that in written form?

Thx

Scott

#2 Fred1

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Posted 16 June 2013 - 02:38 PM

I don't have springs in my Four-Screw-Adjusting-Secondary, but I just keep them "finger tight snug." Adjusting them is simple enough and with a little practice I was able to find just the right "finger tight snugness" to obtain a quick spot-on collimation with my Glatter 2" laser. Tweaking further with my CatsEye XLK Autocollimator often isn't necessary as long as I get the focuser thumb screw torqued properly so as not to unstack the images in the autocollimator.

If your "scope holds collimation well during use" it seems you got it right. I check my collimation at least once during an observing session and it rarely needs more than just a tweak after several hours of slewing around the sky. I hope this helps.

EDIT: I just remembered that there's a thread on the Teeter Yahoo Group about collimation drift that, I believe, was started several months ago. You might consider checking further there.

EDIT: found it Teeter Collimation Drift Thread

#3 Pinbout

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Posted 16 June 2013 - 04:39 PM

Where do you have a spring?

#4 Starman1

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Posted 16 June 2013 - 06:02 PM

Where do you have a spring?

The advantage of four screws is that you can move the secondary in 8 directions--the four screws, and the 2:2 directions halfway between.
I find it much easier to adjust than a 3 screw arrangement.

As for tension, I tighten the screw until it takes quite a bit of force to turn it, but not so tight I cannot turn it if I need to. Therefore, the screws are not loose, and to even unscrew one requires a bit of force. But if I have to tighten it another 1/16 turn, I can.

What spring?

#5 Fred1

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Posted 16 June 2013 - 07:59 PM

Where do you have a spring?



What spring?


I don't know what the OP has but one of my club buddies has an older 18" Obsession that has a secondary holder with a 3-Point system and tension springs. I think the manufacturer is no longer in business and I don't recall the name that he told me. He believes that that 3-Point system was a much better design than the new 4 screw design that Obsession and probably most every other Dob builder now provides.

#6 JMW

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Posted 17 June 2013 - 12:07 AM

I just had my first experience with 4 knobs on the secondary. It's a Webster D14. The mirror movement is much easier than the 3 knobs on our Obsession 20 f5. I had to read the instructions to know to loosen with one hand while tightening the opposite knob with the other hand.

#7 Pinbout

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Posted 17 June 2013 - 06:10 AM

Where do you have a spring?

The advantage of four screws is that you can move the secondary in 8 directions--the four screws, and the 2:2 directions halfway between.
I find it much easier to adjust than a 3 screw arrangement.

As for tension, I tighten the screw until it takes quite a bit of force to turn it, but not so tight I cannot turn it if I need to. Therefore, the screws are not loose, and to even unscrew one requires a bit of force. But if I have to tighten it another 1/16 turn, I can.

What spring?


I guess I shouldn't drink and type... :lol:

no this is why I asked it...

This Teeter is a high end scope. All the secondary elements, spring, etc seem in appropriate near new condition. The scope holds collimation well during use. I use a glatter laser then Catseye tools and the two-hole auto collimation.



what spring?

#8 Fred1

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Posted 17 June 2013 - 09:11 AM

I just had my first experience with 4 knobs on the secondary. It's a Webster D14. The mirror movement is much easier than the 3 knobs on our Obsession 20 f5. I had to read the instructions to know to loosen with one hand while tightening the opposite knob with the other hand.


Yeah, my apologies to the OP. I forgot to mention about loosen one while tightening the other in my earlier post. It's just such a natural thing to do for me now that I've been using it for almost 2 years that I just never think about it. It's become second nature when collimating.

#9 tannehill

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Posted 17 June 2013 - 10:52 AM

My apologies, there is no spring. I was typing too fast and meant to say the hardware was all in top shape; threads, nuts, washers, etc.
I've experimented with the 4 screw adjustments. If there is some instruction sheet, I'd love to see it! I'm sure it's just my inexperience with this.
S

#10 Pinbout

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Posted 17 June 2013 - 11:19 AM

My apologies, there is no spring.



so whose on first? :lol:

#11 dscarpa

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Posted 17 June 2013 - 02:48 PM

Sounds like quite the scope! Aligning the the primarys of my long gone newts was easy, it was the the 3 screw spiders that drove me bats. I was considering the 4 vane 4 screw spiders Teeter uses. Rob told me the 3 vane 3 screw spiders he uses on the STS aren't hard to adjust and hold collimation well so I decided to go with it. David

#12 ausastronomer

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Posted 17 June 2013 - 10:28 PM

Hi Scott,

Good to see you still observing :jump:

When I hadn't seen you on the forums in years I figured that downunda observing trip to Australia/Coonabarabran you made in October 2007 with Dave Kriege, Bob Rose and Don Wyman, plus others, had caused permanent irrepairable damage to your retina. This is a common medical condition suffered when observing in Australia, due to the very dark skies and the multitude of very bright targets :grin:

I have 3 newtonians. The 18" has a 4 screw Astrosystems secondary holder and the 10" & 14" scopes have the Protostar heated secondary holders which have 3 screws.

I find adjustment of them all to be fairly similar, regardless of the number of adjustment screws. That having been said it becomes intuative when you have been doing it a long time.

It's difficult to explain in words :confused:

I could explain it all in about 15 seconds flat standing at the telescope. Regardless of the number of screws they all need to be under moderate and reasonably equal load. You do this by feel. They need to be slightly firm to tighten further, but you do not want them too tight as this may cause permanent warping of the plate. They need to be tight enough so there is a slight mild resistance when you go to loosen one of the screws. You need to play with them all tightening one loosening another until you get this about right. Once you get it right its pretty easy. You will usually only need minor tweaks each time you set up the scope.

Maybe you need to come back to Coona in March next year with Kreige and Bob Rose and I can give a practical demo :lol:

Cheers

#13 dscarpa

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Posted 18 June 2013 - 11:42 AM

Thank for the tips! David

#14 tannehill

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Posted 22 June 2013 - 09:29 PM

Thanks John! Yes, indeed, a year in Oz did spoil me. Took me couple years for the post-antipodean doldrums to fade. After Eta Carina and 47 Tuc....what's M13 and M51, anyway?

I will dabble with this secondary. I wonder if its already damaged........

Scott






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