Hi everyone,

 I have a home built 10 inch dobsonian that was built by my astronomy society some years ago. I am having problems getting it to focus on anything, I have tried low powered lenses with barlows to try to increase my focal length and I still have no luck. My focuser is a standard 1.25 inch Orion focuser, and it seems to me that it does not have enough focus travel to bring anything into focus. Can I get a focuser with a longer travel length or is there something else wrong with the telescope? I have collimated  the mirrors, and cleaned and polished them and everything looks good. Any suggestions from anyone would help.

Thanks,

Alan Corey

alancorey1979:

Hi everyone,

 I have collimated  the mirrors, and cleaned and polished them and everything looks good. Any suggestions from anyone would help.

Thanks,

Alan Corey

It could be the focuser, or it could even be the eyepieces you're using. It could even be the mirror.  

Are you allowing it at least an hour to (cool) equalize to the outdoor temperature when you take it outside?  I don't even bother trying to collimate mine until the OTA reaches amient temperature.

What is the focal length of the telescope? 

What is the focal length of the eyepieces you're using? 

How many years is some years ago when the telescope was first built? 

Exactly how did you go about cleaning and polishing your mirrors?

Have A Nice _________ 

HI Alan,

When You said that you cleaned and polished your mirrors, I gasped,  I hope that you did not polish any mirrors!

Mirrors must be cleaned with great care and with the right approach.  If you polished any Mirror you just damaged the optical coatings.

You need to check to see if the problem is insufficient inwards or outwards focuser travel.

To determine which is the problem,  try inserting your lowest power eyepiece ( the higher the length in mm on the side of the eyepiece barrel, the lower the power)

Point it at a star or the Moon, do not pick a close by terrestrial object.

while looking thru the eyepiece rack the focuser all the way in and then all the way out. you should see the image get smaller as you approach focus then as you pass focus the image will appear to enlarge again.

If the image of a star gets smaller as you rack the focuser outwards then stops and never reaches focus before the draw-tube outwards travel stops, then all you need is an extension tube.

If it gets smaller as you rack the focuser inwards,  then its inwards travel that's the problem.

 The only way to correct this is to remove the main mirror cell and reposition it slightly more towards the secondary mirror. or, Get a lower profile focuser.   Please note that using a barlow will make finding things much harder.

You can also try moving the eyepiece by hand completely out of the focuser to see if its outwards travel that is needed

Are you sure you collimated it correctly?

lets hope that yo did not damage the coatings on your mirrors!

Dennis

Thanks for your suggestions. To answer your questions, I don't know when the telescope was built. It was built before my time in my astronomy society. From what I have been told it has never worked since the day it was built, so instead of troubleshooting it, it was just stored away. My guess is a simple miscalculation in the length of the tube.

That was then and now I am the president of this astronomy club. I was asked to troubleshoot this scope because I am familar with reflector scopes.

Rest assured, when I said "polish" I didn't mean using chemicals. I shouldve said buffed with a extremely soft cloth. I cleaned them with a very mild soap and water. However, I should say that when I acquired this scope, both mirrors looked as if they were cleaned with harsh chemicals. They had yellow and blue smudges all over them. So as to whether or not the coating have been damaged, I do not know.

I do let the scope acclimate to the surrounding temperature just as I do with my 5" newtonian. I do know that the scope is collimated correctly. I have my center mark on my primary so that I can easily spot it while looking through my home made collimator, which basically is a 9/64 hole drilled in the center of a lens cover cap. I found that a works very well for collimating. I just leave the center marker on my primary since that spot in the mirror is in the shadow of the secondary anyway.

Now, to sum up what I found out. I finally just took the focuser completely off the telescope. I then just took my lowest power lens and manually moved it towards the secondary mirror. I should note, that I had the scope aimed at a ski slope about 15 miles away. I can easily see the ski lifts in a pair of binoculars so I thought this would be a good target. I found that my lense had to be 1 15/16 inch from the mirror in order to be focused. Which the focuser was NEVER going to do since that number is physically inside the scope.

So my solution to this problem was to move the secondary mirror. I thought if I can't take the lenses to the mirror, I will take the mirror to the lenses. So, I went and bought a carriage bolt and a few nuts and went to work. I had to move the secondary mirror a total of 2 1/2 inches toward the focuser in order for it to focus properly. A low profle focuser wouldn't have worked because the top of the lens piece would have to be inside the scope to work properly. So once I got the ski lift in focus, I had re collimate the scope and once I did, I waited till night fall. I pointed at the moon and it came into focus with a 40mm with little effort at all. I then tried all my other lenses to see if I can get a focus. I used a 25, 20, 12.5 and a 7mm lens. All of them come into focus each one needing finer and finer focusing ability. I found the 12.5 is about all I am going to get with this scope which isn't suprising. I've never been able to use a 7 or a 4 on any scope. So why do they exist?

After all this, I have learned a thing or two about building a scope. This almost seems like the builder of this scope picked a length of the tube he liked and tried to make it work. Which of course if anyone ever played with a magnifying glass as a kid knows that in order to burn things, you need to move the lenses up or down to make the light a fine point. Which should be the same procedure on building a scope. So that is why I think there was a simple miscalculation in the length of the tube.

Thanks everyone for your suggestions. Next time I know.

Alan C.

If the secondary mirror was centered in the OTA and on center with the focuser then it was in the proper possition and probably properly collimated to the focuser.  Usually an off axis or clear aperture Newtonian has the secondary on the oposite side of the OTA from the focuser.
You are probably correct the there may have been a miscalculation in the length of the tube to the focal ratio of the primary mirror.  Who knows, you might have a f/4 mirror in an f/6 tube or something like that.  
What you could have done is to remove the primary mirror and shorten the tube moving the primary mirror closer to the point of prime focus.  Without knowing the primary mirrors focal ratio it is hard to tell how close you would need to adjust the focal length of the OTA.  So you would have to experiment a little and move it in small steps.
And being old and the discoloration you describes tells me that the coating have been degrated. Getting the mirror refinished and tested probably wouldn't hurt.

Have A Nice _________