newtonian collimation problem

[Duke2k]

Hi all

I have recently acquired a Newtonian reflector 150/750 and I am having a few issues with it. First of all I have read numerous pages on collimation but I don't know what is wrong with this scope.

First off I align the secondary with the focuser by eye and using a collimation cap make sure that I can see the 3 primary retaining clips. Then using a Cheshire eyepiece I align everything up as  it should be.

When I place a good collimated laser in the eyepiece the laser is way off centre on the primary and in actual use stars and planets wont focus and the flare is awful.

If I then use the laser  to collimate then objects look better but still not great and in this position the secondary holder is leaning away from the focuser and not aligned with the tube.

The Cheshire then shows the centre of the primary way off alignment from the circular secondary mirror.

Any ideas? Should a laser and a Cheshire agree that a scope is aligned?   

I will try to load some images tomorrow but its difficult to get what I can see on camera.

[MarkyD]

I went through the same pains as you when first collimating my 200P-DS.  In the end after a few hours it just fell into place.  The key is to start from the very beginning and set everything again from the focuser to the spider to the primary.  I now have the same results from a barlowed laser as the Cheshire with both the cross and the primary ring centered.  It took me a couple of days tbh

--

Mark

[Thalestris24]

Hi

Astrobaby's guide is one of the best. You can compare what you see to her images at each stage.

Louise

[astronymonkey]

Hi duke,

what make of scope is it? Im wondering if the issue may be a misaligned focusser .cheers

 

 

 

 

 

[Owmuchonomy]

To rely on a laser then that needs to be collimated too.

[andyboy1970]

18 minutes ago, astronymonkey said:

Hi duke,

what make of scope is it? Im wondering if the issue may be a misaligned focusser .cheers

 

 

 

 

 

Worth checking that the focuser is squared up properly, I found my 200p & 150p to be off and both had to be shimmed.

 

http://astro-tom.com/telescopes/newtonian.htm

 

[theastrodragon]

If you're going to use a laser, its worth checking its accurate.

What I do with mine is use the laser in daytime, then do a star collimation until its perfect. Then slip in the laser and see what it says. Sometimes this can be quite illuminating (sic)

A lot of laser collimators have quite a bit of slop, and as has been pointed out, the focus mount might need shimming. Rotating the laser in the tube may show a sloppy fitting; if it seems fine, then the next culprit is the focus mount not being square.

[Duke2k]

Thanks for the info.

How do I check that the focuser mount is square? I have held a sliding set square across the front of the tube while the laser is on and lowered the ruler part down both the focuser side and the opposite side of the tube and it appears to be within 1mm but as for rotationally square I'm not sure.

The telescope has the name Arena on the side so its probably a low cost model.

I have found that the secondary mirror wasn't set to 45 degrees which was causing it to lean away from the focuser but that's now corrected.

[Duke2k]

Sorry I didn't notice the link to the astro tom site.

After checking the focuser for alignment I found that it was ok so the only thing was the diagonal set at 50 degrees which I have fixed.

I have now managed to get everything aligned so that the cheshire and laser agree. I'm still getting some flare off to one side of stars which simply won't go away but on the whole its an awful lot better than it was.

I wondered if it could be the chrome focuser tube where it enters the scope thats causing the flare?

Would flocking the tube be worthwhile? 

[theastrodragon]

8 hours ago, Duke2k said:

Sorry I didn't notice the link to the astro tom site.

After checking the focuser for alignment I found that it was ok so the only thing was the diagonal set at 50 degrees which I have fixed.

I have now managed to get everything aligned so that the cheshire and laser agree. I'm still getting some flare off to one side of stars which simply won't go away but on the whole its an awful lot better than it was.

I wondered if it could be the chrome focuser tube where it enters the scope thats causing the flare?

Would flocking the tube be worthwhile? 

It's certainly worth a try, nothing in the light path should be chrome plated!

[brantuk]

Flocking is a good idea but please don't see it as a fix for this problem. If your collimation is out, spider bent,  or optics pinched you'll still get dodgy shaped stars irrespective of flocking. I would be inclined to identify and fix the problem first.

[Duke2k]

Thanks I will blacken the focuser tube first. The best way to describe the star appearance is comet shaped, the way they appear around the edges of the field of view when coma is present but in the centre of the field. Adjusting the primary mirror doesn't change the angle of the flare or reduce it.

The main mirror isn't pinched but could a pinched secondary cause this?

My wife keeps telling me to bin it but I hate giving up on it if it can be fixed.

 

[CraigT82]

Hi, I've seen exactly the comet shaped flare you describe in my newt when it is warm (taken straight from the house outside to view).  First time I saw it I thought something was seriously wrong with the mirror but once the scope cooled down the flare disappeared.

[Moonshane]

are you allowing the scope time to cool outside?

you need roughly 5 minutes per inch of aperture to get neat and tidy views, especially at high power.

[MarkyD]

I flocked my 200P-DS.  Wether it helped/improved I don't know as I didn't really use it in anger before doing so but the difference in colour is remarkable.  There is paint black and there is flocking black - it made the paint look gray.  It also gave me the opportunity to set everything square and true asI reassembled it

--

Mark

[Tinker1947]

Is this coma your seeing, mostly on the outer stars with them all pointing towards the centre........

[theastrodragon]

5 hours ago, Duke2k said:

Thanks I will blacken the focuser tube first. The best way to describe the star appearance is comet shaped, the way they appear around the edges of the field of view when coma is present but in the centre of the field. Adjusting the primary mirror doesn't change the angle of the flare or reduce it.

The main mirror isn't pinched but could a pinched secondary cause this?

My wife keeps telling me to bin it but I hate giving up on it if it can be fixed.

 

That sounds rather like coma

[Duke2k]

Thanks guys, I havent managed to look at it since but as soon as the sky clears I'll have a look.