Back to forums

Not point-shaped stars with perfect guiding

Sky-Watcher Explorer 150PDS Player One Uranus-C Equipment Guiding Troubleshooting
17 replies•516 views
Rodion Geist avatar
Rodion Geist avatar
Hello everyone,
I noticed a small problem with my first light with my SW 150pds. The stars are not point-shaped with a perfect guiding of 0.7 arcseconds per pixel.
I use the SW 150pds with the Player One Uranus-C camera (0.8 angular sec per pixel) and as a guiding setup I use my old SvBony sv165 guidescope with 30mm diameter and 120mm focal length with the ZWO ASI 120mc-s.
Why are the stars not point-shaped (the stars are always elongated in one direction)?
Could it be because of the big difference in focal lengths? And should I buy an off axis guider (OAG) or a larger guidescope (without replacing the two asi120mc-s camera)?

If I will need to buy a guidescope, will this one be a good choice for my setup?:
https://www.astroshop.eu/guidescopes/ts-optics-guidescope-deluxe-60-mm-guiding-finder-scope-with-micro-focusing/p,51114

Thank you and CS,
Rodion
Christian Bennich avatar
Christian Bennich avatar
Have you taken a look at tilt in your image train?

Could you share an example image?

I am not an expert, but if you have perfect guiding, I don’t see any reason to invest in an OAG as the first step. 

Tilt and backfocus can also materialise themselves as elongated stars.
Well Written Insightful Respectful Concise Engaging
Christian Großmann avatar
Christian Großmann avatar
There are several things coming to mind reading your post. Are the stars elongated in the corners only, or are they elongated across the whole image. Did you use a field flattener?

You did not mention any field flattener. So maybe the bad shapes are coma coming from the Newton telescope itself. If the stars are ok in the center of the frame and elongated in the corners, thats what is to be expected.

The guide scope is quite small for a Newt with 750mm focal length. Theoretically it should work. Myself, I always use the suggested TS guide scope type (I use SVBony 106s with 60/240mm) for all my setups without any issues. Depending on your mount, the weight they add to your setup is not exacly light. But for me they always worked.

Another thing might be flexure. I had some issues once with my guide scopes and the machanical stiffness of the mountings. If they are able to bend a tiny little bit, you might get elongated stars as well even on good guiding. But I assume this should be no issue with the little guidescope you used. Make sure, the center of the image of your main telescope is aligned with the center of the image of your guide scope. They should point to the same spot in the sky. If the difference is to large, this could cause problems, too.

It would be helpful to see an axample of one of your bad images. Is it possible to show one of them to us?

CS
Christian
Helpful
Brian Diaz avatar
Brian Diaz avatar
hi

could you unload an example image

There can be many reasons you may get trailed or elongated stars in your image

1- tracking and guiding 
2- optical problems
3-polar alignment 
4- mechanical problems
5-periodic error
6-differential flexure
7-bad seeing
8 -balance, backlash and oscillation 
9-mirror flop 
10-dragging cables
11- incorrect auto guider setting 
also wind, junk or dirt in the gear ,truck on a nearby road, instability from a wooden deck or balcony  and etc. 

CS, Brian
andrea tasselli avatar
andrea tasselli avatar
Guiding depends on the mount's quality. So, what is the mount?
Well Written Respectful
Rodion Geist avatar
Rodion Geist avatar
Christian Großmann:
There are several things coming to mind reading your post. Are the stars elongated in the corners only, or are they elongated across the whole image. Did you use a field flattener?

You did not mention any field flattener. So maybe the bad shapes are coma coming from the Newton telescope itself. If the stars are ok in the center of the frame and elongated in the corners, thats what is to be expected.

The guide scope is quite small for a Newt with 750mm focal length. Theoretically it should work. Myself, I always use the suggested TS guide scope type (I use SVBony 106s with 60/240mm) for all my setups without any issues. Depending on your mount, the weight they add to your setup is not exacly light. But for me they always worked.

