Author Topic: Dob Collimation Accuracy  (Read 52 times)

vistadussi

  • Active Astronomer
  • ***
  • Posts: 119
  • Activity:
    10.67%
  • Reputation: +0/-0
    • View Profile
Dob Collimation Accuracy
« on: December 24, 2017, 08:58:05 AM »
My new Orion 12" dob was sent yesterday and on checking collimation I discovered it to be horribly out - bad enough that the factory made no attempt of working in any way, or it was dropped off a cliff somewhere between here and China.  No big deal - with the added collimator, and a bit of work, I got the laser dead-center at the primary mirror and inside a couple mm onto the collimator target.

But in the course of doing this, I realized that very little in this particular extent could be known as "exact" in terms of tolerances.  I am able to rotate the collimator a quarter twist in the focuser, re-tighten the set screw, and get a completely different outcome.  There is quite a bit of play at the focuser itself (with the set screw un-tightened), meaning the laser can be thrown a couple inches off to the main mirror by loosening the set screw a small, rotating the collimator a quarter turn, and tightening back down.  The struts holding the mirror have some play in them too, which I suppose is to be expected (when hitting the side of this extent, even marginally, the mirror will bounce around, but seems to return more or less to its original position when settled).  All this gives me the impression that collimation on this extent is really not likely to be "exact," it is more of a "horseshoes and hand grenades" exercise.

Had pretty good viewing last night also, and after plenty of cool-down time, I ran the star test on polaris and a couple of others with a 9mm plossl as best I know how.  It was, in my very amatuer opinion, sort of middle-of-the-road.  I didn't find anything that caused me to serious concern.  But I wasn't able to resolve anything at that power to some tack-sharp stage, possibly (my also new - LX70 refractor kicks this scope's @** in that respect, but I expected that).  De-focusing the celebrity at dead center showed slight asymmetry, but just slight.  I also did detect quite lousy "streaking" of stars close to the edge of the viewpoints (even at reduced power) but attributed that to my economical eyepieces.

So how "exact" is collimation on a range such as this?  Is there a way to get it perfect?  I realize optics on a range such as this are a "get-what-you-pay-for" proposal, so just how much can alignment play to imperfections noticed in celebrity analyzing, versus shortcomings of the optics themselves?  I should probably say that I'm not in any way unhappy with this extent, especially considering what I paid for this only a "tinkerer" by nature and love to push equipment to its limitations.



Jairo Zilinskas

  • Active Astronomer
  • ***
  • Posts: 130
  • Activity:
    13.33%
  • Reputation: +0/-0
    • View Profile
Re: Dob Collimation Accuracy
« Reply #1 on: December 29, 2017, 03:00:14 AM »
Use 'Barlowed' collimation. Then these focuser wiggle issues disappear.

multalumiff

  • Active Astronomer
  • ***
  • Posts: 117
  • Activity:
    12%
  • Reputation: +0/-0
    • View Profile
Re: Dob Collimation Accuracy
« Reply #2 on: December 29, 2017, 05:31:00 AM »
The spider and secondary shouldn't move at all. Something isn't right. Also, those cheap lasers that come with the scope are better suited as a cat toy then they are for collimating...usually, the collimator needs collimating itself before use, and checked regularly.

Roger Dixon

  • Active Astronomer
  • ***
  • Posts: 130
  • Activity:
    13.33%
  • Reputation: +0/-0
    • View Profile
Re: Dob Collimation Accuracy
« Reply #3 on: December 31, 2017, 03:13:37 AM »
The spider vanes should be good and tight.

The collimator should not be describing a circle when rotated. If it is then it will need to be aligned. This should be checkedby rotating in a v-block rather than in the scope.

Using the Barlowed laser technique will work in the interim to align the primary which is the most critical, but it will still leave the secondary misaligned.

Sometimes the laser itself is well aligned, but there can be registration issues with theway itsits in the focuser. I have the same problem with the Cheshire site tube...

Rather than being a cat toy,the 1.25"Zhumell laser that came withour scope gives results about as good as the 2" Glatter/Tublug combo...without even Barlowing the Zhumell.

