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How critical is focuser alignment

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#1 Nippon

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Posted 18 September 2019 - 09:09 PM

I have a 4" f/7 triplet. It performs well but I have determined the focuser is slightly out of alignment. It does have provision for adjustment. Is it worth it when the misalignment is very slight?

Thanks, Paul

 

 


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#2 J.LAMBIE

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Posted 18 September 2019 - 10:43 PM

If you are only using it for low power, probably not. But you really notice it at higher mags. Another thing you can do is manipulate the diagonal. I've got an f5 120mm that I can take to to 250x on double stars if the diagonal is properly adjusted. Check page 3 of this PDF. 

 

http://philjay2000.t.../adventures.pdf

 

The useful bit is the images for good and poor collimation. You need a cheshire. With the front lens cap ON the refractor, point the cheshire at the sun so that the silver disc is fully illuminated. When you look through it, you'll get a version of those images. If it's not good, I loosen the the locking screws on the diagonal, and slide it in or out, wiggle it side to side until I get the 'good collimation' picture. The tighten the screws and Bob's your uncle. Makes a huge difference. I get a crisp Saturn w/Cassini and central cloud band at 240x and, on nights of 4 to 5 seeing, split doubles as close as 1.5"

 

This is the cheshire I use with the wire cross hair retaining ring removed. 

 

https://smile.amazon...68864557&sr=8-2



#3 Nippon

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Posted 19 September 2019 - 12:23 AM

I have a similar Cheshire and that is how I discovered it is very slightly off. I use the scope at high power a lot and it is quite sharp. I checked it without the diagonal. It's so close just putting a tiny bit of pressure on the Cheshire in the right direction puts it dead on and that's why I'm wondering if it's worth messing with.



#4 jp071848

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Posted 19 September 2019 - 06:52 AM

I have a 4" f/7 triplet. It performs well but I have determined the focuser is slightly out of alignment. It does have provision for adjustment. Is it worth it when the misalignment is very slight?

Thanks, Paul

Hi Paul...

 

I can sympathize, I struggled for far longer than I should have with a dodgy focuser on my very early model ES127, and only recently gave up on it and went Moonlite. The new CFL has TWO different focuser collimation systems built in and was a breeze to setup and align.

 

I take solace from a passage in Suiter's fantastic "Star Testing Astronomical Telescopes", where he has devoted an entire Chapter 6 to "Misalignment". He has a graphic, Figure 6-14, where he shows a representation of two Cheshire views through a 6" f/12 APO. One image shows perfect alignment with concentric rings, the other shows a clearly somewhat misaligned optic, probably worse than what you are seeing and very clearly off. He then states " Neither alignment gave noticeably different images at the eyepiece". I don't know if the fact that he is using an f/12 as his example, and yours is an f/7 (mine is an f/7.9) impacts his message, but I know on my ES127, I have my alignment better than what he shows as his misaligned example, but NOT perfect, and I still get pinpoint stars, and high power, even with my bit of misalignment.



#5 Nippon

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Posted 19 September 2019 - 07:51 AM

Thanks I do get pinpoint stars and star test of the scope is excellent and does not show any misalignment. I'll probably adjust it to satisfy my OCD personality. The focuser is a stock 2.5" Stellarvue and is a very good focuser with simple straight forward adjustments. This is just the battle between my two key personality flaws, OCD and profound laziness:)


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#6 MalVeauX

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Posted 19 September 2019 - 07:54 AM

Heya,

 

If your stars are pin point away from the center at high power, then just leave it well be. If it was truly not collimated, those stars would not be round and the airy disc pattern would show oblong shapes. 

 

Very best,



#7 BravoFoxtrot

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Posted 19 September 2019 - 06:36 PM

If the performance is acceptable to you, it may be advisable to leave it as is.  

 

With that being said...knowing that my optical system was out of alignment would drive me absolutely nuts (think Edgar Allen Poe "Tell Tale Heart").  I'd fix it or have it fixed.  

 

Caveat:  I completely understand that my post provided almost no value and likely made your opinion of the situation worse.  smile.gif



#8 Nippon

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Posted 20 September 2019 - 09:06 AM

If the performance is acceptable to you, it may be advisable to leave it as is.  

 

With that being said...knowing that my optical system was out of alignment would drive me absolutely nuts (think Edgar Allen Poe "Tell Tale Heart").  I'd fix it or have it fixed.  

 

Caveat:  I completely understand that my post provided almost no value and likely made your opinion of the situation worse.  smile.gif

Nah. My OCD won and I aligned it. It was quite simple actually and I was rewarded with the same stunning performance the scope was giving before:)


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#9 Scott in NC

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Posted 20 September 2019 - 10:15 PM

:funny:

 

I would have done the same thing. :grin:




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