Coma corrector = square stars!

[StargeezerTim]

Hi there,

Used my Baader Coma corrector last night for the first time. It has cured the come but the stars are squarish. Anyone any ideas as to how to cure this? I attached it to my DSLR straight to the T Ring with the stopper collar mounted.

Hope the the pic is clear. There was also a double horizontal star spike on the star on the right. It is a single sub, not the stack. Cheers, Tim. 

[StargeezerTim]

Heres another crop of the effect!

[Magnus_e]

Not 100% shure, but it looks like bad tracking.

All the stars are pointing in the same direction, so seams like "drift"

Was it a guided sub? How long was the exposure?

[StargeezerTim]

Hi Magnus,

150 secs. It was guided. I'll check my settings but up to no my guiding with the MGEN has been trouble free...Tim. 

Only noticed this since the Coma Corrector was added... Does look like tracking though...

[Magnus_e]

At 150 sec you would / could get that trailing if guiding was off

I have had some nights with guiding not working even with settings that normaly works.

You could have been of in balance or something? I allways do "East heavy balancing"

I would try some shorter exposures next time to see if it's tracking.

Maby you will have perfect guiding next time and round stars.

"Don't know about a reason why a coma corrector would affect the stars in that way, but someone may come along and give a better ansver"

[Tinker1947]

Go back to square one, redo the balance, then the PA, the coma corrector likely as not has nothing to do with the wonky stars.....

[StargeezerTim]

 I allways do "East heavy balancing"

Magnus, How do you do that? Is that having the tube heavier than the counterweight, or the balance of the tube, front vs back, or is there another balance I am missing? Tim

[Magnus_e]

East hevy balancing is to keep the side that is pointing to the east slightly heavier than "the other side"

So if you are imaging to the south befor the meridian flip the counterweight will be to the east and the scope to the west, so counterweight should be slightly heavier.

After the meridian flip the scope would be on the east side of the mount, so move the counterweight slightly up on the counterweight bar and the scope would be heavier.

By doing this the motor and gears of the mount will always be pushing the scope, avoinding backlash issues.

[StargeezerTim]

East hevy balancing is to keep the side that is pointing to the east slightly heavier than "the other side"

So if you are imaging to the south befor the meridian flip the counterweight will be to the east and the scope to the west, so counterweight should be slightly heavier.

After the meridian flip the scope would be on the east side of the mount, so move the counterweight slightly up on the counterweight bar and the scope would be heavier.

By doing this the motor and gears of the mount will always be pushing the scope, avoinding backlash issues.

Thank you, perfect explanation for me! 

[Scott]

is the spacing not critical for dslr+coma correctors? I never had one when I used the dslr so I honestly don't know, possibly a tilt in the imaging train? Just thoughts

just had another look at the second image and to me, it doesn't look like trailing, the stars look more like crescents or even seagulls

[Tinker1947]

is the spacing not critical for dslr+coma correctors? I never had one when I used the dslr so I honestly don't know, possibly a tilt in the imaging train? Just thoughts

Its set at (i think 54.5mm) which is the space when the Baader is attached to the DSLR via the T-Ring.....

[Scott]

Its set at (i think 54.5mm) which is the space when the Baader is attached to the DSLR via the T-Ring.....

cheers, I may be way off the mark, but it just doesnt look like trailing to me. the tips of the stars seem too pointy to me. maybe balance i guess.

[StargeezerTim]

cheers, I may be way off the mark, but it just doesnt look like trailing to me. the tips of the stars seem too pointy to me. maybe balance i guess.

I'll try the 'East heavy' way tonight, assuming there is a cloudless night! 

[Tinker1947]

I'll try the 'East heavy' way tonight, assuming there is a cloudless night! 

This shows it to be clear in NW Norfolk

http://astronomy.tools/

[StargeezerTim]

OK then,

I did some more imaging last night. Tried the 'east heavy' balancing of the scope. All went well as far as guiding was concerned but I still have stars that look a bit like saucers or discs with a couple of bites taken out of them. 

Here is a crop of some stars before the Coma Corrector was purchased.

As you can see, generally round!

Here is a crop of a sub from last night (5 mins guided)

As you can see, the stars are mis-shaped. The coma corrector has definately got rid of the coma but this seems a bit of a price to pay. 

Anyone have any other thoughts or suggestions. I am considering returning it, though I would have to do a more rigorous before/after comparison before doing that.

I did read a thread on the Skywatcher coma corrector where something similar happened and someone was advised to take a hacksaw to the focusing tube, but that seems pretty extreme!

All thoughts and ideas welcome! Tim. 

[Magnus_e]

In this image guiding defenetly seams to be spot on.

The missshaped stars almost look to have several difractionspikes, with most of them pointing down and to the rigth.

This is out of my reach... Hopfully someon knows how to fix it.

[kalasinman]

Just a check here, the proper shoulder of CC to chip distance is 55mm. If you are using your Canon , that leaves 11mm from camera flange to shoulder of CC. Suggest you try to get the spacing as close to spot on as possible.

The multiple and non-symetrical diffraction spikes usually result from collimation issues.

[poogle]

To me it looks like the collimation is off. Try to collimate it before using the hacksaw

/Patrik

[StargeezerTim]

To me it looks like the collimation is off. Try to collimate it before using the hacksaw

/Patrik

I hope to keep the hacksaw well away from it. Collimation is OK though, collimated before the previous nights imaging, which showed similar shapes.

[poogle]

I hope to keep the hacksaw well away from it. Collimation is OK though, collimated before the previous nights imaging, which showed similar shapes.

Ok How did you collimate it?

When I had star shapes like that I tried a laser to colliamte it, but it made no difference. I think the secondary mirror was tilted somehow and the focuser not aligned properly. I had to collimate it properly using this as a reference: http://www.astro-baby.com/collimation/astro%20babys%20collimation%20guide.htm

I still had some bad star shapes after that, but that was due to pinched primary mirror screws.

Just a thought, I might be way off here. But maybe try taking some images with and without the CC the same night to rule out collimation.

/Patrik

[ollypenrice]

I'm not sure why guiding has occupied so much attention here. The very severe abnormalities in the diff spikes of the first image don't come from guiding. They could, just about, arise from backlash if the mount spent some time on one side of mesh making one image then dropped to the other side and made another, slightly offset, image during the same exposure. But I don't think so. One diff spike is clearly split into a double while the other isn't. Double diff spikes can be caused by focus but then both are affected, so something else is wrong in your Newt. This isn't something I know much about. What I would do is re-post post the first image with a request for Newt experts to tell you what's wrong with it.

Olly

[Thalestris24]

Hi Tim

It's very easy to introduce tilt in the imaging train on a newt. Double check that camera + coma corrector are seated square and tight in the focus tube.

Hth

Louise

[kalasinman]

Certainly not an expert. It's just that I had split spikes on mine and found that one of my spiders was not parallel to the optical axis. Further the OP hasn't explained the details of his collimation method. Is he starting with a cap or Cheshire, then going to a well collimated laser? An f/5 scope is reasonably sensitive I believe the "sweet spot" is just a couple mm. It is possible to believe one is collimated when one is not. I know, I've had that experience.

[kalasinman]

Hi Tim

It's very easy to introduce tilt in the imaging train on a newt. Double check that camera + coma corrector are seated square and tight in the focus tube.

Hth

Louise

+1

[Thalestris24]

Just to add... Clearly any collimation error will be present with or without the cc. If you get the same problems without the cc then it's likely to be a collimation problem, I think.

Louise