How to collimate refractor?

[bobson (Bob)]

Hi,

I have 102x1000 Skywatcher refractor on EQ3 mount that I recently bought here.
I removed diagonal, checked collimation with laser and the red dot was away from centre quite a bit?

Star test shows concentric circles both ways?
I removed dew shield but can't see any adjusting screws as some people suggest on the net? On the other end where the focuser is, there is nothing to be adjusted either? I can unscrew the focuser and corrected it a bit but still out from centre. And it gets worse with diagonal in and then laser.

Is there any way to collimate this scope?

cheers

bob

[xnomad]

According to the Skywatcher manual that comes with the EQ3,

http://www.skywatchertelescope.net/s...1=1&class2=120

An adjustable refractor should have the screws visible when you remove the dew shield. The aren't hidden behind some removable ring are they?

[bobson (Bob)]

Hi Mate,

Well, I have that manual from Skywatcher website, but I can't see any screws. I removed dew shield so I can see lense but no screws?

I'll take a picture when I can and post it here.

thanks

bob

[bobson (Bob)]

As you can see there is no adjusting screws.
This scope is branded as Starspanner but it is Skywatcher.

bob

[5ash (Philip)]

i think all of the smaller skywatchers dont have collimation screws.If you want to line up the laser with the centre of the objective you could try removing the focusser (3 screws normally) and rotate the focusser in the tube until the holes line up and see if this improves things.If not make the holes in the tube longer using a file so that you can tilt the whole focusser in any direction to achieve collimation.
philip

[anj026]

I don't think it is appropriate to use a laser for a refractor. Try using a cheshire eyepiece.

Cap the objective then put the cheshire in the focuser without a diagonal. Shine a torch onto the side opening of the cheshire. When you look through the eye hole you should see some tiny concentric circles.

If your star test shows concentric circles then it is most likely that all is well and no adjustment is required. An f10 refractor like this is not as critical for colimation as an f5 reflector or short focal length apo.

[chrisp9au (Chris)]

I would suggest first checking that your laser is collimated, before touching your scope.

Cheers
Chris

[anj026]

Good idea to collimate the laser Chris but it will still only be good for use in collimating a reflector.

[dannat (Daniel)]

I'm pretty sure those skywatcher refractors do not have a collimatable lens cell - it is something which is disappearing on the cheaper refractors

[Merlin66 (Ken)]

I recently built a 90mm f10 for solar work, and used both a laser ( pre-collimated!) and the Cheshire.
The laser does help to align the axis of the focuser to the centre line of the objective ( I used a white plastic cover marked in the centre over the end of the tube infront of the objective) the laser dot can then be lined up with the centre of the objective by tilting the focuser body relative to the tube.
The Cheshire with a bright LED torch shining in the side, should show shaded concentric rings reflected from the rear of the objective lens elements. Adjust the tilt of the objective and lock in place.
Suiter cover's the Cheshire method very well in his book " Star Testing Astronomical Telescopes", p121 -125

[anj026]

Good point Merlin.
I have used a laser for lining up the focuser too.
You can check the collimation of your laser by loosening the focuser set screw just enough to rotate the laser body in situ. If the dot on the lens (or mirror) moves in a circular pattern as you rotate the laser then you know the laser needs adjusting.

[GeoffW1 (Geoff)]

Originally Posted by dannat

I'm pretty sure those skywatcher refractors do not have a collimatable lens cell - it is something which is disappearing on the cheaper refractors

Hi,

Yes, I have the SW1000 as a grab n go, this one

http://www.tasco.com.au/Products/tab...1/Default.aspx

It has no collimation geegaws I can find. Seems to be fairly well set up without.

Cheers

[bobson (Bob)]

Hi guys

Laser collimator is collimated. I use it for my 12" dob.
I suspected something is wrong when I tried to look at Omega Century with this refractor and I could not resolve it at all? My 12" dob gives much brighter view of Moon and Saturn. OK, its bigger aperature, but I thought I could see Omega Century with this refractor, or can I?

I know this size refractor is for Moon and planets only, but big DSO like Omega. Plus blue_skys that I got it from claims she enjoyed looking at Tucane with the same size refractor.

I do have Chesshire too and will try it over weekend.

thanks

bob

[GeoffW1 (Geoff)]

Yeah,

So, if you had one of these cheaper refractors without nibnobs sticking out everyplace, and you were a bit suspicious of the optical alignment, what could you do?

Now you might say, "sell it", but that's a begging the question a bit. Are there any really advanced techniques like bending the OTA over your knee?

I suppose that we here would have to be really brave to take out the primary lens and pack it a bit this way or that, but what would a well qualified techo do in the case where one of these scopes showed up a star test which was not the best? Someone with an optical bench I mean.

Cheers

[Blue Skies (Jacquie)]

Originally Posted by bobson

I suspected something is wrong when I tried to look at Omega Century with this refractor and I could not resolve it at all? My 12" dob gives much brighter view of Moon and Saturn. OK, its bigger aperature, but I thought I could see Omega Century with this refractor, or can I?

I don't think you've quite come to grips with the difference in aperture between the 4" refractor and the 12" reflector. The difference in resolution is large, despite what refractor afficiandos will say. And I have looked through a top of line 4" Astrophysics so I have some idea what the best refractors can do. Because the refractor is a lens system there is some loss of light brightness as the light from the stars passed through the lens, and then if you're using it in the city you'll have light pollution on top of that.

