33 replies to this topic

[gcnadam2906]

(10" zhumell dob f5)Long story short I pulled my telescope out of the building for the first time in awhile to view the eclipse.  The mirrors were very dirty so I cleaned them for the first time ever.  After reassembling I was having a hard time lining up the secondary mirror with the focuser when I noticed the previous owner added a .25 inch washer and plastic spacers between the secondary adjustment screws and the plate.  I removed them, collimated the scope the best I could and enjoyed the amazing eclipse.

 

Well now that the scope is cleaned up I have been using it more, and when I try to view stars with the 9 or 5mm eyepiece I get odd shapes.  I'm worried after the major issue I found I don't have things set quite right.  (Moon looks fine with even the 5).  Can someone lead me in the right direction here?  I attached an example of what a star looks like.

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[frwaf bfds]

Yes, the star shape is due to coma caused by collimation issues (of the primary mirror).

 

Star shapes will go back to being round and views of all targets

including the moon will become significantly sharper than before after a quick collimation.

[Asbytec]

What tool are you using to align the primary mirror? 

 

Also, can you take a picture showing the bottom of the focuser draw tube and the secondary mirror? A white paper background behind the diagonal opposite the focuser helps us assess your alignment.  

[gcnadam2906]

What tool are you using to align the primary mirror?



Also, can you take a picture showing the bottom of the focuser draw tube and the secondary mirror? A white paper background behind the diagonal opposite the focuser helps us assess your alignment.

I'll try to take a picture tonight. I was using a Cheshire collimating tool. But it wasn't working well to position the secondary mirror, so I purchased a collimation cap that's supposed to be coming today. I also have a laser but I didn't use it this time.

[gcnadam2906]

Here is the view down the scope.  I can see the collimation is way off, but I'm questioning the secondary mirror position.  Can you see anything from this?

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[gcnadam2906]

Trying again

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[Asbytec]

Can't really see it on my cell phone at work, but looking better.

It's dark in there. Put a white paper background opposite the focuser.

This is taken through your Cheshire combo tool? Try the collimation cap.

[gcnadam2906]

Can't really see it on my cell phone at work, but looking better.

It's dark in there. Put a white paper background opposite the focuser.

This is taken through your Cheshire combo tool? Try the collimation cap.

Ahh I see what you are saying... I was putting the paper over the opening lol.  

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[Asbytec]

Much better view. I don't have the ability to assess at the moment, but I'm sure someone can. Vic normally pops in with annotations showing alignment. You're close at least. I'll look closer after work.

If the cross hair of the Cheshire is collimated to the primary center mark, the reflection of the primary mirror must be centered under the focuser. You can use this fact to assess the position of your diagonal. If it is centered in the diagonal, it will be concentric with the focuser, too.
 

Let's assume the focuser axis is aligned to the primary center. The primary reflection will be centered under the focuser. So, you can see your diagonal mirror should or can move toward the primary mirror on the right and maybe tilt upward a tiny bit. But! That is only true if the cross hair in on the primary center.

 

Edit: Yea, you are close. I cannot see your primary center marker. I presume it's centered on that black dot? If so, a slight tilt of the diagonal will bring the blue cross hair directly onto the black dot and center the primary reflection in the green circle (use one tilt screw in line with the focuser). The red circle is where your diagonal should be, but it's fine as is because you can see the entire primary reflection with room to spare. Then, realign the primary center mark to the Cheshire or collimation cap. 

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Edited by Asbytec, 20 April 2024 - 04:22 AM.

[Starman1]

(10" zhumell dob f5)Long story short I pulled my telescope out of the building for the first time in awhile to view the eclipse.  The mirrors were very dirty so I cleaned them for the first time ever.  After reassembling I was having a hard time lining up the secondary mirror with the focuser when I noticed the previous owner added a .25 inch washer and plastic spacers between the secondary adjustment screws and the plate.  I removed them, collimated the scope the best I could and enjoyed the amazing eclipse.

 

Well now that the scope is cleaned up I have been using it more, and when I try to view stars with the 9 or 5mm eyepiece I get odd shapes.  I'm worried after the major issue I found I don't have things set quite right.  (Moon looks fine with even the 5).  Can someone lead me in the right direction here?  I attached an example of what a star looks like.

This drawing shows coma, modified by astigmatism.  When astigmatism is really bad, coma makes stars look like seagulls, but this astigmatism is mild, so you get the images you see.

The points should all point toward the center of the field.  That is, IF this is a drawing of a star at the edge of the field.

 

If, on the other hand, a star in the center of the field looks like that, then the scope is seriously out of collimation.

 

I'll try to take a picture tonight. I was using a Cheshire collimating tool. But it wasn't working well to position the secondary mirror, so I purchased a collimation cap that's supposed to be coming today. I also have a laser but I didn't use it this time.