Stephen Artman

  • Active Astronomer
  • ***
  • Posts: 115
  • Activity:
    13.33%
  • Reputation: +0/-0
    • View Profile
Re: Dob Collimation Accuracy
« Reply #4 on: January 03, 2018, 04:57:09 PM »
Quote
The spider and secondary shouldn't move at all. Something isn't right. Also, those cheap lasers that come with the scope are better suited as a cat toy then they are for collimating...usually, the collimator needs collimating itself before use, and checked regularly.

Yeah, I wondered about that (the collimator's accuracy). Thanks.

Manish Konakanchi

  • Active Astronomer
  • ***
  • Posts: 112
  • Activity:
    12%
  • Reputation: +0/-0
    • View Profile
Re: Dob Collimation Accuracy
« Reply #5 on: January 04, 2018, 10:32:39 PM »
Well, it sound like there's a lot going on there. It's not particularly surprising that your scope arrived out of collimation. But it is a precision instrument. The Orion scopes are definitely built to a price point but they do work well. IMO, all of their limiting points are relatively easy upgrade paths and there are several paths to chose.

You should check to see that your laser is true. The Orion Lasermates are renown for being out of collimation and they all have an elongated laser "dot" in any case. Fix your laser in V-blocks of some sort and rotate it, watching for the dot to stay still in one place or move as it is rotated. Those warning stickers that promise your warranty will be voided are covers for the collimation screws.

I replaced the focuser on my XT10i because I didn't like the image shift it presented. When collimating, most recommend the set screw be tightened just enough to lightly hold the collimation tool or 1.25" adapter in place.

The secondary spider vanes should be solidly fixedlinearly. That is how they locate the position of the secondary mirror in space. That they vibrate if tapped is a function of their design for minimal interference with the light path and isn't a problem. They should always return the secondary to the same location when the vibration settles. The spider retaining screws on the outside of the tube should be snug and torque-equalized.

The result of a successful Newtonian collimation should be quite precise. These scopes will allow that. Otherwise they could not be as popular as they are and would not produce the views that they do. That said, a Dob is a tinkerer's dream. Upgrades can be really effective. Your 1st consideration might be to your collimation tools. A Glatter laser/Tublug/Parallizer combo might get you all the way there on your collimation problem.

Bryan Sonian

  • Jr. Astronomer
  • **
  • Posts: 96
  • Activity:
    8.67%
  • Reputation: +0/-0
    • View Profile
Re: Dob Collimation Accuracy
« Reply #6 on: January 08, 2018, 01:18:12 AM »
@havasman - Yes! That's the main reason I bought this scope, actually (to tinker with and upgrade over time). So that's very good to hear.

So I just checked the collimator in a home-made v-block (made with a right angle from some scrap lumber and a clamp), and just to make sure I then drilled a 1.25" hole in a 2x4 and tested it again with that - and yep, that's definitely problem #1. This thing is way out. Thanks for the tip on where to find the collimation screws - I was going to toss it and buy a new one since I didn't see any at first glance, haha.

On the spider vanes - they're tight, definitely, and the retaining screws seem to be at close to equal torque (I haven't measured this exactly, just "hand-checked" them). One thing I did notice was that they're (all 4) not exactly straight. That was actually the first thing I noticed when I started trying to collimate yesterday. They're bent, slightly, right at where they connect to the secondary mirror housing. Does that concern you?

And last, thanks for the reassurance that this can and should be precise.  Makes it worth the effort of getting it there, even if that means some parts replacement.

