42 replies to this topic

[Santola]

Hello guys. I'm starting out in this new hobby and I recently bought my first telescope, a 127/900mm.
 

But I started to notice that when I observe the Moon, “circles” appear around it. I even thought that my telescope was extremely decollimated, but it wasn't that.
 

Currently I don't have much idea what this is and what would be causing it.

 

Thanks!

Edited by Santola, 28 September 2023 - 02:22 PM.

[Couder]

Only a guess on my part - but it looks pretty much like mine do when there is a lot of moisture in the air.

[UnityLover]

is the 127 the powerseeker 127? If so, return it if you can.  It is a bird-jones type, which means that it has a spherical mirror and a barlow. Essentially, it gives blurry images at higher magnifications. (saturn wont show cloud bands or the cassini division, jupiter loses detail too.

 

If you have something else, please tell us what you have.

Edited by UnityLover, 28 September 2023 - 02:34 PM.

[kgb]

Specific model of scope would be helpful. The ring you are highlighting is not concentric which would likely eliminate an effect due solely to the effects of atmospheric moisture and likely point to a problem with optical alignment.

Edited by kgb, 28 September 2023 - 02:56 PM.

[Santola]

is the 127 the powerseeker 127? If so, return it if you can.  It is a bird-jones type, which means that it has a spherical mirror and a barlow. Essentially, it gives blurry images at higher magnifications. (saturn wont show cloud bands or the cassini division, jupiter loses detail too.

 

If you have something else, please tell us what you have.

No, it's not a Powerseeker 127. Regarding the planets, I can observe them well. Jupiter I see its equatorial spots easily and Saturn I see its rings perfectly.

[Santola]

Specific model of scope would be helpful. The ring you are highlighting is not concentric which would likely eliminate an effect due solely to the effects of atmospheric moisture and likely point to a problem with optical alignment.

This telescope is from a famous ATM here in Brazil.
 

I recently aligned the telescope mirrors, but I didn't check that they were aligned 100% correctly because I didn't do a star test. I will do this today to confirm whether or not it is collimation.
 

But I believe that this type of effect on the Moon would only appear if it was extremely decollimated, or am I wrong?

Edited by Santola, 28 September 2023 - 03:03 PM.

[Santola]

Only a guess on my part - but it looks pretty much like mine do when there is a lot of moisture in the air.

I didn't know about this atmospheric effect caused by moisture. I live in a coastal city, so it could even be one of the factors.

Edited by Santola, 28 September 2023 - 03:06 PM.

[kgb]

Without inspecting your scope, I am going to say that you are correct. It appears that this is a collimation problem. A star test will be difficult to interpret unless you know what to look for. Do you have a Cheshire eyepiece that you can use instead? Check out the links pinned in the reflector forum for a lot of detailed information on collimating a Newtonian.

https://www.cloudyni...ors/#entry75721

[kgb]

I didn't know about this atmospheric effect caused by moisture. I live in a coastal city, so it could even be one of the factors.

I think it is actually both. You are seeing the effects of light reflecting off of the atmosphere, but that your optics aren't properly aligned making that ring non-concentric. Otherwise, that ring you are seeing would be concentric with the moon. Was that image of the moon taken when it was perfectly centered in the eyepiece?

[UnityLover]

I didn't know about this atmospheric effect caused by moisture. I live in a coastal city, so it could even be one of the factors.

I have the effect uniformly around the moon, due to it being bright. Probably just the moon's glow offset from the moon due to collimation.

[Santola]

Without inspecting your scope, I am going to say that you are correct. It appears that this is a collimation problem. A star test will be difficult to interpret unless you know what to look for. Do you have a Cheshire eyepiece that you can use instead? Check out the links pinned in the reflector forum for a lot of detailed information on collimating a Newtonian.

https://www.cloudyni...ors/#entry75721

 

 

I think it is actually both. You are seeing the effects of light reflecting off of the atmosphere, but that your optics aren't properly aligned making that ring non-concentric. Otherwise, that ring you are seeing would be concentric with the moon. Was that image of the moon taken when it was perfectly centered in the eyepiece?

Unfortunately I don't remember.
 

Regarding the Cheshire eyepiece, I don't have it, but I intend to buy it soon.
 

Thanks for the help, Apollo!

[kgb]

Start with making sure the scope is well collimated. Then once you are certain, repeat the test on the moon and see if your results are improved. Make sure the moon is centered in the field of view. If there is still a ring evaluate if it is now concentric? Then move the moon off axis in one direction. Is it still concentric or does it now flare off to one side? If it is still concentric then you are done. If not move the moon off axis to the opposite side of the field of view. Did the ring switch directions and flare to the other side? If so, you have a problem that requires further evaluation. Try to repeat the test on a night that is clear and with very low humidity. Ideally, you should no longer have a ring surrounding the moon. If that ring persists regardless of viewing conditions then you might have an irreparable problem with the figure of the mirror.

[UnityLover]

Do you have a collimation cap? I collimate my f/5 reflector with one. You have an f/6 one, which means your collimation can be less precise.

[briansalomon1]

Make sure the telescope is cooled off completely and

 

This telescope is from a famous ATM here in Brazil.
 

I recently aligned the telescope mirrors, but I didn't check that they were aligned 100% correctly because I didn't do a star test. I will do this today to confirm whether or not it is collimation.
 

But I believe that this type of effect on the Moon would only appear if it was extremely decollimated, or am I wrong?

