Do I need to be concerned?

[cajen2]

On my two-month-old SW 150p Flextube dob, there's discolouration on many parts of the edge on the secondary mirror. This was so straight out of the box. I always assumed that it didn't matter as it wasn't on the flat reflecting surface but now I'm starting to worry.

I've been having trouble getting sharp images of stars: they often look out of shape, even with the likes of a Pentax XW 14. I've collimated the scope without much effect. Could there be any connection with the secondary edge?

 

[Orange Smartie]

Hi. Do you mean the sides of the secondary (appear frosty in your picture)? These aren't going to be in the optical path so shouldn't make any difference. You may be experiencing coma, especially at the edges of the field of view (especially with lower-power eyepieces) - in my 10" dob, the coma can be quite bad (the dreaded "seagulls"!), but as I'm normally concentrating on the centre of the field of view, I tend to ignore it. Obviously that doesn't happen when you look at a photo.

Have you tried a star test to make sure your collimation is ok? Find Polaris and switch to a high power eyepiece. When the star is slightly de-focussed, you should see the rings all nicely concentric - if they're off-kilter, you may need to tweak the collimation. Search this forum for "star test" or see this:

https://www.skyatnightmagazine.com/advice/how-to-star-test-a-telescope/

Pete

[Orange Smartie]

.....or this:

https://popastro.com/documents/PA_jan-mar2009_p12-13_telescopetopics_startesting.pdf

Which mentions the "bleeding effect", which I hadn't realised was attributable to inadequate cooling. You learn something new every day....

[Stu]

23 minutes ago, cajen2 said:

On my two-month-old SW 150p Flextube dob, there's discolouration on many parts of the edge on the secondary mirror. This was so straight out of the box. I always assumed that it didn't matter as it wasn't on the flat reflecting surface but now I'm starting to worry.

I've been having trouble getting sharp images of stars: they often look out of shape, even with the likes of a Pentax XW 14. I've collimated the scope without much effect. Could there be any connection with the secondary edge?

 

Can you explain a little more about the problem.

Can you get sharp stars on axis but are seeing coma off axis? This is normal for a fast ish newt.

Are stars on axis not sharp? If flaring off to one side that is likely still collimation. Do you get concentric rings on a defocused star on axis? A star test is the best and quickest way to check your collimation is good.

It can also be a little tricky getting accurate focus due to the slight flex in the focuser, possibly made a bit worse by a heavier eyepiece such as the XW. I found that you can inadvertently either push or pull the focuser when focusing, so that when you release it it springs back a tiny bit, taking you out of focus again. A very gentle touch is required here.

The final part could be tube current related. I found that your breathing can affect the views due to the open tube, so a shroud can really help to cut this out, as well as reducing glare.

I’m sure the marks around the secondary edge are nothing to be concerned about, they look quite normal to me.

[Orange Smartie]

Just for your peace of mind, here's my Heritage 150p secondary - looks just the same as yours. 

[cajen2]

Thanks for that, Stu. Yes, I always use a shroud.

The views of stars tend to look like a close double on occasion and can thus be difficult to focus. I've recollimated again, so I need to wait for a clear night to do another star test.

I also need to source an Allen key for the secondary collimation.

[cajen2]

1 minute ago, Orange Smartie said:

Just for your peace of mind, here's my Heritage 150p secondary - looks just the same as yours. 

Thanks, OS, so that's another thing to tick on the list.

[cajen2]

34 minutes ago, Orange Smartie said:

Hi. Do you mean the sides of the secondary (appear frosty in your picture)? These aren't going to be in the optical path so shouldn't make any difference. You may be experiencing coma, especially at the edges of the field of view (especially with lower-power eyepieces) - in my 10" dob, the coma can be quite bad (the dreaded "seagulls"!), but as I'm normally concentrating on the centre of the field of view, I tend to ignore it. Obviously that doesn't happen when you look at a photo.

Have you tried a star test to make sure your collimation is ok? Find Polaris and switch to a high power eyepiece. When the star is slightly de-focussed, you should see the rings all nicely concentric - if they're off-kilter, you may need to tweak the collimation. Search this forum for "star test" or see this:

https://www.skyatnightmagazine.com/advice/how-to-star-test-a-telescope/

Pete

Sorry, OS, missed this answer. No, the edge of field coma is not pronounced. The effect is as much on-axis as anywhere else.

