Double moons

[wookie1965]

Why when I look at Jupiter's moons It looks like they a double stars if I focus in they turn clockwise if I focus out the turn counter clockwise, I have read it could be pinched optics but I have not touched the mirror clips. I took the whole cell out when I flocked the tube so what could it be any ideas please.

This is my scope collimated so why am I getting double moons. 

Paul 

Edited November 13, 2023 by wookie1965

[wookie1965]

Is there  no one that can help me unusual on this site. 

[badhex]

Sorry Paul, I'd love to help if I could but not my area of expertise unfortunately. Hopefully someone with ideas will chime in soon. 

Edited November 14, 2023 by badhex

[doublevodka]

Certainly looks collimated to me, all appears to be lined up nicely.

Next suspect would be the mirror clips/pinched optics although this would usually make them misshapen/comet shaped but it's worth a check.

Seems like you are already familiar with removing the cell so pop it back out and back the mirror clips out a little, probably only needs a quarter of a turn, appreciate you haven't touched them but that doesn't mean it was correctly assembled in the factory  Ideally should be able to get a piece of paper between the mirror and the clip, some people say a playing card, whichever floats your boat 👍

[John]

Do you get double images of stars as well ?

Is the effect showing at high and lower magnifications, or just high ?

The fact that they rotate one way and then the other as you travel through focus and back again must be a clue I guess ?

Sorry that you have not had a response in 48 hours - it's not an issue that I have encountered with my dobs to be honest with you.

 

 

 

 

Edited November 14, 2023 by John

[Mr Spock]

Your scope isn't collimated. The primary doughnut should line up with the secondary vanes, I can see it is a long way out.

The secondary looks as though it's under the focuser ok. So you will need to check first the alignment of the secondary, you can do this with a Cheshire or laser - laser is easier, just make sure the laser is pointing in the centre of the primary doughnut.
You can then do the primary, adjust it until the laser hist the Centre of the collimator.

[bosun21]

As has been said already the collimation requires a tweak or two however it's certainly not far enough off to cause rotating moons. My guess is that the mirror cell clips are too tight on one side of the primary mirror. I would slacken them all off and then re tighten them using a playing card as a spacer. I would also tighten them slowly doing one then its directly opposite clip. Good luck.

[dweller25]

1 hour ago, Mr Spock said:

Your scope isn't collimated. The primary doughnut should line up with the secondary vanes, I can see it is a long way out.

The secondary looks as though it's under the focuser ok. So you will need to check first the alignment of the secondary, you can do this with a Cheshire or laser - laser is easier, just make sure the laser is pointing in the centre of the primary doughnut.
You can then do the primary, adjust it until the laser hist the Centre of the collimator.

This 👍

Edited November 14, 2023 by dweller25

[dweller25]

Try adjusting the secondary until the reflection of the primary moves in the direction of the arrow

[wookie1965]

I've collimated it with a laser now going to do a star test then if it's the same I will take the cell out. 

The stars where not points of light but I am used to refractors so I expected them to be diffracted with the vanes but not to the degree they were. 

Jupiter looked very soft with 25mm and 15mm BST, s

[dweller25]

As said above, you should loosen the mirror clips to allow you to slide a piece of flat paper under each one.

You will need to check collimation again visually in the day after you put it all back together and again on a star and then you should be good to go 🤞🤞

Edited November 14, 2023 by dweller25

[dweller25]

@wookie1965

Another couple of thoughts…

Do you fully cool the mirror down before observing ?

Do you observe with the cooling mirror running ?

[wookie1965]

12 minutes ago, dweller25 said:

@wookie1965

Another couple of thoughts…

Do you fully cool the mirror down before observing ?

Do you observe with the cooling mirror running ?

I put the cooling fan on for about 10 minutes before I unplug and start viewing. 

[dweller25]

Good as the fan can cause micro oscillations that could give you a double image

[bosun21]

I don’t think 10 minutes is enough for a 8 - 10” mirror. I used to give mine at least 30 minutes and even then the views improved as the night went on.

[doublevodka]

@wookie1965 have you managed to give it a test after the clip adjustment and collimation, I'm quite curious now to know if it's behaving after the changes?