There is no substitute for aperture. The image of Omega I always keep in my mind was with a 17.5" telescope and a 12mm Nagler - high power in a big light bucket. Omega usually appears washed out to me compared to the that!

Answering this is making me think about Meade ETX90's - a relatively small scope that was marketed as doing amazing things. They are only 90mm aperture, for goodness sake! Just about everything but the moon looks disappointing in them to me.

Despite being a bit defensive because I was the seller, I really think you need to compare the scope to other scopes of similar size in the same situation and get a better idea of what their limits are before you start blaming the colimation of the refractor.

[Robh (Rob)]

Bob,
You cannot check collimation with a laser collimator on a refractor as you have no return image. On a reflector telescope the laser should hit the centre of the mirror, focus, then return back to the collimator centre.
In the factory, the refractor objective lenses should have been seated to focus the image approximately at eyepiece centre. As 5ash points out the focuser tube won't be exactly parallel to the axis of the telescope tube so using a laser will tell you nothing about the seating of the objective lenses. The laser will simply exaggerate the off-parallel over the focal length of the telescope. However, for an eyepiece the slight off-parallel should be within acceptable tolerances. No system is designed perfectly. Probably the primary lenses axis is not exactly parallel to the telescope tube either.

Refractors generally shouldn't need collimation. Do not remove the objective lenses! If the star test shows concentric circles both ways, this is a good sign.
Here's a simple test you can try. Sight Alpha Centauri with around a 15mm eyepiece (1000/15 = 67 magnification).
Can you see two clearly separated point stars.
Now try alpha Crucis (Acrux) with around a 6mm eyepiece (1000/6 = 167 magnification).
Can you see two clearly separated point stars again?
If yes, there can't be too much wrong.

Regards, Rob.

[GeoffW1 (Geoff)]

Originally Posted by Robh

Bob,

Refractors generally shouldn't need collimation. Do not remove the objective lenses! If the star test shows concentric circles both ways, this is a good sign.
Here's a simple test you can try. Sight Alpha Centauri with around a 15mm eyepiece (1000/15 = 67 magnification).
Can you see two clearly separated point stars.
Now try alpha Crucis (Acrux) with around a 6mm eyepiece (1000/6 = 167 magnification).
Can you see two clearly separated point stars again?
If yes, there can't be too much wrong.

Regards, Rob.

Hi,

Works well for me

Cheers

[bobson (Bob)]

Jacquie

You don’t have anything to be defensive about. I got brand new scope from you, still in the box for the good price. And I am happy for it. You didn’t make this scope so it can’t be your fault if it’s not perfectly aligned. And the fact is it’s not. To prove this I found which way the focuser should be tilted to make it in the centre. And while I was looking at couple of objects I loosen the focuser screw on top so I could tilt the whole focuser and the quality of viewing improved. Not dramatically but still noticeable.
In my free estimation it is approximately 10-15% better when I find sweet spot by tilting the whole focuser, that’s all.
The reason I mentioned you in my post is that when we spoke while we checked that everything is in the box, you mentioned that Tucane looked great through this scope, that’s all. Not 17.5 reflector, not Astrophysics nor Meade, but this particular scope. Of course Omega looked great through some scopes I looked through recently. Scopes like 16” Meade, 18” Obsession, Takahashi, 14” Celestron with full set of Ethos eyepieces.
Anyone who looked through those scopes would notice what difference aperature makes. Maybe you were talking about some other scopes you looked through when you told me that Tucane looked great. Maybe I misunderstood. I also have 12” Bintel Dob, and I didn’t buy this refractor for DSO anyway. I was just curious to find out what it can or not do, that’s all. The last thing on my mind is to blame you or anyone for that matter.

Rob,

I did the test as you recommended and it did resolve the double stars, but just. When I found sweet spot by tilting whole focuser it gets slightly better, something I will not worry about. I experimented with taking dew shield off and holding a mirror flash on top of the tube so the laser has return beam
I also tried with piece of paper cut to size and make a hole in the centre, so I knew where the centre is, and then use Chesshire.

Philip,

That sounds like the way to go mate. But when I removed those 3 screws I could not take it apart, looks like its glued together too. I don’t want to force it or damage it. I tried grabbing it really hard and then pull or twist but no go. The only way I could centre it was by loosening the focuser bolt on top inside focuser itself so I can tilt it. But unfortunately I have to hold it with my hand this way otherwise its very floppy.

Thanks everyone for help

Cheers

bob

[bobson (Bob)]

Originally Posted by GeoffW1

Are there any really advanced techniques like bending the OTA over your knee?

Cheers

I didn't consider bending the OTA over the knee not yet anyway...

cheers

bob

[Robh (Rob)]

Originally Posted by bobson

Hi,
And it gets worse with diagonal in and then laser.

Bob,

Another thought just occurred to me. Have you got a spare diagonal or can you focus an eyepiece in the scope without the diagonal. A mate of mine had his scope for months, complaining about the poor image before we realised that the diagonal was the problem. It turns out the mirror in the diagonal was not seated properly i.e. off angle.
Give it a check.

Regards, Rob.