TheSkySearchers

Can anyone please share a few simple tips on how to check a refractor's optics? Key word being simple, something I can do at home.
Maybe a star test and if so, what am I looking for?

What refractor do you have?
The two main things are checking the alignment of the focuser to the tube and then checking the collimation of the lens.
Do you have a laser collimator and a refractor collimation eyepiece?
https://agenaastro.com/agena-1-25-colli ... ctors.html

Thanks for the link Lady Fraktor. I was not aware there were Cheshires for refractors. The telescope is a Celestron DX 102AZ. It's new and I think the optics are OK but I'm curious to know how one might check this.

Ps. I do have a collimated laser for my dob.

Make a crosshair target for over the objective lens and with the laser in the focuser (without diagonal) see if it hits the center of the target.
If it does its good, if not is it focuser tube droop or misalignment.
If from the focuser use the three screws holding it to the tube to adjust.
Once you have the collimation eyepiece for it the you can easily check alignment of the doublet.

Does the telescope have collimation screws by the objective?

It's a probably a good idea to check a refractor's collimation. Admittedly I have never done so. So far, the images in my achro have been tack sharp with stars as tiny pinpoints.

The best test would be a star test: looking at a high magnification image of a bright star. You do this in focus, but also out of focus, both in- and outside of best focus.

Look here for a few easy tests: https://starizona.com/tutorial/star-tes ... pe-optics/

Search Google for more star testing tricks. Pinched optics for instance is a frequently occurring aberration.

Lady Fraktor wrote: ↑Mon Mar 23, 2020 3:41 pm
Does the telescope have collimation screws by the objective?

Thank you LF. I removed the dew cap and I do not see any screws by the objective lens but there is a black ring over it. I'll try the
star and laser tests before doing anything else. I believe it's actually fine as is, just curious at this point.

Ruud wrote: ↑Mon Mar 23, 2020 3:50 pm The best test would be a star test: looking at a high magnification image of a bright star. You do this in focus, but also out of focus, both in- and outside of best focus.

Look here for a few easy tests: https://starizona.com/tutorial/star-tes ... pe-optics/

Search Google for more star testing tricks. Pinched optics for instance is a frequently occurring aberration.

Thanks Ruud. That's a very useful article on Starizona, especially this part:

..."Almost without exception, all telescopes from reputable manufacturers will perform well enough that you could never notice any errors. If you plan on using your telescope to look at high-power, out-of-focus star images, maybe a star test is a practical examination of the scope’s optics. But in reality, as long as the telescope shows good images under normal use, a star test doesn’t mean too much."

Testing at 40x per inch of aperture does not always give great results, using 50x or 60x is much better.
You are testing the telescope optics so leave the diagonal out when testing, the diagonal can be tested for alignment/ collimation on its own.
1st mag stars work well but a 2nd mag will work as well if available.

If your telescope gives pleasing views of Moon, planets and DSO without introducing any obvious aberrations to the view it is a good telescope.

Duly noted LF.

Will try on Sirius on the next clear night without the diagonal. I have a 12mm Starguider (55x) and a 9mm PL (73x). Will be interesting.

What diagonal are you using?
If the stock one you should check it for alignment as well.
Place your laser collimaor into the diagonal and hold it against a mirror, the laser should bounce back to center and if not you can shim the mirror.

Lady Fraktor wrote: ↑Mon Mar 23, 2020 7:02 pm What diagonal are you using?
If the stock one you should check it for alignment as well.
Place your laser collimaor into the diagonal and hold it against a mirror, the laser should bounce back to center and if not you can shim the mirror.

I'm using a 1.25" Astro-Tech diagonal. It is held by a single screw on his telescope rather than a compression band so potentially it could be misaligned slightly but thankfully it is spot-on center with the laser on the diagonal.

It is snowing here today so no star tests tonight.

Lady Fraktor wrote: ↑Mon Mar 23, 2020 2:33 pm What refractor do you have?
The two main things are checking the alignment of the focuser to the tube and then checking the collimation of the lens.
Do you have a laser collimator and a refractor collimation eyepiece?
https://agenaastro.com/agena-1-25-colli ... ctors.html

A collimating eyepiece for refractors is new to me! Am I the only one? I just read one of the statements in the link:

"Use of this item assumes that your focuser is aligned properly with your telescope tube."

I would like a collimating eyepiece to point out any mis-collimation and not be based on assumptions.

Hmm so how does this work? Comparing to collimation of a Newt, mis-collimation of the primary lens of a refractor should be the equivalent of mis-collimation of the primary mirror of a Newt. Accepted. But mis-collimation of the focuser of a refractor, what is the equivalent for a Newt? Is it comparable to the mis-collimation of the secondary mirror of a Newt?

Trying to figure this out. Until then, I stick my Howie Glatter into the frac focuser tube and look at a dot that I put in the middle of my lens cap to see if it is in line. Sometimes I roll the frac on the ground and project on the door to see if it wiggles up and down.

When looking through the eyepiece you will see circles, you align the lenses using the push/ pull screws until the circles are overlapped and centered.

Once you know the focuser is in alignment and the collimation of the lens is correct the next step is a star test.

Ok but there are at least 4 degrees of freedom. One is the alignment of the primary objective (2 DOFs), the other is the alignment of the focuser (2 DOFs). If the circles look centered but the focuser is off, then the objective must be off as well. When aligning a Dob it is very clear which is which. If the focuser of the frac is off but I must assume that it is not, the alignment of the objective will be off.

Not necessarily, the lens in cell could be fine but the focuser could be out of alignment.
Cheap rack and pinion focusers usually have slop in the (up/ down) as they are moved in and out but the view is centered close enough to the center of the lens that it is not very noticeable.
The longer the focal length the less you will notice focuser misalignment. Short tube refractors can even show coma if the focuser is not aligned.

It is also a good idea to try splitting a few well-known double stars. If you can split the Double Double (Epsilon Lyrae) you are doing well. A 120mm refractor should be able to do this. Others are Albireo, Mizar, Porrima and Castor.

Baurice wrote: ↑Sun Mar 29, 2020 12:33 am It is also a good idea to try splitting a few well-known double stars. If you can split the Double Double (Epsilon Lyrae) you are doing well. A 120mm refractor should be able to do this. Others are Albireo, Mizar, Porrima and Castor.

Yes, thanks Baurice. That is what I have done and I am pleased with the results. Stars are round and pin pointy, and with my 5.5mm Meade I' m able to split the doubles I tried so far. Will check Epsylon Lirae on the next clear clear night.

I am happy with the 102AZ telescope optics as they are and finding it pairs well the Z8.

helicon wrote: ↑Mon Mar 23, 2020 3:44 pm It's a probably a good idea to check a refractor's collimation. Admittedly I have never done so. So far, the images in my achro have been tack sharp with stars as tiny pinpoints.

I was initially not happy with my Orion branded (identical to Celestron & Meade) 120mm f8.3 achromat's performance. Collimation made it brilliant. I did it myself. No need to return.