Double images

[steelfixer]

Hi all.

I am new to all this forum stuff and I need an answer to a simple question.

I have tried to check the collimation my 8 inch reflector using a laser. I note that the laser beam is not a dot but is being scattered into a 10mm sized blob on the primary mirror.

Is this normal or do I have a problem with the secondary mirror?

The reason for the check is I have started getting double images through the eye piece and using my philips 900 camera to get decent photos is impossible.

Thanks

Graham.

[Moonshane]

Welcome to the group!

One thing you may not be aware of is that lasers can and do go out of (or arrive out of) collimation. here's a way to check How-to Collimate a Laser Collimator - How To but you could rig up something more simple I suppose.

Personally I prefer a sight tube, collimation cap and Cheshire eyepiece to do my collimation.

Just as a quick check try the following:

• take the plug out of the focuser and the cap off the end
  
• do all this at a shallow angle in case anything drops down the tube
  
• looking back from a distance, you'll be able to see if the secondary shape is round - if not then you'll have to turn it slightly to get a round shape
  
• place your eye close to and as centrally as possible in the focuser
  
• do you see all three mirror clips? if you do then it's close enough if not then adjust (I presume you have a guide to collimation - maybe Astrobaby's guide?)
  
• then assuming you have a barlow (any quality will do), put the barlow in the focuser and insert the laser - switch on
  
• you should just be able to see the large shadow of the doughnut on the face of the laser collimator - if not then move one of the primary collimation screws a little and you should see it. centre this dark shadow on the hole in the collimator face and you should then be collimated.
  

bad collimation should not cause double images as far as I am aware, but a lack of cooling might or even the plume of central heating outlets etc. or even being a little out of focus.

if you follow the above then an out of collimation laser or slop in the focuser tube have no effect.

hope this helps.

[steelfixer]

Hi there

Thanks for that I will try it out when I get home.

The problem in more detail is when I was looking at Saturn the other night i could not get a sharp focus on it. I would get a double image and adjusting the focus even a tiny bit would resule in Saturn becoming a blury blob.

Whilst getting the double image the actual view of Saturn was pretty good rings and all.

Confusing or what?

Cooling should not have been a problem as it was pretty cold and as there are no thermal outlets any where near my telescope I can rule that one out.

thanks again.

[DaBozUK]

Hi there

Thanks for that I will try it out when I get home.

The problem in more detail is when I was looking at Saturn the other night i could not get a sharp focus on it. I would get a double image and adjusting the focus even a tiny bit would resule in Saturn becoming a blury blob.

Whilst getting the double image the actual view of Saturn was pretty good rings and all.

Confusing or what?

Cooling should not have been a problem as it was pretty cold and as there are no thermal outlets any where near my telescope I can rule that one out.

thanks again.

I get exactly the same from Saturn with my basic 26mm plossl that came with my scope. I have spent hours collimating the scope to perfection and the 10mm eyepiece shows no such double images.

So it might not be the mirrors that's the problem, might be EP.

[steelfixer]

Strange you should mention the 26mm plossl i was using a 25mm at the time.

Forgive my ignorance but EP ?????

Thanks

[ollypenrice]

You should not be getting this huge dispersion of the laser beam. Whether or not the laser is collimated is irrelevant with regard to that. If the laser were out of collimation you'd just get a tight beam in the wrong place, that's all.

I assume the laser, pointed at a distant wall, is still giving a tight beam? In that case have a look at the secondary. Does it perhaps have a centre dot which might be scattering the beam before it gets to the primary? Or could bit be damaged or very dirty? This should be visible.

Olly

[steelfixer]

Hi there.

Laser is the proper tool for the job and yes it does give a pinpoint dot on a wall 12 feet away.

One thing I have noticed is every place I read about this is tells me I should be able to see all three primary mirror fixings through the eye piece tube.

Well I cannot unless I look from side to side in the tube. Even then it is only just possible.

reasons for this would be appreciated.

Thank you.

[DaBozUK]

Strange you should mention the 26mm plossl i was using a 25mm at the time.

Forgive my ignorance but EP ?????

Thanks

EP=Eyepiece, sorry. Already I'm using acronyms that I puzzled over a couple of weeks ago!

[steelfixer]

Thanks pretty self explanatary rearly if you think about it.

did you only get this problem with Saturn. I ask because I can focus on stars with no problem.

thanks again

graham

[DaBozUK]

Thanks pretty self explanatary rearly if you think about it.

did you only get this problem with Saturn. I ask because I can focus on stars with no problem.

thanks again

graham

Now you mention it, the stars are ok, it's just big bright Saturn with the double image, one slightly offset from the other, and changes as you move Saturn around in view.

Odd.

[Moonshane]

I wonder if it's internal reflections in the eyepiece?

have you tried it on the moon?

[steelfixer]

the moon is no problem.

i did a complete collimisation with and without the laser. whilst it is a lot better it is still not perfect.

time to try something else.

ho hum