A collimation cap is a primary mirror adjustment tool and won't help as much as the Sight tube/Cheshire for aligning the position of the secondary.

[Vic Menard]

...Let's assume the focuser axis is aligned to the primary center. The primary reflection will be centered under the focuser. So, you can see your diagonal mirror should or can move toward the primary mirror on the right and maybe tilt upward a tiny bit. But! That is only true if the cross hair in on the primary center.

 

...you are close. I cannot see your primary center marker. I presume it's centered on that black dot? If so, a slight tilt of the diagonal will bring the blue cross hair directly onto the black dot and center the primary reflection in the green circle (use one tilt screw in line with the focuser). The red circle is where your diagonal should be, but it's fine as is because you can see the entire primary reflection with room to spare. Then, realign the primary center mark to the Cheshire or collimation cap. 

I agree with your assessment. That said, the collimation is "close enough" to deliver good performance in the center of the field of view, unless something else is impacting the image performance.

 

This drawing shows coma, modified by astigmatism.

I suspect this is true too. Could be the primary mirror is being pinched in the cell, or the secondary mirror may be pinched in its holder, or both. I wonder what eyepieces the OP was using?

 

Annotated view below, agrees with Norme's annotated view and also assumes the primary mirror center marker is in the center of the Cheshire. (All circles are concentric.)

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[gcnadam2906]

Asbytec, on 19 Apr 2024 - 10:36 PM, said:

Much better view. I don't have the ability to assess at the moment, but I'm sure someone can. Vic normally pops in with annotations showing alignment. You're close at least. I'll look closer after work.

If the cross hair of the Cheshire is collimated to the primary center mark, the reflection of the primary mirror must be centered under the focuser. You can use this fact to assess the position of your diagonal. If it is centered in the diagonal, it will be concentric with the focuser, too.

Let's assume the focuser axis is aligned to the primary center. The primary reflection will be centered under the focuser. So, you can see your diagonal mirror should or can move toward the primary mirror on the right and maybe tilt upward a tiny bit. But! That is only true if the cross hair in on the primary center.

Edit: Yea, you are close. I cannot see your primary center marker. I presume it's centered on that black dot? If so, a slight tilt of the diagonal will bring the blue cross hair directly onto the black dot and center the primary reflection in the green circle (use one tilt screw in line with the focuser). The red circle is where your diagonal should be, but it's fine as is because you can see the entire primary reflection with room to spare. Then, realign the primary center mark to the Cheshire or collimation cap.

It looks to me(in that picture) like the secondary mirror is not centered side to side. Is this something I should try to adjust? Also should the secondary mirror be in the center of the dark red circle up and down the tube too. It looks like it may be a little close to the main mirror.

[Vic Menard]

It looks to me(in that picture) like the secondary mirror is not centered side to side. Is this something I should try to adjust? Also should the secondary mirror be in the center of the dark red circle up and down the tube too. It looks like it may be a little close to the main mirror.

For reference (note that all circles are concentric except the reflection of the secondary mirror/violet circle):

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Edited by Vic Menard, 20 April 2024 - 10:47 AM.

[Skywatchr]

For reference (note that all circles are concentric except the reflection of the secondary mirror/violet circle):

And the offset is towards the focuser.

[Starman1]

And the offset is towards the focuser.

The offset in the properly-positioned secondary is toward the primary and AWAY from the focuser.

Unless you're referring to gncadam2906's post .

In his image, the secondary appears to be slightly mis-rotated and a bit too high in the tube toward the spider vanes (assuming the grey crescent is part of the secondary's reflective surface).

 

The same offset is achieved with a centered-in-the-tube secondary by the process of collimation, as it results in the primary's optical axis being tipped slightly toward the focuser.

Perhaps that is what you were referring to.

[Skywatchr]

The offset in the properly-positioned secondary is toward the primary and AWAY from the focuser.

Unless you're referring to gncadam2906's post .

In his image, the secondary appears to be slightly mis-rotated and a bit too high in the tube toward the spider vanes (assuming the grey crescent is part of the secondary's reflective surface).

 

The same offset is achieved with a centered-in-the-tube secondary by the process of collimation, as it results in the primary's optical axis being tipped slightly toward the focuser.

Perhaps that is what you were referring to.

Yes, the side to side (of the ellipse) needs to be centered.  And that is assuming the focuser is squared to begin with.  From top to bottom (toward or away from the primary) isn't as critical as long as the focus drawtube is square, and there is enough FOV to encompass all the light cone.  Centerline of the optical axis is what is critical.  Now an "exact" sized elliptical secondary for the light cone will not have any wiggle room so you must get it right, or miss out on some illumination.  That will also cause some reflection artifacts, and some illusions the brain struggles with.

[Starman1]

It might be mentioned that the focuser need not be perpendicular to the tube.  There are even scopes where the focuser is purposely angled to see a secondary positioned higher in the tube.

