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how often do you collimate your SCT?

collimation equipment SCT astrophotography
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#26 Axunator

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Posted 14 December 2018 - 10:16 AM

I have a suspicion that frequent tweaks to collimation become a self-fulfilling prophecy because tweaking it makes things just a tad more likely to loosen up a bit.

Well... maybe your comment was made in jest, but if the collimation is not off, why to tweak? If it is off, why to keep using a scope that is not well collimated when it could be tweaked just right? shrug.gif

 

Last thing inexperienced beginners reading these forums is a comment that make them even more afraid of collimating than they are to begin with... flowerred.gif


Edited by Axunator, 14 December 2018 - 10:21 AM.


#27 bumm

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Posted 14 December 2018 - 01:02 PM

Well... maybe your comment was made in jest, but if the collimation is not off, why to tweak? If it is off, why to keep using a scope that is not well collimated when it could be tweaked just right? shrug.gif

 

Last thing inexperienced beginners reading these forums is a comment that make them even more afraid of collimating than they are to begin with... flowerred.gif

I think a lot of the problem is that the screws have to be reasonably snug to hold collimation.

                                                                                                                          Marty



#28 Axunator

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Posted 14 December 2018 - 01:25 PM

I think a lot of the problem is that the screws have to be reasonably snug to hold collimation.

                                                                                                                          Marty

I agree, but Steve's post may be read like saying that adjusting the collimation screws makes the scope more prone to lose collimation (well, at least I read it that way, don't know about others). Sure, if you leave them loose. But why to do that? It's not that difficult to finish collimation  by making sure that every screw is snug.

 

And if your SCT is out of collimation, at least don't leave it like that ouf of fear of making it somehow less capable of keeping the collimation (which it didn't even keep in the first place if we are having this discussion!). If you don't feel like collimating, at least pick a good excuse, like laziness grin.gif.



#29 AxelB

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Posted 14 December 2018 - 11:47 PM

People that collimate frequently are probably chasing collimation do to poor technique. Once properly collimated, an SCT can go months to years without collimatoin.

I think my C14 went 5 years without collimation. I would check it every now and then, but it never changed (and should not change with normal handling).

Technique has a lot to do with minimizing the effects of mirror slop, which means always approaching final focus using CCW turns of the focuser knob.

Once collimated though, the scope should hold collimation for months to years, depending on handlng and transport.


No, not every sct are the same. For exemple mine suffers from mirror flop. It would fare better on an alt-az mount but I use it on a gem so my collimation needs to be optimized on one side of the meridian.

You’re very lucky indeed if yours is super stable.
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#30 highfnum

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Posted 15 December 2018 - 06:09 PM

i have a bunch of SCTs
most stay stable but my c8 from 70's
seems to need collimation a bit more often

#31 Stargazer3236

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Posted 16 December 2018 - 03:00 AM

I have not had to re-collimate my Nexstar C8 at all. I had aligned it about 6 months ago and have not had to re-do it. When I bought the scope about two years ago, it came in very good alignment. Since then, I have had to only re-collimate it maybe two times. It holds its collimation very well.



#32 Gregory2012

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Posted 16 December 2018 - 12:41 PM

In general SCT's tend to hold collimation well.  The frequency needed to check and tune collimation is driven more by how the OTA is handled and sot so much by use as it is only the secondary mirror you would can tweak.  For instance, I have collimated mine maybe 3 or 4 times in 10+ years, but my OTA only travels from mount to house and back and unless I bump it or take it on a road trip, collimation holds up well and is left untouched.  I primarily use the SCT for planetary (f/20 and up) so I typically check collimation at the beginning of planetary "season" and tune it as required. 

 

A star test is your friend...as it is the best "teller" while in the field (or Obs) of when to do it, we don't have a dog or cat. 

 

Keep in mind this should be done on nights of good seeing.  There's a plethora of SCT collimation threads here on CN if and when you get the itch (or your pets tell you) to look at a defocused star near the zenith at high magnification on a night of good seeing.    

Hi,

 

Can someone tell me what tool is used to collimate an SCT? Also, can you collimate with a star?

Thanks,

Gregory Gig Harbor, WA.



#33 tjz

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Posted 16 December 2018 - 05:05 PM

Hi,

 

Can someone tell me what tool is used to collimate an SCT? Also, can you collimate with a star?

Thanks,

Gregory Gig Harbor, WA.

If you have occasional good viewing, your eyeball and a star are the best option. If you can go for months without a night of good seeing, I have used a reflection off a rounded shiny surface (chimney cap, power line poles, etc) in the daytime. You can also get an "artificial star" for use at night. I have this one and it does the job: http://www.hubbleopt...cial-stars.html.

