29 replies to this topic

[Jeff L]

Is there a test I can perform on my new xx14g mirror to see if I got a good one or do I have to send it to someone?

[BradFran]

Star test and Ronchi test. An inside and outside of focus star at around 1mm exit pupil (eyepiece focal length equal to your f/ratio) will show how good your mirror is. It depends on having good collimation, thermal equilibrium and steady atmospheric seeing though. A Ronchi test is similar, but holding a screen in front of your focuser and analyzing the lines on the grating. Search around here for more info.

[junomike]

Is there a test I can perform on my new xx14g mirror to see if I got a good one or do I have to send it to someone?

Look at the Planets and/or Globulars at 300X - 400X.  If the Image is sharp and offers a "wow", ya got a good one.

[Jeff L]

Look at the Planets and/or Globulars at 300X - 400X.  If the Image is sharp and offers a "wow", ya got a good one.

And if not what do I do?

[kfrederick]

I live in Bedford Pa stop by I test it 

[junomike]

And if not what do I do?

See what max magnification you can obtain.

[junomike]

I live in Bedford Pa stop by I test it 

This sounds like a good solution

[Asbytec]

If the Image is sharp and offers a "wow", ya got a good one.

Yep, and a similar test is the snap to focus test. Poor scopes (or conditions) will tend to mush through focus. A good scope (under good conditions) will have a more convincing focus. Focus will not be in doubt as it tends to "snap" into focus instead of mushing through focus. Good scopes will take higher magnification while with poor scopes the degradation becomes visible at around 1x per millimeter of aperture, give or take. 

 

You can do a simple star test by scrolling through focus a little on both sides (3 or 4mm) when the scope is well collimated and at thermal equilibrium. If the defocused image looks pretty much the same on both sides at about the same defocus, your scope is probably okay. It probably won't look exactly the same on either side, but similar enough is good enough. 

Edited by Asbytec, 30 November 2020 - 08:37 AM.

[Bean614]

All of these well meaning suggestions,  of course, Assume you know how to properly Collimate your scope!  If it's not properly collimated,  then ANY of those tests are completely meaningless. 

[GUS.K]

Don't worry about testing yet, if you don't already know, learn how to collimate it and just enjoy the views.

[RobertMaples]

Look at the Planets and/or Globulars at 300X - 400X.  If the Image is sharp and offers a "wow", ya got a good one.

That's not a good test unless you know the seeing is very good.  You could have an almost perfect mirror and have the image break down well below 300x with bad seeing.

[Asbytec]

That's not a good test unless you know the seeing is very good. You could have an almost perfect mirror and have the image break down well below 300x with bad seeing.

Yes, that's true. But, if the image is clean and sharp, then seeing and the scope are good. If the image breaks down below 300x, then it's probably seeing. If so, any image wavering should be an indication of poor seeing and should make testing for quality more difficult. I understand this is your point well taken.

[Jeff L]

All of these well meaning suggestions,  of course, Assume you know how to properly Collimate your scope!  If it's not properly collimated,  then ANY of those tests are completely meaningless. 

I use the hotech lazer collimater. It’s quite simple to collimate with it.

[Jeff L]

Yep, and a similar test is the snap to focus test. Poor scopes (or conditions) will tend to mush through focus. A good scope (under good conditions) will have a more convincing focus. Focus will not be in doubt as it tends to "snap" into focus instead of mushing through focus. Good scopes will take higher magnification while with poor scopes the degradation becomes visible at around 1x per millimeter of aperture, give or take. 

 

You can do a simple star test by scrolling through focus a little on both sides (3 or 4mm) when the scope is well collimated and at thermal equilibrium. If the defocused image looks pretty much the same on both sides at about the same defocus, your scope is probably okay. It probably won't look exactly the same on either side, but similar enough is good enough. 

This is my second xx14g. The first one I bought used it was about eight years old. I sold it then bought another brand new one. This one doesn’t seem to “snap” to focus. Ive only had it out twice both times the seeing wasnt excellent by any means. The stars were dancing anyway. I forgot about the star test focus deal. I’ll try that next time. Hopefully the moon won’t be in the way either.

[Asbytec]

This is my second xx14g. The first one I bought used it was about eight years old. I sold it then bought another brand new one. This one doesn’t seem to “snap” to focus. Ive only had it out twice both times the seeing wasnt excellent by any means. The stars were dancing anyway. I forgot about the star test focus deal. I’ll try that next time. Hopefully the moon won’t be in the way either.

Even if the image doesn't snap to focus during lesser seeing, the star test being more similar than dissimilar on both sides of focus will be telling. You may be able to read through some level of lesser seeing. Be sure the primary is thermally stable, the star test is that sensitive. 

[Jeff L]

https://optcorp.com/...iece-10-101-150

Anybody try one of these ?

[Asbytec]

No, but now I have to get one, too. Thanks. 

 

I've used 133 LPI Ronchi gratings, just slap it over the focuser. 

 

Remember, this is a null test. Straight is good. 

[esd726]

https://optcorp.com/...iece-10-101-150

Anybody try one of these ?

🤔

Edited by esd726, 04 December 2020 - 06:43 AM.

[Starman1]

https://optcorp.com/...iece-10-101-150

Anybody try one of these ?

I got one years ago to test optics.  A great tool.  You can evaluate the optics in a scope quite easily with one, and see zones and bumps in the mirror surface, and evaluate correction at the focal plane.

One thing to remember is that you are evaluating both mirrors together.

[Spikey131]

This is my second xx14g. The first one I bought used it was about eight years old. I sold it then bought another brand new one. This one doesn’t seem to “snap” to focus. Ive only had it out twice both times the seeing wasnt excellent by any means. The stars were dancing anyway. I forgot about the star test focus deal. I’ll try that next time. Hopefully the moon won’t be in the way either.

A 14” scope in Easton, PA is not going to “snap” into focus very often.....

[Pinbout]

https://optcorp.com/...iece-10-101-150

Anybody try one of these ?

a good mirror looks like this with 2 lines

https://youtu.be/z3xQhmI81kw

 

if it wasn't good you'd see the lines go like )( to () or the other way depending on the direction of the correction error.

 

with the ronchi you have to go 2 line inside of focus to 2 lines outside of focus... keep flipping to help see if the lines do )( to ()

 

also you could have a secondary that isn't flat enough... that will look like astig in a star test very close to focus...

 

the every so slightly defocused star will look like - then |  going from inside to outside of focus

[Starman1]

You might like to see what can be seen with a Ronchi grating mounted in an eyepiece:

https://www.teleskop...nchi-manual.pdf

[Jeff L]

You might like to see what can be seen with a Ronchi grating mounted in an eyepiece:

https://www.teleskop...nchi-manual.pdf

I ordered the one in my post, #16 

[tboss70]

I ordered one about a 4-6 weeks ago....still waiting.

[Jeff L]

I ordered one about a 4-6 weeks ago....still waiting.

Did you get yours yet? I ordered one on Dec 4 still waiting