what's this ??

[sharpie78]

Hi all,

Just wondering if anyone can explain what's going in the the images I've linked to below ;

I've done quite a bit of reading up but I'm not sure whether it's my collimation routine, astigmatism, poor mirror quality, coma, mis-aligned focuser  etc etc etc.....for all I know this could be prefectly normal for my scope.

I have a SW Quattro 8S by the way and I am using a Baader coma corrector.

I've obviously de-focused the images quite a bit to try and show the effect more clearly.

I'm still at the lower end of the learning curve for Astrophotography and I'm cropping my finished images so the effect at the outer edges of the image isn't a massive problem. I would like to know what the cause is though.

If anyone can help.........Thanks in advance

[LukeSkywatcher]

It looks like an out of focus star showing the secondary mirror in the centre.

[sharpie78]

lol. I reckon I've got that part, I aint totally brain dead, not yet anyway

......It's the rugby ball shaped stars at the fringes of the FOV in the other images I was referring to.

[sharpie78]

at one of the edges (top left if i remember correctly) the secondary swaps sides depending on which isde of focus it's on

[LukeSkywatcher]

Ah ok. That looks like the secondary mirror is turned and is misaligned with the primary.

[sharpie78]

so....rotational error ??

[LukeSkywatcher]

Not sure it could be something else to do with imaging.

[Pompey Monkey]

It looks like the stars at the edge are so far off the optical axis that the end of the tube is partially shadowing the primary. This is one cause of vignetting.

[sharpie78]

What would cause that pompey monkey please ?

I had my primary out of the scope recently to re-spot it with a hot-spot.

Would me not getting it centred properly cause these odd shapes ?

[Peter Drew]

Looks rather like vignetting to me as well, you get the same effect from the baffle tube in a SCT if you place a defocused star well off axis. 

[cjdawson]

That looks like Coma to me.  Basically, your scope is great in the middle, then as you progress towards the edge of the fov, the problem gets worse.  Looking at the image, it's fairly uniform, so a coma corrector (make sure you get the right one) will help.

[sharpie78]

I've already got a Baader coma corrector in CJ so I'm kinda leaning towards the whole vignetting theory. My quattro 8S does have baffles in it Peter so maybe this is the cause of the problem ?

I'd never done a star test before so never noticed this issue until a couple of days ago.

My understanding of baffles is that they increase contrast by reducing stray light bouncing around in the tube. If I removed the baffles and flocked the inside......would this correct the problem or just lead on to other issues ? which would be better.....baffles or a flocked tube ?

[D4N]

Is the Baader coma corrector you are using designed to be used at f4? Is the spacing to the imaging sensor correct?

There is a skywatcher one specifically designed for that scope.

/Dan

Sent from my iPad using Tapatalk

[cjdawson]

Not all coma correctors are the same.  If it's not the right one, or the spacing between the corrector and the camera (or the primary mirror and the coma corrector for that matter) isn't right, then it won't properly solve the problem.

Vignetting is the darkening of the image towards the edges, this is to be expected with all scopes.  There's not really a way to stop it, but you can correct for it by using flat frames.

[Waldemar]

The spheric aberration that is present in your scope is blown up by being out of focus, which makes it look a lot worse then it is when in focus.

Like it has been said before the distance between your comacorrector and the camera may be off a little bit, maybe only a millimeter or less. This is very critical.

The effect is hard to see in the focused image you also sent, so it cannot be much. The weird shape on the edge of one of your images is probably a combination of factors:

out of focus + spheric aberration + vignetting + maybe a cut off by the edge of a baffle. and last but not least, sensor size. Maybe the sensor is larger then the scope's (coma corrector's) corrected field.

Do not take the baffles out... most work better then flocking i.m.h.o. (if they have the right size)

[Waldemar]

not to forget: collimation...

[sharpie78]

Thanks for your replies.....

I've got the Baader mk III coma corrector and it's advertised as having no vignetting down to f3.5.

It has a thread for direct coupling to a Canon EOS t-ring so one would assume it's corrected field is large enough to accomodate EOS sensor sizes. (I'm using an EOS 450D by the way).

I might just try it with and without the optional adapter that comes with it and see if that makes any difference. The adapter looks like it probably changes the spacing between the coma corrector and the sensor by a mm or 2 so maybe that will help. Only one way to check I guess.

It's not something I would have ever thought about on my own so thanks guys. I had no idea the spacing was so critical to it working properly.

A quick question on the baffles........I'm now thinking about flocking just the top section of the scope, the bit between the top of baffles and the aperture of the scope, and might need to remove the baffles so I can get my hands in there easily. Is re-positioning of the baffles important when I put them back in or will roughly where they were before removal be ok ?

(The baffles in my Quattro are just plastic ring mouldings that stack on top of each other)

[Waldemar]

Well, if you can take them out easily you can always give it a try,,, For re-positioning you can measure the distances in between them, before taking them out.

If you want to use self-adhesive flocking material, you can put it in as a roll without attaching it to the scope's inside, so you can take it out easily again.

With a little care it will stay in place long enough to get an idea about the difference with baffles. Maybe use little pieces of double sided tape to keep it in place.

That will give you a better insight on how to position the flocking material as well.

I believe your canon has an APS-c size sensor, yes? so you will need a corrected circle of about 27 mm. Basically you should have a nice flat image with that sensor on your Baader corrector. Maybe  investing in a set spacer rings like this: http://agenaastro.com/baader-t2-delrin-spacer-ring-set.htmlgives you the playroom you need to find the sweet spot of the corrector to camera distance.

Wishing you luck and patience :-)

Waldemar

[sharpie78]

Thanks for that info waldemar. I'll do a few experiments and see what results I get.

Patience wasn't one of my virtues but since having kids and taking up astrophotography..........now.......I've got it in spades 

You're correct in saying my sensor is an APS-c size sensor. Dialling in the coma corrector should be a good test of my patience eh.

The link is handy. Pretty reasonably priced too.

Thanks to everyone for the help. SGL community living up to it's reputation again.