Another thing might be flexure. I had some issues once with my guide scopes and the machanical stiffness of the mountings. If they are able to bend a tiny little bit, you might get elongated stars as well even on good guiding. But I assume this should be no issue with the little guidescope you used. Make sure, the center of the image of your main telescope is aligned with the center of the image of your guide scope. They should point to the same spot in the sky. If the difference is to large, this could cause problems, too.

It would be helpful to see an axample of one of your bad images. Is it possible to show one of them to us?

CS
Christian

I have usedthe baader mpcc mark iii cc, so this shouldn't be the problem (the backfocus is also the right).
I use the SW HEQ-5 Pro mount.
Here's a link to all of the subexposures: first light 150pds
Thank you for your help and cs,
Rodion
Christian Bennich avatar
Christian Bennich avatar
That does look strange indeed. 

No real tilt as measured by ASTAP.


It is as if the elongation turn counterclockwise if you look at the frames over time.



Not something I have seen before - sorry. 
Hope someone else has some meaningful input.
Rodion Geist avatar
Rodion Geist avatar
Christian Bennich:
That does look strange indeed. 

No real tilt as measured by ASTAP.


It is as if the elongation turn counterclockwise if you look at the frames over time.



Not something I have seen before - sorry. 
Hope someone else has some meaningful input.

It does look pretty strange…😶. But thank you for your time and help! 
CS,
Rodion
andrea tasselli avatar
andrea tasselli avatar
Poor correction. Could either be a spacing issues or poor collimation to start with.
Rodion Geist avatar
Rodion Geist avatar
andrea tasselli:
Poor correction. Could either be a spacing issues or poor collimation to start with.


Ok, that sounds plausible, because I collimated the scope with my guide camera and without the CC.
I’m interested in buying the Ocal from noctutec, do you have any experience with this thing and is good?
cs,
Rodion
Christian Bennich avatar
Christian Bennich avatar
Rodion Geist:
andrea tasselli:
Poor correction. Could either be a spacing issues or poor collimation to start with.


Ok, that sounds plausible, because I collimated the scope with my guide camera and without the CC.
I’m interested in buying the Ocal from noctutec, do you have any experience with this thing and is good?
cs,
Rodion


I use it for my newt - it’s a great tool. Took a little while to get used to. 

Not too many good videos om how to use - but I found this to be the best: 
https://m.youtube.com/watch?v=KKGFGQ4rplk

From the video, you can also find a better piece of software to use with the ocal. 
https://astro.stefanhohenegger.de/de/software/
Rodion Geist avatar
Rodion Geist avatar
Christian Bennich:
Rodion Geist:
andrea tasselli:
Poor correction. Could either be a spacing issues or poor collimation to start with.


Ok, that sounds plausible, because I collimated the scope with my guide camera and without the CC.
I’m interested in buying the Ocal from noctutec, do you have any experience with this thing and is good?
cs,
Rodion


I use it for my newt - it’s a great tool. Took a little while to get used to. 

Not too many good videos om how to use - but I found this to be the best: 
https://m.youtube.com/watch?v=KKGFGQ4rplk

From the video, you can also find a better piece of software to use with the ocal. 
https://astro.stefanhohenegger.de/de/software/


Ok, then I’ll probably buy this tool!
Danke!😁
CS,
Rodion
Christian Bennich avatar
Christian Bennich avatar
Rodion Geist:
Christian Bennich:
Rodion Geist:
andrea tasselli:
Poor correction. Could either be a spacing issues or poor collimation to start with.


Ok, that sounds plausible, because I collimated the scope with my guide camera and without the CC.
I’m interested in buying the Ocal from noctutec, do you have any experience with this thing and is good?
cs,
Rodion


I use it for my newt - it’s a great tool. Took a little while to get used to. 