 

This is a good reference: https://astromart.co...sct-collimation


Edited by tjz, 16 December 2018 - 05:06 PM.


#34 mosentos

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Posted 16 December 2018 - 05:39 PM

Greg,

If you have access to a 3d printer, try one of these (duncan mask) and an artificial star:

https://www.thingive...=75c16d3e7f3694

 

I had good luck with fine adjustments using these two.

Good luck!



#35 Gregory2012

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Posted 17 December 2018 - 11:53 AM

Thanks.

 

If I can't see any real stars from my viewpoint, then artificial stars work as well?

 

Thanks,

Gregory Gig Harbor, WA.



#36 choward94002

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Posted 17 December 2018 - 12:11 PM

A hubble artificial star reflected off a convex mirror works OK if you don't have a real star ...



#37 Asbytec

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Posted 17 December 2018 - 07:13 PM

Thanks.

 

If I can't see any real stars from my viewpoint, then artificial stars work as well?

 

Thanks,

Gregory Gig Harbor, WA.

Any tiny glint of sunlight during the day and small enough to present nearly a point source works well. A car bumper or chrome or a electrical pole often has sources of glints. A small plain Christmas tree bulb works. There is likely something you can find that'll work. I've even used a glint of something shiny on a neighbor's roof tile. If you can find a similar glint at night, near a streetlight for example, it'll will work too. It does not have to be at infinity, but should be far enough away you can focus on it. And it should be bright enough to see well when slightly defocused. An artificial star also works. Buy one or make one with a flashlight with a tiny pin hole in some aluminum foil. 


Edited by Asbytec, 17 December 2018 - 07:18 PM.


#38 Asbytec

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Posted 17 December 2018 - 07:17 PM

A hubble artificial star reflected off a convex mirror works OK if you don't have a real star ...

I understand why you recommend a convex mirror, but I don't believe it's really necessary for collimation. 



#39 Kokatha man

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Posted 17 December 2018 - 09:41 PM

Crikey! I collimate before every imaging session - if there is considerable shift from one planet to another when we image multiple planets, I'll re-collimate again for the 2nd etc...despite pretty effective primary mirror locks, seeing conditions are always dynamic regardless of elevation - & the seeing might present a better chance to fine tune the collimation later in the session...star-collimating on a nearby star wherever possible.

 

I made a good artificial star years ago set up on an old EQ mount for fine adjustments for targeting - SCT's need a heck of a lot of distance (in a park etc) to get a decent result...worked much better spacing it just down the driveway for the Newt. smile.gif

 

Seems like I know diddly-squat about collimating our C14...better delete that page on our website on some of the finer aspects* of collimation of an SCT: http://momilika.net/...3Processing.htm - must be doing things completely wrong by the contents of some of the posts in this thread...I wondered about some of the points I make there about various helpful aspects to look out for - such as the inner rings becoming more illuminated around the Poisson Point with a star defocused to a dozen or more rings...& that bit about a "strobing" effect (Pat calls that "helicoptering!") around the image - another confirmation that you're getting close! 

 

They never seem to be mentioned in any of the "guides" I've ever looked at: then again, most of what you reference are C-G images etc...but taking some of the posts here seriously, I think we must be delusional! rofl2.gif

 

*Ok, a little bit of an admission - our C14 spends about 1/2 its' time in the back of our 4WD vehicle so it does get a bit of "rock & roll"...but we do exactly what I said in my first paragraph here regardless, even if it is used continuously over time when set up on the mount in the backyard: we want maximum results dependant upon whatever seeing we encounter. (below a certain quality we simply pack it in)

 

I intend to expand our website guide to fully encompass collimating an SCT from "go to whoa" which essentially means the earlier stages & a simple means of working out what to do from the very start...note that this is all an "in camera" approach which does make it very easy for a single person to collimate & know what they are doing throughout - simply turning the laptop screen to face you as you work on the collimation. But its' essence can easily be translated to an ep collimation.

 

But those little "pointers" I mentioned in para.3 here are real & do assist in knowing you're well down the correct path...some folks who have permanent setups might well find that their collimation holds reasonably well over extended periods of time: but that sort of level of collimation would never satisfy us quite frankly...if you consider any combination of metal & glass etc componentry, it will inevitably experience at the very least mechanical expansion & contraction over time  due to temperature, let alone if you utilise primary mirror focusing (which we do not)  as well as (possibly) other types of stress so that in any circumstances the fine alignments you are enabling when collimating is never going to remain static or "set & forget" - well, certainly nowhere near well enough for our own demands!


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#40 choward94002

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Posted 17 December 2018 - 10:00 PM

I understand why you recommend a convex mirror, but I don't believe it's really necessary for collimation. 