Jody Mukherjee

  • Active Astronomer
  • ***
  • Posts: 106
  • Activity:
    7.33%
  • Reputation: +0/-0
    • View Profile
Re: Dob Collimation Accuracy
« Reply #7 on: January 08, 2018, 05:02:13 PM »
The accuracy of the collimation in a Newtonian is dependant on the focal ratio. If the focal ratio is say an F/8 or more then a mere off focus star test is good to go. If the focal ratio is an f/4.5 or less then you need that barlowed collimation technique!

bolgsorchumsdea

  • Active Astronomer
  • ***
  • Posts: 116
  • Activity:
    12%
  • Reputation: +0/-0
    • View Profile
Re: Dob Collimation Accuracy
« Reply #8 on: January 10, 2018, 10:44:53 AM »
Quote
The spider and secondary shouldn't move at all. Something isn't right. Also, those cheap lasers that come with the scope are better suited as a cat toy then they are for collimating...usually, the collimator needs collimating itself before use, and checked regularly.

Generally i agree, although when i bought a cheapy for 29 bucks i was surprised to find it spot on. I guess even a stopped clock is right twice a day ;-)

bijstentetal

  • Active Astronomer
  • ***
  • Posts: 124
  • Activity:
    16%
  • Reputation: +0/-0
    • View Profile
Re: Dob Collimation Accuracy
« Reply #9 on: January 11, 2018, 04:13:22 PM »
So yeah, the moral of this story is: check your dadgum collimator before trying to collimate anything, haha. Made a rig involving a 1.25" hole drilled through a block of wood, a vice, and a dot & circle on the floor to get the collimator's alignment dead-on, and then was able to get the dob's alignment very precise. No more screwy circles when rotating it in the focuser (and that was without the barlow).

What was kind of funny though, was how close I actually had it even with the collimator way out of alignment. Just pure luck, but still funny.

I'll star test again tonight; looks like decent weather. I am thinking about replacing the spider in this scope with something a little less "cheap"-feeling. Eventually intend to replace both primary and secondary optics with something very nice, like Zambuto etc, so might wait until then as long as I don't notice any problems with it slipping way out after moving it around.

Thanks to all for the help/suggestions.

ricoperte

  • Active Astronomer
  • ***
  • Posts: 120
  • Activity:
    14.67%
  • Reputation: +0/-0
    • View Profile
Re: Dob Collimation Accuracy
« Reply #10 on: January 12, 2018, 03:06:19 AM »
Quote
On the spider vanes - they're tight, definitely, and the retaining screws seem to be at close to equal torque (I haven't measured this exactly, just "hand-checked" them). One thing I did notice was that they're (all 4) not exactly straight. That was actually the first thing I noticed when I started trying to collimate yesterday. They're bent, slightly, right at where they connect to the secondary mirror housing. Does that concern you?
Close to = is likely just fine. Bent spider vanes can still work fine. They may catch a tiny bit more light but that's a small incremental matter.

A really good replacement system, if you decide to go that way, can be had from https://www.astrosystems.biz/
Randy will likely tell you that his product won't fit your scope. What's required is that you have to drill new holes for the spider bolts. Folks have screwed that up and then blamed him. Theprintson his website are spot on. That's what makes the whole thing relatively simple to configure.

​Definitely have yourprimarytested before you decide to replace it. Often the primaries in Orion Dobs are fine. Secondaries not s'much.

Owen Richter

  • Active Astronomer
  • ***
  • Posts: 126
  • Activity:
    15.33%
  • Reputation: +0/-0
    • View Profile
Re: Dob Collimation Accuracy
« Reply #11 on: January 13, 2018, 01:16:39 AM »
On the same vein, I've been wondering about the impacts of collimation errors. If the primary is sending most of the light that comes in the tube onto the secondary and through the field stop on the eyepiece, is there too much more we can hope for? Is there a way to understand the real effect of a 1cm or 1 inch error in the projection of the eyepiece axis onto the primary mirror?

Another way to ask would be to wonder "how close is close enough"? Lots of people say "I got the laser inside the donut" or the ATMs say "the laser spot stays inside the donut all the way to the horizon", but what is the real impact on observing quality if it wanders to the outside of the center dot, or even a bit off to the side?