[wookie1965]

14 minutes ago, doublevodka said:

@wookie1965 have you managed to give it a test after the clip adjustment and collimation, I'm quite curious now to know if it's behaving after the changes?

No afraid not had rain for past 3 days it's overcast now I have not seen a star since Saturday I will of course update this thread as soon as I can get out. 

Thank you 

Paul 

[Captain Scarlet]

Another thought is the side-grubscrews in the mirror cell, restraining the mirror laterally. They also need to be made to contact, then 1/8 turn reversed. With my old 300p I once cleaned the mirror then tightened up those grubs, but not by much. That night my brother in law asked “why are all the stars triangular?”. Mirror-clips and those grubs have a much greater effect than you’d think .

Cheers, good luck with the diagnosis, Magnus

[Ricochet]

What exactly do you mean by “double stars”? Do you mean that you get a double image of each star/moon as in two points?

Alternatively, do you mean that each star/moon  looks like an oval? When you change from outside focus to inside focus is the direction of the oval rotated by 90 degrees? If so this is astigmatism and there are several possibilities for the cause. 
 

1. Your eyes. When you observe using large exit pupils the aberrations in your own eye are the dominant factor. Try changing eyepieces to a lower exit pupil and see if the problem disappears once you go below a certain exit pupil.
2. The eyepiece(s). Many eyepieces have some level of astigmatism, particularly towards the edges. Centre the object in the view and see if it improves and try different eyepieces of similar focal lengths to see if it makes a difference. 
3. The mirrors. It is possible, but unlikely, that one or both of your mirrors have some level of latent astigmatism. However, many mirrors that are perfectly fine at ambient temperature will become astigmatic whilst they are cooling. Given that the cooling time was only ten minutes with a fan blowing onto the mirror then I think it is highly likely that this is the culprit. The solution is to allow the scope more time to acclimatise before observing. 

[wookie1965]

10 hours ago, Ricochet said:

What exactly do you mean by “double stars”? Do you mean that you get a double image of each star/moon as in two points?

Alternatively, do you mean that each star/moon  looks like an oval? When you change from outside focus to inside focus is the direction of the oval rotated by 90 degrees? If so this is astigmatism and there are several possibilities for the cause. 
 

1. Your eyes. When you observe using large exit pupils the aberrations in your own eye are the dominant factor. Try changing eyepieces to a lower exit pupil and see if the problem disappears once you go below a certain exit pupil.
2. The eyepiece(s). Many eyepieces have some level of astigmatism, particularly towards the edges. Centre the object in the view and see if it improves and try different eyepieces of similar focal lengths to see if it makes a difference. 
3. The mirrors. It is possible, but unlikely, that one or both of your mirrors have some level of latent astigmatism. However, many mirrors that are perfectly fine at ambient temperature will become astigmatic whilst they are cooling. Given that the cooling time was only ten minutes with a fan blowing onto the mirror then I think it is highly likely that this is the culprit. The solution is to allow the scope more time to acclimatise before observing. 

No it was two distinct moons very close like a 1.5° split double star they rotated as I focused in and out I have never seen this ever. 

[Ricochet]

5 hours ago, wookie1965 said:

No it was two distinct moons very close like a 1.5° split double star they rotated as I focused in and out I have never seen this ever. 

When you say rotate, did they gradually rotate as you changed focus, or did they jump from one position to another as you moved through the focus point? I think any issue with your mirrors would have to jump from one point to another. 

[wookie1965]

3 minutes ago, Ricochet said:

When you say rotate, did they gradually rotate as you changed focus, or did they jump from one position to another as you moved through the focus point? I think any issue with your mirrors would have to jump from one point to another. 

No they gradually rotate as a focus in and out. 

[Mr Spock]

Can you grab a phone image of it? It would help with diagnostics.

[Richard N]

I’m pretty certain that some of the visual artifacts that I see are actually due to my old eyes. Not suggesting this is the case here though. 

[mikeDnight]

It makes me wonder if either of the mirrors are under strain, causing two foci? It might be worth loosening everything off and starting collimation from scratch. If its worked perfectly in the past then that's all I think it could be. Anoher cause could be an Astigmatic secondary.

Edited November 17, 2023 by mikeDnight