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Questions about autocollimator (esp. 1.25” variety)

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#1 Henry from NZ

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Posted 07 December 2017 - 02:36 PM

I would like to consider an autocollimator to perfect the collimation of my Takahashi e130d

There is no easy way to use 2” accessories with the e130d unless one buys an expensive custom adaptor. I wonder how much a disadvantage is with a 1.25” AC if things are pretty close to begin with?

If 1.25” is feasible, what would be a good one? I know Cateye is probably the best or most well known brand but it only comes in 2”

If a use a 1.25” to 2” adaptor so that a 2” AC will fit physically (but obviously not getting the full view) is it going to work?

I do not want to open up the OTA at this stage to put a new centre mark. Is it possible to use the circular ring center mark with AC effectively? Specifically how difficult would it be to discern the different reflections?

Any other practical tips specific to the Epsilon?

#2 nirvanix

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Posted 08 December 2017 - 08:54 AM

I use a 1.25 inch AC from Astrosystems. I feel its well made and does the job. There's been lots of discussion at CN about how useful an AC really is after already collimating with a chesire. Personally I think it helps tweak a little more out of the scope for high mag lunar/planetary views.



#3 kingjamez

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Posted 08 December 2017 - 09:01 AM

I've found that for an autocollimator to be most effective, the mirror needs to be at or near the focal plane. If you can do that with the 1.25-2" adapter, I think it will work just fine. 

 

The more narrow field of view won't matter much if you are already close. I think you'll still be able to do CDP without issue.

 

-jim



#4 Starman1

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Posted 10 December 2017 - 05:12 PM

Realistically, you'll have trouble getting an accurately-made 1.25" autocollimator.

You might find a used Tectron that would be accurate.

With the other brands, you might have to buy several of them to get one that is sufficiently accurate.

 

They're simple to test:

--once collimated perfectly per the AC, loosen the set screw, rotate the tool 90° and retighten the set screw.

The scope should still show as perfectly collimated.  If it's not, the AC is insufficiently accurate to use as a collimation tool.

 

If you plan to use an adapter, you have the additional complexity of testing the accuracy of the adapter, which can be fairly easily done using a Cheshire,

rotating the adapter only and retightening, and seeing if collimation changes.  If it does, the adapter is insufficiently accurate to use with collimation tools.

 

Finding an accurate adapter will prove difficult, especially a 2" to 1.25" step-down adapter.

 

You might be better off buying several of the brand you pick and testing them all and keeping the one that's accurate.



#5 kingjamez

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Posted 10 December 2017 - 05:25 PM

They're simple to test:
--once collimated perfectly per the AC, loosen the set screw, rotate the tool 90° and retighten the set screw.
The scope should still show as perfectly collimated. If it's not, the AC is insufficiently accurate to use as a collImating tool.

I'd modify this to say don't tighten down the set screw at all. Put the AC in and turn it to see if the reading changes. The setscrew's differential pressure is often enough to throw off A perfectly good AC if you perform the above test.

-Jim

Edited by kingjamez, 10 December 2017 - 05:25 PM.


#6 Starman1

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Posted 10 December 2017 - 05:31 PM

There is no AC-to-focuser fit tight enough to do this.  Not tightening the setscrew at least a bit on the AC will allow the weight of the AC tool itself to cause it to become crooked in the focuser.

Will ALL collimation tools, you MUST tighten the screw or the tool will be misaligned in the focuser.

The exception would be a tool that fits so tightly you have to turn it to get it in and where tightening the setscrew makes no difference at all.

In 54 years, I've only seen two tools that tight.

Otherwise, the tightening of the set screw is essential to registering the tool in the focuser.

 

Technique?

Press the center of the tool to make sure the top of the tool is pressing on the top of the focuser all the way around, then lightly tighten the setscrew on the tool.

Only enough pressure to register the tool against the drawtube wall.


Edited by Starman1, 10 December 2017 - 05:32 PM.


#7 kingjamez

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Posted 10 December 2017 - 05:55 PM

Our experiences differ I guess. I let the top of my Infinity XLKP (or whatever the letters are) index against the top of the focuser. When doing tests like evaluating an AC, of course you'd have the focuser vertical so that gravity keeps it indexed.

For the general case when not doing critical measurements and the focuser isn't vertical, sure give the setscrew a small turn.

#8 Old Dinosaur

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Posted 10 December 2017 - 09:03 PM

There's some pretty good self centering draw tube adapters around that are nice for things like the autocollimator.

Many draw tube adapters have varying bore sizes.

I use the same one all the time for mine, a wrap of .005" shim stock makes for a nice smooth fit without using locking screws.



#9 Starman1

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Posted 11 December 2017 - 11:39 AM

Our experiences differ I guess. I let the top of my Infinity XLKP (or whatever the letters are) index against the top of the focuser. When doing tests like evaluating an AC, of course you'd have the focuser vertical so that gravity keeps it indexed.

For the general case when not doing critical measurements and the focuser isn't vertical, sure give the setscrew a small turn.

I've never seen a vertical focuser in a dob.  Most are horizontal to some degrees above the horizontal, but it still boils down to the tightness of fit in the focuser.

If it's tight enough, you don't have to tighten the thumb screw.  It's just really rare to have any tool fit that tightly.



#10 kingjamez

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Posted 11 December 2017 - 12:07 PM

 

Our experiences differ I guess. I let the top of my Infinity XLKP (or whatever the letters are) index against the top of the focuser. When doing tests like evaluating an AC, of course you'd have the focuser vertical so that gravity keeps it indexed.

For the general case when not doing critical measurements and the focuser isn't vertical, sure give the setscrew a small turn.

I've never seen a vertical focuser in a dob.  Most are horizontal to some degrees above the horizontal, but it still boils down to the tightness of fit in the focuser.

If it's tight enough, you don't have to tighten the thumb screw.  It's just really rare to have any tool fit that tightly.

 

Right, me either. We're talking about a Takahashi e130d in this thread.

 

-Jim



#11 kingjamez

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Posted 11 December 2017 - 12:09 PM

I would like to consider an autocollimator to perfect the collimation of my Takahashi e130d

There is no easy way to use 2” accessories with the e130d unless one buys an expensive custom adaptor. I wonder how much a disadvantage is with a 1.25” AC if things are pretty close to begin with?

If 1.25” is feasible, what would be a good one? I know Cateye is probably the best or most well known brand but it only comes in 2”

If a use a 1.25” to 2” adaptor so that a 2” AC will fit physically (but obviously not getting the full view) is it going to work?

I do not want to open up the OTA at this stage to put a new centre mark. Is it possible to use the circular ring center mark with AC effectively? Specifically how difficult would it be to discern the different reflections?

Any other practical tips specific to the Epsilon?

To answer another question from the OP. I tried to use a standard circular ring "paper donut" with my AC. It works, but it's more difficult to detect P3 since you can't tell if it's inverted with a paper ring. In that case just look for the reflection that doesn't move when you adjust the primary mirror. I used a flashlight pointed down the tube to illuminate the donut and P3 was far more visible. So yes, you can use a paper donut, but it's slightly harder than something like a hotspot.

 

-Jim


Edited by kingjamez, 11 December 2017 - 12:10 PM.


#12 Henry from NZ

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Posted 11 December 2017 - 04:14 PM

Thank you for all the input. I have placed an ad for a tectron unit. In the meantime I think there may be an off the shelf adaptor that will let my 130d take 2” so I may be able to use Cateye after all.


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