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Collimating an 8" Meade

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#1 mantrain

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Posted 12 July 2018 - 05:42 PM

Is it possible to get a good collimation at home during the day by that method where you look into the OTA to see that everything is rounded, etc?  IE you look at the primary reflection around the secondary, to see that the primary region is equidistant all around? 



#2 carolinaskies

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Posted 12 July 2018 - 05:52 PM

A daytime adjustment can be done with an artificial star if you're talking an SCT.  You can make your own star using a flashlight and drilling graduated holes in it and focusing and adjusting the secondary.   Any adjustment is best star tested at night and tweaked as necessary.  

For an 8" Newtonian a laser collimater makes it simple. 

 



#3 mantrain

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Posted 12 July 2018 - 06:02 PM

I have an artificial star but, I had trouble with that method. Doesn't it need to be like 6 x the FL? 



#4 carolinaskies

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Posted 12 July 2018 - 06:25 PM

You need to have a small enough star size and may need to even dim down the star to get a good read on collimation. 



#5 n2dpsky

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Posted 12 July 2018 - 06:40 PM

Distance is important for evaluating optics only.  An artificial star that is too close will cause you to see spherical aberration, so know that going in and ignore the rings and just center the secondary shadow (on both sides).   It only needs to be far enough away to be able to focus.  

 

Don't look at SC optics the way you do newtonians.  They don't need to be setup before collimation.  Your mirrors are mechanically centered by the hole in the corrector plate/primary.  You can only adjust tilt, which is best done on a medium bright star at relatively high power (6 or 7mm would suffice).   



#6 mantrain

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Posted 12 July 2018 - 07:12 PM

Is it reasonable to go out at night an collimate on site, at theeveningg when you are going to observe?



#7 n2dpsky

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Posted 12 July 2018 - 07:27 PM

Sure.  I do it all the time.  Every single time a scope is transported it could go out a bit and touching it up is something I'm always prepared to do once it's on the tripod.   SC's are pretty good about holding collimation, but they need to be tweaked periodically.   This is a 2-3 min process max.   

 

After I align, I find a good star, center it, defocus and see what it looks like.   If the shadow of the secondary is not centered, I hold my index finger in front of the aperture next to each screw as I look through the scope.  That helps tell me which screw I need to turn to move the secondary.  It won't tell you direction, but when you start turning, you can see if the shadow is going to center or not.  If not, turn the other way.   Re-center the star after each adjustment.  Just take it slow and pay attention to what's happening when you make adjustments.   After you've done this 4 or 5 times, this will be a 1-2 min exercise.  Make sure you look at both sides of focus.   

 

I want to point out that these are small adjustments and should not require multiple turns of the screw.  Often, you're only turning the screw 90 degrees at a time, if that.   I've heard of people backing out all the screws until the secondary drops (onto your primary).  You don't want that and there is nothing holding the secondary mirror captive except those three screws.     It's pretty hard to back them out enough to disengage but not impossible, so just understand that.   The screws themselves are quite long and shouldn't disengage during normal collimation adjustments.  


Edited by n2dpsky, 12 July 2018 - 07:33 PM.

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#8 carolinaskies

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Posted 12 July 2018 - 07:29 PM

Yes you can collimate onsite. Depending on adjustments necessary it may only take a few minutes.  Getting use to the procedure is the more involved part.  If you read through some of the online sources there are a few tricks to help ID which screw to turn, etc.  


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#9 Bill Barlow

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Posted 12 July 2018 - 08:56 PM

Is it possible to get a good collimation at home during the day by that method where you look into the OTA to see that everything is rounded, etc?  IE you look at the primary reflection around the secondary, to see that the primary region is equidistant all around? 

That method wouldmhelp to see if the primary and secondary mirror remained.  But for collimation, use a star about 45 degrees above the horizon and adjust the secondary mirror if needed when the seeing is decent.

 

Bill



#10 mantrain

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Posted 13 July 2018 - 04:59 PM

I have the Meade 8", it has three inner and three outer adjustment screws. Does anyone know what the diff is bw the inner and outer? thank you!



#11 n2dpsky

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Posted 13 July 2018 - 05:15 PM

That's the new type of holder.  The inner 3 are supposed to be collimation and outer 3 are from the secondary holder assembly (don't touch those).

 

Here is the LX600 manual, which shows that type of holder on page 46.  Make sure the photo matches your scope.

 

https://www.meade.co...ctionManual.pdf


Edited by n2dpsky, 13 July 2018 - 05:16 PM.



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