Star diagonals are all out of alignment

Bobbyoutback · #1 23-07-2019, 11:15 AM

After getting all my refractors collimated I then tested my mirror star diagonals & prism diagonal in the scopes to see they all are out of wack .

How can I collimate them without using a laser ?
They are all Chinese made .

Cheers
Bobby

bojan · #2 23-07-2019, 11:21 AM

Long time ago I used a pieces of thin paper as spacers to put the prism or mirror of diagonal in alignment.

However, small misalignemt is not that anoying.. at least not to my taste.

Wavytone · #3 23-07-2019, 11:22 AM

IMHO it’s one more thing for those with OCD to obsess about. Unless you have a test lab most of us have no way to measure these things quantitatively and you take pot luck with what you buy.

Most have no means of adjustment beyond inserting shims under one side or another, ie trial and error. You’re more likely to degrade the mirror or prism by futzing with it than actually achieve much.

As Bojan said I don’t see it as a big issue for visual observing.

If it really bothers you go buy a better quality one from the likes of Lumicon, AP, APM or Televue.

BTW There was a report recently somewhere where a lab tested a large sample of diagonals to measure their orthogonality, transmission, wavefront error and other parameters. Misalignment is not the major problem.

Bobbyoutback · #4 23-07-2019, 11:56 AM

Thanks guys I will have a go at using shims to align them with the optical train .
Don't have enough spare funds to buy top of the range diagonals .

Cheers
Bobby

LewisM · #5 23-07-2019, 01:28 PM

I got my Tak prism diagonal for $100, and my Lumicon 2" diagonal for about the same - you don't need to buy new.

Bobbyoutback · #6 23-07-2019, 09:09 PM

Quote:

Originally Posted by LewisM

I got my Tak prism diagonal for $100, and my Lumicon 2" diagonal for about the same - you don't need to buy new.

Love to get that sort of deal ' in the mean time I will try to figure out how to make these Chinese diagonals adjustable , using shims would be a pain for each as they need to be individually figured out .

Have Fun
Bobby

Ukastronomer (Jeremy) · #7 24-07-2019, 07:31 PM

Can you adjust diagonals, Mine are either William Optic Carbon 2" or Baader Click lock, I thought cheaper (sorry) chinese ones were set, glued together.

I know William Optics make a point of stating their can not, do not need alignment.

Ukastronomer (Jeremy) · #8 24-07-2019, 07:33 PM

Quote:

Originally Posted by Bobbyoutback

Thanks guys I will have a go at using shims to align them with the optical train .
Don't have enough spare funds to buy top of the range diagonals .

Cheers
Bobby

You don't need great funds to buy "top range" ones

Skywatcher diagonals are very good

Bobbyoutback · #9 24-07-2019, 11:02 PM

I did this tonight

Collimated my 2" diagonal , didn't use shims .
After removing the back plate I fitted a 3mm thick plastic foam material that wants to spring back after you apply pressure .
This was cut out a bit smaller then the plate ' then I screwed it back on but not tight .

Now instead of the laborious task of star testing then adjusting again & again'''''''' I just used a cheshire eyepiece ( the short one without the cross hairs ) you leave the lens cap on & adjust the angle of the cheshire to pick up light that will travel up the tube through the objective lens elements & show a bulls eye pattern through the peep hole .

Without the star diagonal I see a perfect concentric bulls eye , with the diagonal I see two bulls eyes overlapping .

I then start tightening one of the four screws whilst looking through the peep hole , I keep trying different screws until I merge the pattern but I want them perfect so I tape half over the 45deg silvered section of the cheshire , now I can see that the two halves of the bulls eye need to be finely touched up to make it perfect .

Also fixed up my inch & a quarter diagonal that was out of wack , the mirror in this was fixed to the back plate so had to fit two rails of the 3mm stuff up the sides to be able to have some shift , anyway got that spot on also .

Have Fun
Bobby

Wavytone · #10 25-07-2019, 08:17 PM

Sounds to me like you've also applied strain too, the force from behind due to the foam will be flexing the mirror.

Bobbyoutback · #11 25-07-2019, 10:07 PM

Quote:

Originally Posted by Wavytone

Sounds to me like you've also applied strain too, the force from behind due to the foam will be flexing the mirror.

That did occur to me also , tonight is clouded over so I'll do a check as soon as it clears up .

Cheers