Jump to content

sgl_imaging_challenge_2021_4.gif.6a323659519d12fc7cafc409440c9dbf.gif

Centring the Secondary Mirror in the Focuser


Recommended Posts

Hello

I have just purchased my first Newtonian (Skywatcher FlexTube 300P f/5) after many years of owning a small refractor. I am starting to work my way through the collimation process, but seem unable to centre the secondary mirror in the focuser view.

I am using a plain collimation cap which I have inserted into the focuser, i.e. no site tube or Cheshire. I can adjust the secondary mirror so that it is centred in the axis that runs between the sides of the tube, but not in the axis that runs towards/away from the primary mirror. It seems that no matter what adjustments I make, the secondary view in the focuser always appears offset towards the primary mirror. I have adjusted the central screw on the rear of the secondary mirror assembly, but I am at the limit of the movement away from the primary mirror.

Am I doing something wrong here?

Many thanks

Mark

Link to post
Share on other sites

It seems that no matter what adjustments I make, the secondary view in the focuser always appears offset towards the primary mirror. I have adjusted the central screw on the rear of the secondary mirror assembly, but I am at the limit of the movement away from the primary mirror.

Hello Mark,

Did you loosen the secondary central bolt?

If you loosened the central bolt, did you also loosen the three set screws?

Loosening the central bolt the three set screws should move the secondary mirror away from the primary mirror unless you are out of the gap between the secondary mirror stalk and the spider vanes central assembly.

Jason

Link to post
Share on other sites

I am using a plain collimation cap which I have inserted into the focuser

How are you judging the position of the secondary mirror with respect to the focuser? Are you using the primary mirror reflection as your guide?

The collimation cap is not the best tool for this task.

Jason

Link to post
Share on other sites

Did you loosen the secondary central bolt?

If you loosened the central bolt, did you also loosen the three set screws?

I loosened the three set screws first before adjusting the central screw. I figured that loosening the central screw would move it towards the primary and tightening it would move it away.

Link to post
Share on other sites

How are you judging the position of the secondary mirror with respect to the focuser? Are you using the primary mirror reflection as your guide?

I've been using a sheet of white paper, either on the inside tube behind the secondary and opposite the focuser, or rested on the top of the black cap covering the primary mirror. I either get a black reflection (primary cap) in the secondary against a white background, or a white reflection against the black background of the tube.

The collimation cap is not the best tool for this task.

What are the shortcomings of a collimation cap? Would you recommend something else?

Link to post
Share on other sites

I loosened the three set screws first before adjusting the central screw. I figured that loosening the central screw would move it towards the primary and tightening it would move it away.

I stand corrected... Wrote that when I woke up.. Still morning in California...

OK, when you tighten the central bolt the loosen the 3 set screws, why in your opinion the secondary does not "appear" to move away from the primary when you know it did?

Jason

Link to post
Share on other sites

What are the shortcomings of a collimation cap? Would you recommend something else?

The right tool for the task is a sight-tube. The problem with the collimation cap is accuracy. The gap between the secondary mirror edge and the bottom edge of the focuser could be too large to make a good judgement.

Jason

Link to post
Share on other sites

If you have a webcam you can use it to get your secondary perfectly aligned.

Hi sunshine185. I was wondering if its not to much trouble, could you explain this process. If not could you direct me to a link that explains it. I have always used a cheshire and i think its pretty spot on though it would be nice to check with my webcam. thanks

Scott

Link to post
Share on other sites

Hi Auspom, here is a link to astronomyshed video on YouTube which shows how its done.

You will need 2 free software programs, sharpcap and mir-de-collimation, the download links for both are provided in the video. Hope this is of help.

if the link above doesn't work, try this one

Link to post
Share on other sites

i had the same problem with my skyliner 200p dob. when i researched this it seemed that it was a common problem and was advised to use a few thin strips from a milk bottle and place them underneath the right side of the focuser base plate (ie. the side furtherest from the top). this sorted the problem out for me and would recommend it.

Link to post
Share on other sites

Hello

I have just purchased my first Newtonian (Skywatcher FlexTube 300P f/5) after many years of owning a small refractor. I am starting to work my way through the collimation process, but seem unable to centre the secondary mirror in the focuser view.

I am using a plain collimation cap which I have inserted into the focuser, i.e. no site tube or Cheshire. I can adjust the secondary mirror so that it is centred in the axis that runs between the sides of the tube, but not in the axis that runs towards/away from the primary mirror. It seems that no matter what adjustments I make, the secondary view in the focuser always appears offset towards the primary mirror. I have adjusted the central screw on the rear of the secondary mirror assembly, but I am at the limit of the movement away from the primary mirror.

Am I doing something wrong here?

Many thanks

Mark

Afew of us with 300p FlexTubes,have found that the secondary holder is too close to the primary mirror, to allow sufficient outward adjustment. I'm willing to bet this can be solved (like ours were) by swapping the secondary holder vanes to the opposite sides of the secondary holder.

If you look at them, you will see that they have an off-set in one direction - In your case, probably downward. Because they are handed, you can't just turn them around, you have to swap them to the otherside of the holder, but the nett effect is that the off-set they impart, will now be up, rather than down the tube. Voilla! Loads of adjustment, allowing you to mess up alignment in all directions!

Russell

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.