Coma corrector = square stars!

[StargeezerTim]

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

Thanks Patrick,

I will do the comparison images before collimating, if we ever get a clear night again! Thanks for the link, Tim.

[Uranium235]

Could be a number of things, check the following:

1: Collimation - Get it as good as you can, with no (or as little as possible) secondary rotation.

2: Focus - Use a B-mask (super important)

3: Adaptors - Make sure everything is flush, no room for wonky or loose fittings.

4: Push-fit connection - Ensure its done up nice & tight and the corrector isnt "popping out" slightly.

5: Spacing - If using the T2 thread, the spacing is somewhere between 55 - 57mm.

6: Spider vanes - Make sure none are twisted or bent.

Do as suggested and take some images with and without the corrector.

[ollypenrice]

I was thinking 'twisted vanes,' I must say, but if the CC started the trouble then it could be that the focuser was disturbed and is no longer on-axis.

Olly

[StargeezerTim]

UPDATE!

Last night I tested the Coma Corrector on two stars, Vega and surround and Sadr and surround, First two without CC and then two with the CC, not guided, 1 min exposures.

There was a very slight deterioration on the Vega pics but no real difference on the Sadr pics. One of the things I noticed when changing the camera fittings around is that the CC nosepiece is shorter and not quite so snug a fit as the other nosepiece I use with the DSLR. I think it is possible to introduce a slight tilt when tightening it with the two screws on the focuser, especially if one is tightened more that the other.

So I am relieved to know that it is unlikely any real problem resides with the CC but that I need to be sure to tighten it evenly. To be honest, the two screws on the the focuser don't really seem that good for such a crucial task. 

I will learn how to do a proper collimation. I recently washed the primary mirror and though it was resited and collimated with the use of a laser collimator, I take the point that it may need to checked and adjusted with more rigour.

I thank you all for supporting this keen, but green amateur. Tim. 

[Uranium235]

Just as I thought, the push fit adaptor being a bit skew-iff.

Time to get the toolkit out... Best thing you can do is remove the pushfit adaptor from the drawtube (it just screws right off), might be a bit tight if youve never removed it - but it does come off. Then you need to drill & tap a third M4 screw hole (you can get the taps from B&Q), use a slightly smaller drill bit than the thread you will be cutting - a guide on drill/thread sizes can be easily found on 'tinternet. Then borrow another thumbscrew from another adaptor/barlow/etc and it will fit right in and hold things in better with three screws 120deg apart.

However, dont use a hand drill - if you have access to a bench drill use that (it will be more accurate). This is quite a common problem (especially with heavy cameras), and replacing it with a compression ring does not solve the problem (in fact it makes it worse!).

This is core reason why I went all-threaded - screws and compression rings really dont cut the mustard.

[ollypenrice]

Another definitive post from Mr Uranium!

Olly

[Uranium235]

Its just what I found out through trial and lots of error. I gave mine a little tweak last night and ive just done a defocused star test, the donut had the shadow slap bang in the middle - a good sign! Just waiting for the first 15min sub from the tulip to roll off... ahhh speak of the devil - here it is!

A few more of these and I'll be in business.... but im trying to resist the urge to swing over to M16 

[StargeezerTim]

Just as I thought, the push fit adaptor being a bit skew-iff.

Time to get the toolkit out... Best thing you can do is remove the pushfit adaptor from the drawtube (it just screws right off), might be a bit tight if youve never removed it - but it does come off. Then you need to drill & tap a third M4 screw hole (you can get the taps from B&Q), use a slightly smaller drill bit than the thread you will be cutting - a guide on drill/thread sizes can be easily found on 'tinternet. Then borrow another thumbscrew from another adaptor/barlow/etc and it will fit right in and hold things in better with three screws 120deg apart.

However, dont use a hand drill - if you have access to a bench drill use that (it will be more accurate). This is quite a common problem (especially with heavy cameras), and replacing it with a compression ring does not solve the problem (in fact it makes it worse!).

This is core reason why I went all-threaded - screws and compression rings really dont cut the mustard.

Ta da....

First bit of metalwork I have done for at least 45 years!

Anybody want a hole drilled and tapped (Only M4 though, thats the only tap I have) get in touch). Lol 

I did find it was pinched optics causing the problem but now my camera is even more securely fastened to the focuser... Tim. 

[eyepod]

Just purchased a coma corrector in FLO's sale so will be doing this "3rd screw mod" before it gets its first light

Thanks for the info

Jason

[Uranium235]

Good to see you got it sorted Tim, now bring forth the images!   (if only it were clear!)

[lensman57]

Heres another crop of the effect!

Star shapes.jpg

Make sure that the CC is held rigid in the focuser tube. It looks if the position of the sensor has changed after the initial exposure as you also have double diffraction spikes. I have had issues retaining the CC and the camera rigid in the SW focusers both with SW CC and Baader MPCC, this is just bad design of both  the CC and the focuser tube. Check the tracking after making sure that the CC and the camera can not move under their own weight.

A.G