Odd star shape.

[Perrywarburton]

Hi. I’m having an issue with my stars.
They seam to be an odd shape. The scope is a TS optics 80mm triplet and I sent it off to ES Reid because of this issue. At first I thought it was pinched optics but Reid confirmed that the optics are in great condition and found that the best backspacing for this scope with my TS flat is 106mm. I have made my spacing 106 but the issue is still there. I have attached a photo of the centre of M34 which shows the odd shape.  I polar align with sharp cap and my guiding is great at 0.19 tot. 
im not sure if it’s tilt or flexure. 
many help would be great. 
thanks. 

[michael8554]

There seems to be a hint of diffraction spikes ?

But a screengrab is not good enough, post a jpg of one sub.

What exposure was that ?

To judge star shape, eliminate guide errors by taking a short exposure, stretched if necessary.

Guiding at 0.19 Total is no help either.

If RA and Dec errors are very different then you will have elongated stars.

What camera ?

Any filters ?

OAG or guidescope ?

Michael

[Adam J]

1 hour ago, Perrywarburton said:

Hi. I’m having an issue with my stars.
They seam to be an odd shape. The scope is a TS optics 80mm triplet and I sent it off to ES Reid because of this issue. At first I thought it was pinched optics but Reid confirmed that the optics are in great condition and found that the best backspacing for this scope with my TS flat is 106mm. I have made my spacing 106 but the issue is still there. I have attached a photo of the centre of M34 which shows the odd shape.  I polar align with sharp cap and my guiding is great at 0.19 tot. 
im not sure if it’s tilt or flexure. 
many help would be great. 
thanks. 

Thats 0.2 pixels total as you have it set to pixels and not arc-seconds and those are guide scope pixels not imaging scope pixels (unless off axis guider) and hence will be at a larger image scale (probably many times that of the imaging scope) so I am going to guess that this may well yet turn out to be guide errors. Set to arc-seconds next time and then compare the RMS / Peak values to your image scale. Also as above try really short exposures, 10s or so, on a star field and see how they look eliminating guide errors. 

Adam 

Edited March 8, 2022 by Adam J

[ollypenrice]

I agree with Adam, you need to know your guide values in arcseconds. However, the spikes across the stars do look like diffraction. Could this be from something in a part of the lightpath that didn't go to Es Reid?

Olly

[wimvb]

1 hour ago, Adam J said:

Thats 0.2 pixels total as you have it set to pixels and not arc-seconds and those are guide scope pixels not imaging scope pixels (unless off axis guider) and hence will be at a larger image scale (probably many times that of the imaging scope) so I am going to guess that this may well yet turn out to be guide errors. Set to arc-seconds next time and then compare the RMS / Peak values to your image scale. Also as above try really short exposures, 10s or so, on a star field and see how they look eliminating guide errors. 

Adam 

 

12 minutes ago, ollypenrice said:

I agree with Adam, you need to know your guide values in arcseconds. However, the spikes across the stars do look like diffraction. Could this be from something in a part of the lightpath that didn't go to Es Reid?

Olly

As I see it, the guide graph is in pixels, but we don't see the Y-scale anyway. The RMS figure in the screen grab shows total RMS = 0.2 (px) but also 0.55". So, unless the OP uses the wrong settings, guiding should be ok. Total guiding RMS is 0.2 px, DEC guiding RMS is 0.14 px and if RA and DEC are fairly orthogonal, RA guiding should also be about 0.14 px.

But I agree that short unguided images are best to evaluate star shapes.

[Adam J]

2 hours ago, wimvb said:

 

As I see it, the guide graph is in pixels, but we don't see the Y-scale anyway. The RMS figure in the screen grab shows total RMS = 0.2 (px) but also 0.55". So, unless the OP uses the wrong settings, guiding should be ok. Total guiding RMS is 0.2 px, DEC guiding RMS is 0.14 px and if RA and DEC are fairly orthogonal, RA guiding should also be about 0.14 px.

But I agree that short unguided images are best to evaluate star shapes.

Unfortunately not, if set to pixels I believe Nina will use the main scope focal length to calculate arcseconds as Nina has no knowledge of guide scope focal length. So the value in brackets is likely not correct from my experience with Nina. Unless using OAG. 

Adam

 

Edited March 8, 2022 by Adam J

[Perrywarburton]

Hi sorry I’ve been away for a few days. I’ve attached an image that is 10s my back spacing is where it should be as mentioned but even if it wasn’t the centre is still wrong. 
I use a zwo 60mm guide scope with a zwo 120m, a zwo 183mm pro main camera and Optolong 7nm filters. 

[Perrywarburton]

This is my setup

[michael8554]

Centre of image seems okay, apart from the hint of diffraction spikes.

Edges show coma, needs more spacing.

Michael

[Perrywarburton]

I’m not sure where the diffraction spikes come from as I’m using a refractor.

