Refractors for Dummies question

[notFritzArgelander]

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.

As a final check be sure that the inside and outside of focus images are symmetrical and identical. I use an artificial star so I don't need to waste observing time.

[GCoyote]

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.

As a final check be sure that the inside and outside of focus images are symmetrical and identical. I use an artificial star so I don't need to waste observing time.

Can you elaborate for the neophytes?

[notFritzArgelander]

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.

As a final check be sure that the inside and outside of focus images are symmetrical and identical. I use an artificial star so I don't need to waste observing time.

Can you elaborate for the neophytes?

Here is more details with pictures,

https://www.telescope-optics.net/star_t ... escope.htm

[GCoyote]

Ah, more bedtime reading!

[notFritzArgelander]

Ah, more bedtime reading!

It's probably more than you need to know. But keep it as a reference. Just look for perfectly identical inside and outside focus diffraction patterns as a quick check. Then refer to the link if there is something to worry.

[The Happy Parrot]

Gosh, thanks nFA! I am slowly crawling through the text as best I can. Thank you.

I didn't realize how much goes into optimizing optics. We've had a week of clouds, rain and snow and no chance to do any further testing yet. Last I looked I was able to split Castor and Mizar. Epsilon Lyrae was too low on the horizon behind my neighbor's house. I can see the four main stars on the trapezium in Orion's nebula but not E and F. In fairness, I rarely see those with my Z8 either due to lp in my area and viewing conditions.

I will read through the text and pay special attention to the unobstructed view diffraction patterns. I will also try LF's suggestions out of curiosity.

In the mean time, I have a question. I've seen the terms over-correction and under-correction used in several places. What does that mean?

[notFritzArgelander]

Gosh, thanks nFA! I am slowly crawling through the text as best I can. Thank you.

You're very welcome.

I didn't realize how much goes into optimizing optics. We've had a week of clouds, rain and snow and no chance to do any further testing yet. Last I looked I was able to split Castor and Mizar. Epsilon Lyrae was too low on the horizon behind my neighbor's house. I can see the four main stars on the trapezium in Orion's nebula but not E and F. In fairness, I rarely see those with my Z8 either due to lp in my area and viewing conditions.

I will read through the text and pay special attention to the unobstructed view diffraction patterns. I will also try LF's suggestions out of curiosity.

They are good suggestions to try.

In the mean time, I have a question. I've seen the terms over-correction and under-correction used in several places. What does that mean?

It's easiest to understand in terms of glasses. Overcorrection means that the lens is "too strong". Undercorrection means "too weak". Optics (and glasses) can be specified by 1) the radius of curvature of the lens surfaces or 2) the diopter (reciprocal of the focal length in meters). For glasses a diopter value is used for the relatively simple lenses involved. For telescopes with more surfaces the radius of curvature is used. In a Fraunhofer achromat each lens has 2 surfaces, 4 different radii of curvature. Other designs might economize by making some of the surfaces have the same curvature.

These different radii combine to give a perfect star test in a perfectly executed scope.With under correction more light rays are misdirected away from focus outside the normal focus point. For overcorrection the reverse is true and some rays come to a focus too early. So the spherical aberration or bad focusing of the light rays has that sort of analogy with corrective glasses.

There's a handy book, very practical, little math: https://www.willbell.com/tm/tm5.htm

And there is a handy calculator that can generate pictures of what the diffraction patterns should look like: http://aberrator.astronomy.net

It's something you can do with your gear on even a rainy night.

[The Happy Parrot]

This very helpful, thanks again nFA. We have a beautiful night here so I'm signing off to see if I can bag a few more Messier objects and double stars with the new difractor.

More on that later.

Be and stay well!

[The Happy Parrot]

Just came in from a beautiful night. Split Almitak and Capella to confirmmore doubles. I then tried a star test on Betelgeuse and the rings are perfectly round both in and out the focus dot. It seems to me, however, that the bullseye black dot is not centered. When I am in, it is slightly in the lower right quadrant and when I am out it is slightly in the upper left quadrant. This was with the star diagonal and a Meade 5.5mm UWA eyepiece on the OTA. It seems some tweaking might be needed but I need to read up on that.

After the test I did a leisurely galaxy 5K with stops in M42, M44, M45, M47, M51, M81 and M82, M94, M101 (first time), M106 and pit stops at the Sunflower and NGC's 864, 881 and an attempt at CY 19 Atlas (not sure i saw it) and a blinding grand finale in Venus and the moon. What a night, it was so clear!

[Lady Fraktor]

It may be due to atmosphere, temp or a few other things.
If the telescope is putting up excellent views I would not worry about it other than checking for curiosity.

A nice tour as well