 

 

Because mirrors reflect all of the light equally, they don't produce chromatic aberrations, so I don't think collimation is the issue.

https://en.wikipedia...onian_telescope

 

Here is a publication by Mike Lockwood that explains how to evaluate your collimation using a bright star. It should be fairly easy to tell if your collimation is very far out of alignment by looking at the diffraction rings produced by a star. They should be concentric and should look like the picture in the publication. The nice thing about using a star for collimation is that there is no error in the diffraction rings at all.

http://www.loptics.c.../starshape.html

 

I wouldn't be surprised if there is some CA in your corrector, or the eyepiece(s) you're using. If you look at the wikipedia description for your design you can see it's intended to be an entry level telescope, and will naturally have some limitations.

 

Glad to hear you're happy with the views of Saturn and Jupiter and welcome to CN.
 

[Santola]

Start with making sure the scope is well collimated. Then once you are certain, repeat the test on the moon and see if your results are improved. Make sure the moon is centered in the field of view. If there is still a ring evaluate if it is now concentric? Then move the moon off axis in one direction. Is it still concentric or does it now flare off to one side? If it is still concentric then you are done. If not move the moon off axis to the opposite side of the field of view. Did the ring switch directions and flare to the other side? If so, you have a problem that requires further evaluation. Try to repeat the test on a night that is clear and with very low humidity. Ideally, you should no longer have a ring surrounding the moon. If that ring persists regardless of viewing conditions then you might have an irreparable problem with the figure of the mirror.

 

Start with making sure the scope is well collimated. Then once you are certain, repeat the test on the moon and see if your results are improved. Make sure the moon is centered in the field of view. If there is still a ring evaluate if it is now concentric? Then move the moon off axis in one direction. Is it still concentric or does it now flare off to one side? If it is still concentric then you are done. If not move the moon off axis to the opposite side of the field of view. Did the ring switch directions and flare to the other side? If so, you have a problem that requires further evaluation. Try to repeat the test on a night that is clear and with very low humidity. Ideally, you should no longer have a ring surrounding the moon. If that ring persists regardless of viewing conditions then you might have an irreparable problem with the figure of the mirror.

The Moon is now rising here and I took the opportunity to do this quick test without changing anything in the collimation.
 

The ring remains, but I have a slight impression that it is weaker than yesterday. As I move the Moon off axis, the ring does not change its intensity, shape or direction.
 

The forecast for today's humidity is 65%, almost 10% lower than yesterday.

[Santola]

Make sure the telescope is cooled off completely and

 

 

 

Because mirrors reflect all of the light equally, they don't produce chromatic aberrations, so I don't think collimation is the issue.

https://en.wikipedia...onian_telescope

 

Here is a publication by Mike Lockwood that explains how to evaluate your collimation using a bright star. It should be fairly easy to tell if your collimation is very far out of alignment by looking at the diffraction rings produced by a star. They should be concentric and should look like the picture in the publication. The nice thing about using a star for collimation is that there is no error in the diffraction rings at all.

http://www.loptics.c.../starshape.html

 

I wouldn't be surprised if there is some CA in your corrector, or the eyepiece(s) you're using. If you look at the wikipedia description for your design you can see it's intended to be an entry level telescope, and will naturally have some limitations.

 

Glad to hear you're happy with the views of Saturn and Jupiter and welcome to CN.
 

Got it!

Sorry, what's "CA"?

Thanks!

Edited by Santola, 28 September 2023 - 03:54 PM.

[Inkie]

Would this scope benefit from flocking?  Is the secondary centered? Is the eyepiece/accessory optic clean?  Slightly turned edge of the primary?

[Inkie]

CA is 'chromatic aberration',  SA is 'spherical aberration'.

[briansalomon1]

CA is 'chromatic aberration',  SA is 'spherical aberration'.

There are others who can explain this better than I can but here's what (believe) I understand.

 

CA, or color aberration or "errors" happen when different colors reach the point of focus (focal plane) in different places, either in front of or behind focus.

 

SA or spherical aberration means all the light being gathered isn't all being directed exactly to the focal plane when the object is in focus. It looks fuzzy, or perhaps the center is in focus but not the outer edge. In that case you could focus on either the center of the object or the outer edge, but not both at the same time.

 

[kgb]

There are others who can explain this better than I can but here's what (believe) I understand.

CA, or color aberration or "errors" happen when different colors reach the point of focus (focal plane) in different places, either in front of or behind focus.

SA or spherical aberration means all the light being gathered isn't all being directed exactly to the focal plane when the object is in focus. It looks fuzzy, or perhaps the center is in focus but not the outer edge. In that case you could focus on either the center of the object or the outer edge, but not both at the same time.

Since the ring was colorless, I was leaning more towards SA, but wanted to rule out alignment error and the effects of the atmosphere first. Not having the scope and additional photos make's troubleshooting this a little difficult, but I think that ruling out alignment errors is a good first step. Also, we can't rule out reflections but these would be impacted by the location of the object in the FOV and according to the OP's latter post it doesn't appear that the reflection is moving with the placement of the object in the field.

[star acres]

Did you peer through panels of glass? That will create repeat images very close to each other.

[Santola]

Did you peer through panels of glass? That will create repeat images very close to each other.

No, I didn't.

[Santola]

Since the ring was colorless, I was leaning more towards SA, but wanted to rule out alignment error and the effects of the atmosphere first. Not having the scope and additional photos make's troubleshooting this a little difficult, but I think that ruling out alignment errors is a good first step. Also, we can't rule out reflections but these would be impacted by the location of the object in the FOV and according to the OP's latter post it doesn't appear that the reflection is moving with the placement of the object in the field.

Spherical aberration occurs in spherical mirrors correct? (Ignoring the eyepieces) 
 

My mirror is parabolic, and I use Celestron's aspherical eyepieces. (23mm | 10mm)

Edited by Santola, 28 September 2023 - 05:40 PM.

[Santola]

I got better images of what is happening to me:
 

 

 

[walt r]

Maybe it is internal reflections. Could be in the Eyepiece. The Moon is very bright.

Does this ring show with a different eyepiece?