Not too many good videos om how to use - but I found this to be the best: 
https://m.youtube.com/watch?v=KKGFGQ4rplk

From the video, you can also find a better piece of software to use with the ocal. 
https://astro.stefanhohenegger.de/de/software/


Ok, then I’ll probably buy this tool!
Danke!😁
CS,
Rodion


and you will not have the same issues as me understanding the details - language wise 🤣🤣
I had to seriously practise my school German again 🤪
andrea tasselli avatar
andrea tasselli avatar
Rodion Geist:
Ok, that sounds plausible, because I collimated the scope with my guide camera and without the CC.
I’m interested in buying the Ocal from noctutec, do you have any experience with this thing and is good?
cs,
Rodion


I don't use those devices and I don't trust them. The easiest way is to collimate the secondary is with a Cheshire EP and then with an autocollimator, or use a collimated laser to center the secondary onto the primary and then do a star test collimation.
Rodion Geist avatar
Rodion Geist avatar
Christian Bennich:
Rodion Geist:
Christian Bennich:
Rodion Geist:
andrea tasselli:
Poor correction. Could either be a spacing issues or poor collimation to start with.


Ok, that sounds plausible, because I collimated the scope with my guide camera and without the CC.
I’m interested in buying the Ocal from noctutec, do you have any experience with this thing and is good?
cs,
Rodion


I use it for my newt - it’s a great tool. Took a little while to get used to. 

Not too many good videos om how to use - but I found this to be the best: 
https://m.youtube.com/watch?v=KKGFGQ4rplk

From the video, you can also find a better piece of software to use with the ocal. 
https://astro.stefanhohenegger.de/de/software/


Ok, then I’ll probably buy this tool!
Danke!😁
CS,
Rodion


and you will not have the same issues as me understanding the details - language wise 🤣🤣
I had to seriously practise my school German again 🤪


Hey there, again😁.
I’m wondering if you need the 55mm backfocus length from the CC to the Ocal or is it irrelevant? I searched the whole internet for how deep the sensor is located, ChatGPT says 7mm, but I’m not sure…
So if it’s 7mm I’ll need 48mm additional, right?
CS,
Rodion
Christian Bennich avatar
Christian Bennich avatar
I have not needed that. Just dropped it into my focuser, with an extender to get a proper “grip” on it. 

Never actually thought about backfocus on the Ocal and I don’t think I have ever seen it or heard it mentioned in any instructional videos. 

My own collimation has never been better, since I added the Ocal to my toolbox. 
Also if I move out of focus looking at the airy discs from stars, they are close to perfect as well. 

Let me know if you come across some solid descriptions of needing backfocus on the device.
I am planning on setting up my equipment again in a month or so, when the darkness start to come back and will also need to recollimate at that time.
Helpful Engaging
Rodion Geist avatar
Rodion Geist avatar
Christian Bennich:
I have not needed that. Just dropped it into my focuser, with an extender to get a proper “grip” on it. 

Never actually thought about backfocus on the Ocal and I don’t think I have ever seen it or heard it mentioned in any instructional videos. 

My own collimation has never been better, since I added the Ocal to my toolbox. 
Also if I move out of focus looking at the airy discs from stars, they are close to perfect as well. 

Let me know if you come across some solid descriptions of needing backfocus on the device.
I am planning on setting up my equipment again in a month or so, when the darkness start to come back and will also need to recollimate at that time.


Thank you for the clarification, happy to hear that I don’t need to buy more extension tubes!
I’ll let you know if I find any information about the backfocus, I just thought it would be better to have the backfocus right, but now that I think about it, it doesn’t make really sense.
Thank you!
Clear skies,
Rodion
Christian Bennich avatar
Christian Bennich avatar
Rodion Geist:
Christian Bennich:
I have not needed that. Just dropped it into my focuser, with an extender to get a proper “grip” on it. 

Never actually thought about backfocus on the Ocal and I don’t think I have ever seen it or heard it mentioned in any instructional videos. 

My own collimation has never been better, since I added the Ocal to my toolbox. 
Also if I move out of focus looking at the airy discs from stars, they are close to perfect as well. 

Let me know if you come across some solid descriptions of needing backfocus on the device.
I am planning on setting up my equipment again in a month or so, when the darkness start to come back and will also need to recollimate at that time.


Thank you for the clarification, happy to hear that I don’t need to buy more extension tubes!
I’ll let you know if I find any information about the backfocus, I just thought it would be better to have the backfocus right, but now that I think about it, it doesn’t make really sense.
Thank you!
Clear skies,
Rodion

The same to you 👍💪