It wasn't the "frosty"edge that concerned me, but the patches of shiny surface too. I hadn't worried about it as, like you say, it isn't in the optical path, but just needed to check!

Edited January 7, 2022 by cajen2

[Stu]

5 minutes ago, cajen2 said:

The views of stars tend to look like a close double on occasion and can thus be difficult to focus

That doesn’t sound right, but check again. Could still be a collimation issue. How are you collimating?

[cajen2]

1 minute ago, Stu said:

That doesn’t sound right, but check again. Could still be a collimation issue. How are you collimating?

Just with a collimating cap, to get it in a roughly correct position.

No, I've just been looking again and the secondary is slightly out of position, so I need to get some tiny Allen keys! Are there Bob's Knobs just for secondary mirrors?

[cajen2]

As to the focuser, that's fairly solid as I did the PTFE tape mod. The eyepieces are rock solid with no flex.

[Adam J]

Don't know what you are trying to describe but there is nothing in that image that would have me worried. 

[lenscap]

52 minutes ago, cajen2 said:

The views of stars tend to look like a close double on occasion

That sounds more like a focus problem than a collimation issue.

Is the problem present with all eyepieces or just at high power?

Can you achieve sharp focus on say a distant electricity pylon in daylight?

I'm wondering if the truss is extended to its maximum to enable eyepieces to come to focus.          

[cajen2]

The problem seems worse on some EPs than others (the OVL is better than the Pentax).

Can't do a day test now - not at home.

The truss is definitely fully extended - I'm very careful about that.

[Ricochet]

Is it coma you are seeing? Perhaps the focuser is sagging a bit under the weight of an eyepiece which would miscollimate the telescope and shift the coma free zone from the centre of the FoV. Coma appears to "point" towards the centre so moving a star around the FoV will reveal where it points to. If it is not the centre try comparing different weight/size eyepieces and see if the position changes according to eyepiece size.

If the star image appears to be an oval one side of focus and a perpendicular oval the other side of focus then you could have astigmatism present. As the Heritage focuser rotates you will also need to loosen the eyepiece clamp and rotate the eyepiece to rule out it being inherent in the eyepiece you are testing, and rotate your head to check whether it is inside your eye (more likely at larger exit pupil, see the Televue chart below). If it is a function of your telescope mirrors check multiple times during a session, it could be induced only while the scope is still cooling.

If the stars appear slightly triangular then you have pinched optics, and usually overtightened mirror clips on the primary are to blame.

However, with just a collimation cap being used so far, I suspect the collimation needs to be looked at in the first instance with a decent cheshire or concenter.

 

[cajen2]

On 07/01/2022 at 19:14, Ricochet said:

Is it coma you are seeing? Perhaps the focuser is sagging a bit under the weight of an eyepiece which would miscollimate the telescope and shift the coma free zone from the centre of the FoV. Coma appears to "point" towards the centre so moving a star around the FoV will reveal where it points to. If it is not the centre try comparing different weight/size eyepieces and see if the position changes according to eyepiece size.

If the star image appears to be an oval one side of focus and a perpendicular oval the other side of focus then you could have astigmatism present. As the Heritage focuser rotates you will also need to loosen the eyepiece clamp and rotate the eyepiece to rule out it being inherent in the eyepiece you are testing, and rotate your head to check whether it is inside your eye (more likely at larger exit pupil, see the Televue chart below). If it is a function of your telescope mirrors check multiple times during a session, it could be induced only while the scope is still cooling.

If the stars appear slightly triangular then you have pinched optics, and usually overtightened mirror clips on the primary are to blame.

However, with just a collimation cap being used so far, I suspect the collimation needs to be looked at in the first instance with a decent cheshire or concenter.

 

Thanks very much for your comprehensive answer, R. I've actually tried most of the tests you recommend with no clear result.

However, I tried recollimating the secondary today and while it was clear tonight, did a star test on Sirius. Clear, round concentric rings!  I also tried all my eyepieces on different targets and all of them show improvements. 🤔🥳

So I hope the problem has been solved, thanks to all for your  input. I've also ordered a set of secondary mirror Bob's Knobs to make it easier next time.