I'm mostly wondering how frequently I need to be checking the collimation on my homemade 4.5f8 Newtonian and how exacting I need to be with it.

xgamodrakmu

  • Active Astronomer
  • ***
  • Posts: 122
  • Activity:
    12%
  • Reputation: +0/-0
    • View Profile
Re: Dob Collimation Accuracy
« Reply #12 on: January 13, 2018, 01:23:31 AM »
Quote
On the same vein, I've been wondering about the impacts of collimation errors. If the primary is sending most of the light that comes in the tube onto the secondary and through the field stop on the eyepiece, is there too much more we can hope for? Is there a way to understand the real effect of a 1cm or 1 inch error in the projection of the eyepiece axis onto the primary mirror?

Another way to ask would be to wonder "how close is close enough"? Lots of people say "I got the laser inside the donut" or the ATMs say "the laser spot stays inside the donut all the way to the horizon", but what is the real impact on observing quality if it wanders to the outside of the center dot, or even a bit off to the side?

I'm mostly wondering how frequently I need to be checking the collimation on my homemade 4.5f8 Newtonian and how exacting I need to be with it.


I have often wondered the same thing... in practice, I have found that an f/8 focal ratio to be forgiving of minor collimation errors... my 10 inch f/5 however is a bit sensitive. When I re-furbished a friend's 6 inch f/8 Newt, I could eyeball collimate the darn thing, and it would give very nice views... I did so because I didn't have a collimation tool at that time, and when I got a tool and collimated it with that, I didn't notice a heck of a lot of difference - and that scope gave excellent views... when I had nearly completed my 10 inch build, I did a rough alignment to get a feel for where things should land, and it was evident that this would not be good enough... before I finished the scope, I had since gotten a set of Cat's Eye tools, did the collimation for first light and wham! All good!

So, to answer your question "I'm mostly wondering how frequently I need to be checking the collimation on my homemade 4.5f8 Newtonian and how exacting I need to be with it." I would have to say that it is not CRITICAL to be exact and to always be checking it for the views to be good.... but for optimal, unquestionable performance, you should get it as close to perfect as possible... it sure won't hurt anything, eh?

This is my considered opinion and others may have something different to say...

Best regards!

CB

Robert Johnson

  • Active Astronomer
  • ***
  • Posts: 131
  • Activity:
    23.33%
  • Reputation: +0/-0
    • View Profile
Re: Dob Collimation Accuracy
« Reply #13 on: January 13, 2018, 05:36:32 AM »
Quote
Quote

On the spider vanes - they're tight, definitely, and the retaining screws seem to be at close to equal torque (I haven't measured this exactly, just "hand-checked" them). One thing I did notice was that they're (all 4) not exactly straight. That was actually the first thing I noticed when I started trying to collimate yesterday. They're bent, slightly, right at where they connect to the secondary mirror housing. Does that concern you?
Close to = is likely just fine. Bent spider vanes can still work fine. They may catch a tiny bit more light but that's a small incremental matter.

A really good replacement system, if you decide to go that way, can be had from https://www.astrosystems.biz/
Randy will likely tell you that his product won't fit your scope. What's required is that you have to drill new holes for the spider bolts. Folks have screwed that up and then blamed him. Theprintson his website are spot on. That's what makes the whole thing relatively simple to configure.

​Definitely have yourprimarytested before you decide to replace it. Often the primaries in Orion Dobs are fine. Secondaries not s'much.
Nice. That looks about a million times better than what it shipped with. And like the price too. I had heard that about these secondaries, and since a fine replacement (looking at Antares) doesn't seem to be that terribly expensive, I'll likely go for that soon. Might as well replace the spider while I'm at it.

After some testing tonight (headed back out in a few to do more), I'm definitely much happier with the high-powered views, but there's plenty of room for improvement. I do want to get the primary objectively tested, but after tonight it actually seems pretty good. Since that's a much higher priced upgrade, I'm going to mess with some other stuff first (replacing collimation knobs, probably the focuser, etc).

reilpipohen

  • Active Astronomer
  • ***
  • Posts: 121
  • Activity:
    14.67%
  • Reputation: +0/-0
    • View Profile
Re: Dob Collimation Accuracy
« Reply #14 on: January 13, 2018, 07:36:52 AM »
In general, how are the secondaries on Zhumell?