Half Moon Star Shapes

[Starwiz]

I attempted to image M1 last week but I'm seeing half moon star shapes in parts of the image.

The first images shows the full frame.

The next shows the left-hand side at about the 1/3rd point.

The final images shows the right-hand side at about 2/3rds.

The 'half moons' are pointing in different directions on either side of centre.

Any thoughts on the cause?  Tracking was good and I didn't see this in the previous session.

Thanks

John

[ollypenrice]

Collimation and focus, I would say. I can't tell if it's a double star or not, but double diffraction spikes usually point to poor focus. How did you focus?

Olly

[alacant]

Hi. A hands-on novice POV:

Left: (a) if cc, it is too far away from the camera sensor and is tilted or... (b) the camera is tilted

Right: the focusing tube is entering the light path.

HTH and clear skies.

[PhotoGav]

How low was the Crab Nebula when you were imaging it? Was it early in the evening and low to the horizon? Or perhaps low over neighbours’ roofs? Just wondering if seeing can have this effect. I recently imaged M2, which is pretty low for me and I saw similar stars, yet my next project, which was higher in the sky, with no changes to the kit, had perfect star shapes. 

[Starwiz]

1 hour ago, ollypenrice said:

Collimation and focus, I would say. I can't tell if it's a double star or not, but double diffraction spikes usually point to poor focus. How did you focus?

Olly

Bahtinov mask, with APT focusing aid.  It showed focus to be as close as I've ever had it.  I locked the focusing tube and rechecked.

Could tightening up the rings too tight cause this?

[cfpendock]

The second picture looks like a collimation problem.  

Chris

 

[Starwiz]

1 hour ago, PhotoGav said:

How low was the Crab Nebula when you were imaging it? Was it early in the evening and low to the horizon? Or perhaps low over neighbours’ roofs? Just wondering if seeing can have this effect. I recently imaged M2, which is pretty low for me and I saw similar stars, yet my next project, which was higher in the sky, with no changes to the kit, had perfect star shapes. 

It was around 30 - 40 degrees.  No nearby rooftops as my back garden is next to a school field.  I've imaged lower and not seen the same problem.

[PhotoGav]

2 hours ago, Starwiz said:

It was around 30 - 40 degrees.  No nearby rooftops as my back garden is next to a school field.  I've imaged lower and not seen the same problem.

You can cross that idea off the list then!

[Davey-T]

First enlarged pic' looks out of focus hence little donuts, if a different part of the same pic is focused OK it looks like some sort of tilt, was the star you focused on in this image or somewhere else in the sky ?

Dave

 

[wimvb]

2 hours ago, Starwiz said:

Bahtinov mask, with APT focusing aid.  It showed focus to be as close as I've ever had it.  I locked the focusing tube and rechecked.

Could tightening up the rings too tight cause this?

Most definitely. Best way to avoid this: don't lock the focuser. I.e. use a motor focuser. If this is not an option, try to anticipate how much focus will shift and stop short focusing so that focus lock will do the rest.

A coma corrector at the right distance should improve star shape, as @alacant suggested.

Good luck

[Starwiz]

59 minutes ago, Davey-T said:

First enlarged pic' looks out of focus hence little donuts, if a different part of the same pic is focused OK it looks like some sort of tilt, was the star you focused on in this image or somewhere else in the sky ?

Dave

 

The star was elsewhere in the sky as I need something bright for the Bahtinov.  This is what I usually do, but haven't seen the same issue previously.

I'll try some test frames next time I'm imaging.  Unfortunately, I broke my collarbone last week, so it could be a few months.

John

[Starwiz]

1 hour ago, wimvb said:

Most definitely. Best way to avoid this: don't lock the focuser. I.e. use a motor focuser. If this is not an option, try to anticipate how much focus will shift and stop short focusing so that focus lock will do the rest.

A coma corrector at the right distance should improve star shape, as @alacant suggested.

Good luck

Sorry, I meant the tube rings.  I had loosened and tightened these to balance the scope early in the setting up (well before focusing).

[alacant]

5 minutes ago, Starwiz said:

I meant the tube rings

I'm gonna say yes. This is based on my experience with a 208/f3.9. Tube flex was eliminated minimised by fitting a wider, longer dovetail mount side and a solid rail along the top of the rings. I think your setup will be less prone to tube movements though. Probably best to do a star test and tweak the primary accordingly just before you start the first exposure; only takes a couple of minutes. 

16 minutes ago, Starwiz said:

I broke my collarbone last week

Ayeeee. ¡Que te mejores!

[Starwiz]

12 minutes ago, alacant said:

¡Que te mejores!

Thanks.

[PhotoGav]

43 minutes ago, Starwiz said:

Unfortunately, I broke my collarbone last week, so it could be a few months.

Ouch - hope you’re back to full fitness as quickly as a jockey!

[Starwiz]

2 minutes ago, PhotoGav said:

Ouch - hope you’re back to full fitness as quickly as a jockey!

Thanks.  No way am I carrying outside an NEQ6 anytime soon.

[wimvb]

10 hours ago, Starwiz said:

The star was elsewhere in the sky as I need something bright for the Bahtinov.  This is what I usually do, but haven't seen the same issue previously.

I'll try some test frames next time I'm imaging.  Unfortunately, I broke my collarbone last week, so it could be a few months.

John

 

9 hours ago, PhotoGav said:

Ouch - hope you’re back to full fitness as quickly as a jockey!

Ouch, indeed. Hope you'll recover soon.

In the mean time, here's an article about collimation (with more good references in the menu section), that may help. But you probably akready know this.

http://garyseronik.com/a-beginners-guide-to-collimation/

[Starwiz]

2 hours ago, wimvb said:

 

Ouch, indeed. Hope you'll recover soon.

In the mean time, here's an article about collimation (with more good references in the menu section), that may help. But you probably akready know this.

http://garyseronik.com/a-beginners-guide-to-collimation/

Thanks.  I have plenty of time to study at the moment.