What IS important is that the secondary be centered under the focuser, however the focuser is pointed.

 

You can see from that that if the secondary is centered in the tube, but NOT centered under the focuser, it's perfectly acceptable to shim the focuser angle to point it exactly at the secondary.

Where I see this is necessary sometimes is when the center bolt of the secondary is not parallel to the tube.

I'm not saying this is the case, here, but I throw it in as a possible solution.

[gcnadam2906]

The offset in the properly-positioned secondary is toward the primary and AWAY from the focuser.

Unless you're referring to gncadam2906's post .

In his image, the secondary appears to be slightly mis-rotated and a bit too high in the tube toward the spider vanes (assuming the grey crescent is part of the secondary's reflective surface).

 

The same offset is achieved with a centered-in-the-tube secondary by the process of collimation, as it results in the primary's optical axis being tipped slightly toward the focuser.

Perhaps that is what you were referring to.

 

The gray crescent you see is not part of the reflective surface. It's the rough side of the mirror itself.

[Vic Menard]

The gray crescent you see is not part of the reflective surface. It's the rough side of the mirror itself.

Here's the pretty much "round", not so much elliptical, reflective secondary mirror surface (green circle), and the outermost blue circle (concentric relative to the green circle), showing small errors in both rotation and offset (probably mostly rotation). But I don't believe these small errors have anything to do with your star shape...

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Edited by Vic Menard, 20 April 2024 - 05:37 PM.

[Asbytec]

It looks to me(in that picture) like the secondary mirror is not centered side to side. Is this something I should try to adjust? Also should the secondary mirror be in the center of the dark red circle up and down the tube too. It looks like it may be a little close to the main mirror.

You do not have to adjust the diagonal centering because you can see the entire primary reflection. So, your field is fully illuminated, so diagonal position is good. What is important for image quality is axial alignment. The cross hair appears to be very close to the primary center and probably within tolerance.

 

Double check the cross hair is on the primary center, then make sure the primary center mark is aligned with the collimation cap by tilting the primary mirror. The latter should get rid of the V shaped stars. If you want, you can try to get a little better diagonal position centered under the focuser later.

 

I was a bit confused over the rough grey crescent, too. Normally the edge of the primary is smooth. You can paint it with a black sharpie to reduce any possible stray light from entering the focuser. It might be helpful to check the clips are not squeezing the mirror if you see astigmatism on axis.

Edited by Asbytec, 20 April 2024 - 05:20 PM.

[Starman1]

The gray crescent you see is not part of the reflective surface. It's the rough side of the mirror itself.

I presumed the black surface to the left of the mirror was the secondary mirror's edge and that the black ring around the outside represented the edge of the reflective surface.

But if the mirror is cut from a cylinder at a 45° angle, that sand-blasted grey surface should extend to the left quite a way.

Is your secondary mirror super thin?

 

It should look like this, where the surface to the left is the edge of the secondary mirror.

How exactly is the secondary mirror attached?

Does your secondary mirror not look like the 2nd picture?:

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Edited by Starman1, 20 April 2024 - 05:10 PM.

[Vic Menard]

I presumed the black surface to the left of the mirror was the secondary mirror's edge and that the black ring around the outside represented the edge of the reflective surface.

I think the OP's secondary mirror is sitting inside a plastic surround that doesn't extend to the front edge of the secondary (hence the clip on the far right). I also suspect the secondary is glued inside the plastic surround, which may actually contribute some astigmatism...

[Starman1]

I think the OP's secondary mirror is sitting inside a plastic surround that doesn't extend to the front edge of the secondary (hence the clip on the far right). I also suspect the secondary is glued inside the plastic surround, which may actually contribute some astigmatism...

Kind of like the original Meade LightBridge scopes in 10" and 12".

[gcnadam2906]

I think the OP's secondary mirror is sitting inside a plastic surround that doesn't extend to the front edge of the secondary (hence the clip on the far right). I also suspect the secondary is glued inside the plastic surround, which may actually contribute some astigmatism...

Yes you are right it is sitting inside a plastic surrounding.  And I have to say I'm not completely happy with it.  If I push on the (gray crescent) that we have been talking about I can move the mirror slightly inside the plastic housing.  I did try to remove the mirror from the housing and I was planning on either gluing or silicone it in place but I couldn't pull it out easily and decided it wasn't worth messing up the mirror at this time.  It is hard to tell, but the mirror does not look extremely thin. As in I don't think it's less than a quarter of an inch and could be thicker.  But I am also guessing the manufacturer went with this design so they could make it thinner than normal.

Edited by gcnadam2906, 20 April 2024 - 06:14 PM.

[gcnadam2906]

I tried taking a pic of the secondary but the file size is too big so I'll have to do it again later.

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