You want to get the artificial star to be as small as possible ... a few microns small.  The Hubble will start you at 50 microns, then you start bouncing it off convex mirrors to further reduce it (there are formula's to tell you how much based on the curvature of the lense) with the stronger the convex figure the smaller the eventual dot.  Using a ball bearing or Xmas ornament will introduce distortion from the surface, so you need to use mirrors ... I use three arranged in a little jig I built using some plywood, light from the Hubble bounces off three 2" convex mirrors from Edmund Optics before it travels about 40ft to my OTA ... otherwise you won't be able to get an airy disk (which remember needs a point, not a sphere) and I might as well collimate off Venus ...


Edited by choward94002, 17 December 2018 - 10:01 PM.

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#41 tjz

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Posted 17 December 2018 - 11:00 PM

I understand why you recommend a convex mirror, but I don't believe it's really necessary for collimation. 

I just duct tape my artificial star to a tripod and walk it 100+ yards or so down the block. It gets plenty small enough to get a nice pattern from. My neighbors think I'm nuts by now, of course.


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#42 Stardust Dave

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Posted 18 December 2018 - 12:21 AM

Not touched since spherical secondary re-coat and general refurb on the Meade 2080 LX-3 in mid or late 2016...

 

Typically just used at home, carried 15 feet and placed on wedge.

 

Standard meade allens on the secondary and all quite snug.  I think that is the key,  gentle handling of OTA and snug enough adjustment screws on secondary.

 

On best nights the collimation still adequate  40-50X inch.

Still check every session .


Edited by Stardust Dave, 18 December 2018 - 12:25 AM.


#43 Asbytec

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Posted 18 December 2018 - 01:35 AM

"...otherwise you won't be able to get an airy disk (which remember needs a point, not a sphere) and I might as well collimate off Venus."

Yes, you want it to be very small. Distance and pin holes work, too, if available.

#44 Noah4x4

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Posted 20 December 2018 - 03:18 AM

I use Hyperstar for EAA which involves removing my secondary mirror which has a tiny screw that fits in a slot in its holder to retain visual collimation of my 8" SCT. After a year of doing this and carrying my fully assembled scope (ex Hyperstar/camera) from my indoor mission control to its outside location its collimation still remains perfect, just as it did during two years of ownership before this. 

 

It is inevitable that some telescopes might require regular collimation, but my two SCTs have never required adjustment in over three + years. However, I did spend a fortune on unnecessary expensive eyepieces and filters in my first two years of ownership trying to beat light pollution. Yes, I admit there was a fear of attempting collimation that deterred me in my novice days. With hindsight, I think that was a good thing as it was never necessary. Having adopted Hyperstar (a brave step) and other advanced stuff it would not now concern me. But it is still not necessary as everything is pin point sharp (or, is it because my eyesight isn't and I might not notice a difference?). 

 

My point is I wonder how many collimate their scopes with greater frequency than necessary for the same poor 'seeing conditions' reason? Darker skies (or EAA/NV) is the only solution to defeat light pollution but we are all driven by whatever urge to attempt other solutions when we encounter 'faint fuzzies'. I wonder how many folk buy "Bob's Knobs" as one of their first 'must have' accessories and get lured into a ritual that is rarely necessary? But I will still check my collimation on next use. It's now almost certain to be out having spouted my two pence worth!


Edited by Noah4x4, 20 December 2018 - 03:19 AM.

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#45 Cali

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Posted 20 December 2018 - 06:06 AM

Four minute YouTube on collimating a MAK. Any thoughts on this approach? Seems low cost.

 

- Cal



#46 Conaxian

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Posted 07 January 2019 - 07:32 PM

Four minute YouTube on collimating a MAK. Any thoughts on this approach? Seems low cost.

 

- Cal

Looking at his out of focus star, I thought his scope did need a tweak. The CO is not centered.

Then he said to loosen the three big screws and adjust via the small screws.

I understand that the three big ones 'PULL' the cell, the three small ones 'PUSH'.

He should have backed off two little ones then turned the big one between them, judge the resultant movement, then either continue or move to the opposite pull screw.

After it's collimated you snug all the disturbed screws to firmly lock the cell in place, then recheck the collimation in case they moved it in the tightening.

Go back and forth til it's collimated and the three small screws are snugged up. It is best to only ever use two of the PULL screws. Using all three can move the entire cell too far forward or too far back, they say.

 

That's my understanding, anyway.



#47 treadmarks

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Posted 08 January 2019 - 10:10 AM

I've had my SCT for 2 years and it needed collimation twice in that time. It arrived out of collimation and lost collimation after about a year. I check it maybe 2-3 times a year.



#48 Astrojedi

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Posted 11 January 2019 - 08:19 AM

I check collimation every time but typically the SCTs hold collimation for years even after trips to dark sites. So, almost never.
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