Stars not round

[Astrosurf]

Hi all, I’ve been using my ASI533MC with my 72mm frac with real success. Now we’re into galaxy season and I’m using a Newt. I currently have two: a SW Explorer 200P and a second-hand Altair 200 from a friend. Both are f5. 
 

With the SW  I get stars that are distorted (tried to get a pic tonight of that but the clouds rolled in) and with the Altair the stars look like spaceships! See poor-quality pic. EDIT: this is a 3s snap from the ASIAir while focusing, not a guided sub. I never get that far. I’ve collimated both scopes to within an inch of their lives with an OCAL. 
 

I use the recommended spacers to make up 55mm back focus as recommended by the ZWO site. I use an all-purpose coma corrector which FLO said should be okay. 

 

To be honest I’m losing the will to live. Any ideas? 

Edited March 30 by Astrosurf

[Chris-h]

How are you guiding? I only had a newt for a short while, but I could never get round stars with a guidescope. The stars improved a lot after switching to a OAG, but they were still not perfect (collimation, CC distance). After a while I gave up, and switched to a refractor

[Backyardscope]

Have you checked the clips on the primary mirror to make sure the mirror is not pinching.

They should be just touching the mirror, not forced down and also they should he enough that you can easily rotate the mirror within the clips.

[alacant]

Hi

To enable us to help, we really need to see where the star is in the frame and so be able compare it to others therein. Maybe post -links to- example frames along with detail of mount, guiding, cc, how the guide telescope or WHY is mounted...

Cheers

Edited March 30 by alacant

[rl]

A 200P plus all the gizmos might be a bit heavy for an HEQ5 (others might disagree on this) but maybe it's worth checking the balance; it should be slightly off balance, just enough to take out the mechanical ambiguity of the backlash. The reflector has more than twice the focal length of the frac so all the sources of mechanical drift have to be beefed up in proportion. An OAG is virtually de rigeur with Newtonians in my experience. Much better guiding at the cost of far fewer guide stars which can be an issue when you're looking out of the plane of the milky way (but you only need one).

Which coma corrector are you using? I use the Skywatcher 4-element aplanatic jobbie which turns out a decent image with a 8" f/4.5 Newt and an APS-C sensor. Some of the simpler, cheaper ones are not as good. 

How about your polar alignment? Using a polemaster (or similar setup using the finder) can really cut down on the amount of work PHD2 has to do.

If you want an easy life, go the frac route. But it's very expensive compared to a secondhand 200P for the same scale factor and speed!

No future in losing the will to live!...how does the main subject in the middle of the frame look when examined at a normal distance? Sometimes we all lose the plot obsessing over the corners....or maybe I've been doing AP long enough to have the last vestiges of perfectionism beaten out of me!

[Varavall]

Hi

14 hours ago, Astrosurf said:

I use an all-purpose coma corrector

Assuming its the Baader corrector take a look at this

I use a SW200pds and a Skywatcher coma corrector on an HEQ5 with  round stars using a guidescope. The mount is probably overloaded but I get a PHD2 RMS of 0.25-0.6, but I have hypertuned the mount and replaced all the bearings with SKF ones. Maybe you could post a PHD2 log file?

Best of luck!

[Rallemikken]

6 hours ago, Backyardscope said:

Have you checked the clips on the primary mirror to make sure the mirror is not pinching.

Exactly! And check the angle of the focus tube, it must be spot-on in both axis. These are the two things you should start with on a newt.

[Astrosurf]

Thank you all! I didn’t make it clear that this pic isn’t from a sub as I didn’t get that far. It’s only a 3s snap while focusing with the ASIAir. I’ve used my old SW Explorer 200P for years with this CC and mount with few issues. It’s just since using this cam.  

[Astrosurf]

Thank you all! I didn’t make it clear that this pic isn’t from a sub as I didn’t get that far. It’s only a 3s snap while focusing with the ASIAir. I’ve used my old SW Explorer 200P for years with this CC and mount with few issues. It’s just since using this cam.  

[Astrosurf]

The lady I bought the scope off is super clever and capable. She would have made sure the focusing tube was lined up properly. 

[ollypenrice]

I'm with Backyardscope and suspect pinching. The star looks distinctly triangular to my eye and that's a classic consequence of pinching. I'd try to relax the mirror clips just a tad.

Olly