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

[Astrosurf]

On 02/04/2024 at 16:53, ollypenrice said:

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

Thank you. If I loosen the bolts off a bit I’m worried the mirror will move. They’re huge bolts! 

[AstroMuni]

13 hours ago, Astrosurf said:

If I loosen the bolts off a bit I’m worried the mirror will move.

There should be screws on the other side (the silvered side) to hold the clips. The bolts in your image seem to be the Adjusting/Locking screws

[bosun21]

Looks very triangular to me as well. The screws responsible for pinching are inside the telescope tube holding the primary mirror in place. The three clips you see when collimating are where the screws are located.

Edited May 9 by bosun21

[Tiny Clanger]

You need to remove the mirror cell from the tube to get to the clips which hold the mirror in the cell.

If the clips are pressing on the mirror, the glass distorts, there needs to be a tiny gap between the mirror surface and the side of the clip that faces it.

It sounds scary, but if you are very careful it's not hard to take the cell out (best mark the outside of the scope tube and the edge of the cell before you start with some masking tape or similar, to help get it lined up exactly the same way round when put back together to minimise any collimation loss) I've managed it with a 250 newt and I'm a weedy ham fisted fool.

The usual advice is there should be a gap big enough to allow a sheet of paper between the glass and the clip.

 

[AstroKeith]

The photo doesn't look like a usual SkyWatcher backend? 

The classic 3 push/pull pairs of knobs and locking screws arent there. Putting the locking knobs halfway between the sprung (pulling) knobs is going to distort the primary cell holder.

[tomato]

1 hour ago, AstroKeith said:

The photo doesn't look like a usual SkyWatcher backend? 

The classic 3 push/pull pairs of knobs and locking screws arent there. Putting the locking knobs halfway between the sprung (pulling) knobs is going to distort the primary cell holder.

I agree, it’s not a good idea to have the locking screws so far away from the adjustment screws. Is it a third party mirror cell?

[PeterStudz]

1 hour ago, tomato said:

I agree, it’s not a good idea to have the locking screws so far away from the adjustment screws. Is it a third party mirror cell?

It’s standard on the Stellalyra Dobs.

https://www.firstlightoptics.com/stellalyra-telescopes/stellalyra-10-f5-dobsonian.html

 

Edited May 9 by PeterStudz

[bosun21]

Both my Stella Lyra dobs held collimation very well. Once I upgraded the springs I barely tightened the locking screws as the springs held the mirror well. It also didn't change collimation when nipping the locking screws.

[woldsman]

Odd if two scopes have bad stars. Most likely causes are the primary & secondary mirrors. Some photos of a bright star either side of focus would help diagnosis.