Coma affect with BST Eyepieces

[Olli]

Hi,

i have been using a 18mm bst with my 130ps and i have been getting quite a lot of star trails with it ( this is visual only) when i use the eyepiece i get too distracted by the star trails and i can't concentrate on the object i am supposed to be looking at and gets quite annoying once in a while. However the image is much sharper with the bst compared to the stock eyepieces so i would like to keep using them the trails usually appear from the edges and go to the centre. So what i am really asking is that is there something wrong with the eyepiece or is it normal to have this much of a affect when using a bst on my type of scope and will it just take  time to adjust to it ? 

Many Thanks 

[Ally8446]

It sounds more like a collimation issue to me but I'm open to be corrected. I used to own a Skywatcher 8" dob and the BSTs were absolutely lovely in it regardless of their focal length. The dob was an F/5 so relatively fast.

Ally

[Olli]

4 minutes ago, Ally8446 said:

It sounds more like a collimation issue to me but I'm open to be corrected. I used to own a Skywatcher 8" dob and the BSTs were absolutely lovely in it regardless of their focal length. The dob was an F/5 so relatively fast.

Ally

Thanks for the reply i forgot to mention  unfortunately my scope can't be collimated  

[Ally8446]

1 minute ago, Olli said:

Thanks for the reply i forgot to mention  unfortunately my scope can't be collimated  

Sorry for my ignorance but..why can it not be collimated. I presume it's a Skywatcher Explorer ?

[Louis D]

If the 18mm BST is anything like my 18mm Meade HD-60, you're seeing astigmatism.  Try racking the focuser on either side of focus to see if the star shape changes from radial to tangential.  Coma usually only shows up in very well corrected wide field eyepieces.  In lesser eyepieces, astigmatism completely drowns it out.

[Olli]

35 minutes ago, Ally8446 said:

Sorry for my ignorance but..why can it not be collimated. I presume it's a Skywatcher Explorer ?

It has no collimation screws ( i think) 

 16:53 i think he says it 

[Ally8446]

Every day is a school day as they say. Thank you for the clarification Olli. It may well be as @Louis D suggested, astigmatism.

[Olli]

49 minutes ago, Louis D said:

If the 18mm BST is anything like my 18mm Meade HD-60, you're seeing astigmatism.  Try racking the focuser on either side of focus to see if the star shape changes from radial to tangential.  Coma usually only shows up in very well corrected wide field eyepieces.  In lesser eyepieces, astigmatism completely drowns it out.

Basically my stars look like comets with long trails i thought that was what coma was whats the difference between that and astigmatism? but i will have a go with the focuser. Thanks.

[Olli]

3 minutes ago, Ally8446 said:

Every day is a school day as they say. Thank you for the clarification Olli. It may well be as @Louis D suggested, astigmatism.

No promblem, that was one of the reasons why i bought the scope  

[Ricochet]

I think you might find the secondary can be adjusted but I think a collimation issue would probably show issues on one side of the image but not the other. I've got an 18mm Starguider arriving soon so once the skies clear again I will see if I can see any issues at f6. 

[Louis D]

Apparently, the 130ps primary is factory aligned and only the secondary is adjustable.  I did read that the rear cover could be removed and there might be some collimation screws and locking screws underneath.

Do you have a cheshire eyepiece (peep hole sight tube) or similar to check for proper collimation?  Is the primary even center spotted?

[Louis D]

2 hours ago, Olli said:

Basically my stars look like comets with long trails i thought that was what coma was whats the difference between that and astigmatism? but i will have a go with the focuser. Thanks.

If you really want to know, read this excellent page describing pretty much all optical aberrations.

From that page, this is coma:

This is astigmatism:

I had the names mixed up, tangential I was calling radial and sagittal I was calling tangential.  Regardless, to distinguish the two, you need to place a bright star at the edge and rack through focus to see if the shape is the same (coma) or different (astigmatism) on either side of focus.

Here is the combination of the two just to complicate matters:

The two together resemble a seagull.  You'll probably also have chromatic aberrations (typically lateral) as well coloring either end of the bird red/blue most typically:

Have fun testing your optics.  I warn you, once you start testing for them, you can't unsee them ever again, and you'll want to correct them with coma correctors and super-premium eyepieces.

[John]

Coma is generally a product of Newtonian optics. It is more apparent in focal ratios of f/5 and faster.

Astigmatism is usually attributed to eyepeices, especially lowish cost wide field types used in fastish scopes.

It's quite possible to be seeing a combination of the two issues but normally in the outer half of the field of view.

If you are seeing distortions in the centre of the field then collimation might be the suspect.

 

 

[Olli]

39 minutes ago, Louis D said:

If you really want to know, read this excellent page describing pretty much all optical aberrations.

From that page, this is coma:

This is astigmatism:

I had the names mixed up, tangential I was calling radial and sagittal I was calling tangential.  Regardless, to distinguish the two, you need to place a bright star at the edge and rack through focus to see if the shape is the same (coma) or different (astigmatism) on either side of focus.

Here is the combination of the two just to complicate matters:

The two together resemble a seagull.  You'll probably also have chromatic aberrations (typically lateral) as well coloring either end of the bird red/blue most typically:

Have fun testing your optics.  I warn you, once you start testing for them, you can't unsee them ever again, and you'll want to correct them with coma correctors and super-premium eyepieces.

Thanks Louis that's quite interesting I will have a look at that link later. Yes I can imagine that you won't be able to unsee them not sure if I want to start that  

[Olli]

29 minutes ago, John said:

Coma is generally a product of Newtonian optics. It is more apparent in focal ratios of f/5 and faster.

Astigmatism is usually attributed to eyepeices, especially lowish cost wide field types used in fastish scopes.

It's quite possible to be seeing a combination of the two issues but normally in the outer half of the field of view.

If you are seeing distortions in the centre of the field then collimation might be the suspect.

 

 

Thanks john for the advice as stated above my scope can't be colimated well properly anyway. Next time I use my scope I'll have to have a proper look at where abouts it's happening I'm sure it's not right in the centre.

 

[John]

I like this web page for more info:

umich.edu/~lowbrows/reflections/2007/dscobel.27.HTML

 

 

[Ricochet]

I've had a couple of sessions to try to test out this eyepiece. At f6 the main issue that I see is field curvature. From half way between the centre and edge of field stars begin expanding into out of focus circles. Towards the edge they are large enough to see some sort of "clipping" on those circles, perhaps because the field exceeds the fully illuminated area or some additional aberration. Winding the focus in would correct the stars at the edge of field.

The first time I attempted to test the eyepiece I was not expecting it to be clear and so had not taken the scope out to cool prior to observing. It had been a hot day that quickly turned to a cold night and unbeknown to me, one of the support bands holding the cooling fan had perished and so the fan was pulled off centre and not cooling the mirror evenly. This introduced a whole load of astigmatism which produced aberrations that seem to resemble your description. 

I have made a diagram showing the issues I could see at the edge of field. The left hand side shows the field curvature dominated effect (but the "clipping" might have appeared on the inside rather than outside, I can't quite remember) and the right hand side shows what happened to that field curvature when astigmatism was added on top of it. Apologies for the quality of my drawing. 

If you see star shapes as on the right hand side of my image I would suspect there is some astigmatism coming from the scope as well as any in the eyepiece. Perhaps a longer cool down period is required given the way the mirror is fixed to the plastic mirror cell.

[25585]

Good for vendors as our A&N friend says. But if you are not happy, get a replacement or refund & buy another make.