ES Reid stated that the most round stars are at 106mm so I’m not sure where to go with the spacing side of things. 

[michael8554]

Well if Reid say the optics aren't pinched, and 106mm is the right spacing.

Then who am I to contradict them...........  😆

Michael     

 

[tooth_dr]

Unless your not referencing that 106mm from the same point?  Definitely agree with Michael that the spacing isn’t correct.  There is no harm in adding a shim or two to see if it helps. 

[Stuart1971]

On 08/03/2022 at 17:03, michael8554 said:

There seems to be a hint of diffraction spikes ?

But a screengrab is not good enough, post a jpg of one sub.

What exposure was that ?

To judge star shape, eliminate guide errors by taking a short exposure, stretched if necessary.

Guiding at 0.19 Total is no help either.

If RA and Dec errors are very different then you will have elongated stars.

What camera ?

Any filters ?

OAG or guidescope ?

Michael

Hmmmm, where have I heard this before….🤔🤔

it makes no odds of RA and DEC figures are very different, as long as the total RMS is under half of your imaging scale, it will not show up in any image…👍🏼

Edited March 11, 2022 by Stuart1971

[Stuart1971]

I get this same effect, in the very edge of my FOV in one corner of my TAKFSQ85, and never found out what causes it, it’s never bothered me as it’s in just one corner, and I usually crop my images anyway…

see here, top left corner of image and full image below it…

 

[michael8554]

57 minutes ago, Stuart1971 said:

Hmmmm, where have I heard this before….🤔🤔

it makes no odds of RA and DEC figures are very different, as long as the total RMS is under half of your imaging scale, it will not show up in any image…👍🏼

Okay, I already got the message 😆

Michael

[Perrywarburton]

Hi all thanks for the replies.

I don’t believe this to be a guide error my guiding is very good. My image scale is 1.1 as calculated in astronomy tool and I’m guiding at 0.19 so it doesn’t matter if my RA and DEC are different. I’ve been doing AP for some years so I know my software. 

For reference ES Reid isn’t  a “company”, “he” is the best optical tester in the uk based in Cambridge anyone who knows will vouch for him. 
 

im going to continue with adding spacing and see what I end up with but because the centre is an odd shape this might be pointing towards the camera. I’m also going to replace my dovetail and try to get the scope centred to the dec because if I push on the camera end there is some give…..🤔

[Same old newbie alert]

You're still talking in pixels... Your guidescope looks to be hanging out the back abit, could this be a flexure issue I wonder...  Have you pulled it out to get focus? Still don't know why you have slight spikes, something in the imaging train

What software are you using to guide?

Edited March 12, 2022 by Same old newbie alert
Added info

[Stuart1971]

2 hours ago, Perrywarburton said:

Hi all thanks for the replies.

I don’t believe this to be a guide error my guiding is very good. My image scale is 1.1 as calculated in astronomy tool and I’m guiding at 0.19 so it doesn’t matter if my RA and DEC are different. I’ve been doing AP for some years so I know my software. 

For reference ES Reid isn’t  a “company”, “he” is the best optical tester in the uk based in Cambridge anyone who knows will vouch for him. 
 

im going to continue with adding spacing and see what I end up with but because the centre is an odd shape this might be pointing towards the camera. I’m also going to replace my dovetail and try to get the scope centred to the dec because if I push on the camera end there is some give…..🤔

You need to refer to your guiding accuracy in arc seconds and not pixels, you say it’s 0.19, this is pixels, you should be quoting 0.55” this is what determines the actual accuracy of the guiding, as it’s a fixed figure, but in pixels it’s different for different cameras….so means nothing…..👍🏼

[Perrywarburton]

Here is a screen grab of my guiding. It’s very good. Well below 1.1

[Stuart1971]

1 hour ago, Perrywarburton said:

Here is a screen grab of my guiding. It’s very good. Well below 1.1

It’s not 1.1, it’s 0.55” in arc seconds what is perfectly average….👍🏼

[Same old newbie alert]

1 hour ago, Stuart1971 said:

It’s not 1.1, it’s 0.55” in arc seconds what is perfectly average….👍🏼

He's saying his image scale is 1.1

 

[Stuart1971]

5 hours ago, Same old newbie alert said:

He's saying his image scale is 1.1

 

He said his guiding was good, well below 1.1. How does 1.1 relate to image scale, but never mind I will bow out now as, I have exhausted my help on this….👍🏼

 

Edited March 12, 2022 by Stuart1971

[scotty38]

He is actually referring to his image scale of 1.1 so is saying his guiding is well below that. 

[Perrywarburton]

Hi all. So I have played with my spacing and I think it’s where it should be. Attached is an image of M48 at 10s with the luminance filter. 
From what I can tell, all of my stars are moving in the same direction now. I can’t seam to work this out. Please help. 

[knobby]

Could it be tilt ? He camera is a long way from focuser ? Plenty of scope for tilt.

Maybe try a star at the zenith to test ?