Terrible corners

[Carl]

Is this the result of coma?
It can't be field rotation i get the same length streak no matter what the exposure. The problem only exsists in the corners, stars in the centre are sharp and round.

Used a C8 with 6.3 reducer same problem. Modified an extension tube to increase distance between reducer and camera and ended up with slightly wider field of view but just as bad a result with streaking stars.

What have i got to do to fix this problem (within affordable costs). Can it be fixed on these type of scopes or are the optics just bad on the C8.

I've heard some good reviews about the new Orion/Bintel f/4 reflectors.$799-$999. Would love an Astrograph but simply out of the question at the moment.

I'm actually happier with my wide field with my Skywatcher 80ed

Any help appreciated.
PS just had a go with my new modded camera . Woah! talk about red!

[leinad (Dan)]

It wasn't mentioned but are you properly polar aligned?

I had the same last night; but my polar alignment was waaay off. Was just testing some adapters with the focal reducer at the time.

[allan gould]

The stars in the corners are actually out of focus, while this in the center are in focus. This result is due to the fact that the spacing for your corrector is not at the right distance.
The correct distance depends upon the make of corrector and is in the region of 85-106mm from film/chip plane to the back of the corrector.
Hope this helps.
Allan

[multiweb (Marc)]

The out of focus effect is most likely field curvature. The trailing likely to be field rotation.

[bmitchell82 (Brendan)]

Can i take a combination of all 3?

slightly out of focus
wrong sensor/reducer distance
field rotation

a few things to work on their cheif plug away and you will be rewarded!

[Carl]

Thanks for the replies.
1. Polar alignment: Ive done a 10min test and my star doesnt move move from the centre of my reticle.
2. Focus: wasn't concerned with focus, this was just a test the rotating stars on the edge corners.
3. Length to camera from reducer: Ive tried it at around 50mm and at 100mm. Streaks just as bad. The only difference was that at 100mm i gained a slightly wider field of view.
If my scope was not aligned properly wouldn't the length of the streaks increase in distance with a longer exposure? At 4 different exposures, 30sec, 1min, 2min, 3min the streaks are exactly the same length.

Any ideas appreciated

[richardo (Rich)]

I'm going to chime in here....
If it were coma, you'd see it all around the edge of the field, angling out.

Have you checked that your camera is square to the incoming light (orthogonal)
You appear to be achieving focus to the centre of the image and to the L/H side.... totally out of focus to the R/H side.
I've seen this before!

I'd be checking all hardware extension tubes and fixtures into the focuser and also checking the focuser.
If your image train has any movement in the focuser or sag, then you need to fix this.
But even before this, you must start at the basics to confirm it's not something else....
Use practical process of elimination.
#1. Is the scope collimated?? (collimate it)
#2. check your focuser for any movement in the draw tube, any movement can relate to sag with camera attached, more extreme to different areas of the sky.. (new focuser??)
#3. Have solid threaded extension tubes into focuser... forget about putting cameras into a poor focuser with a measly single screw. If it's sloppy, you need to find a better solution.

Get these things checked and double check that they are all good then you can work on spacing of reducer....

Hope this helps..
Rich

[renormalised (Carl)]

Problem is, Rich, that pic he posted isn't the whole pic (at least I don't think it is), so it's hard to tell whether it's coma and/or misaligned optics/imaging setup. He's got 100's of little pacmen there, which is indicative of coma as much as it is of misaligned extension tubes etc.

[richardo (Rich)]

Mmm, be nice for people who want help to show exactly the way things are with a full size image and not a crop.... also what camera/ ccd, how its attached... all things that will help others...
This image is so misleading then, how can any one help to nail anything down??

Originally Posted by renormalised

Problem is, Rich, that pic he posted isn't the whole pic (at least I don't think it is), so it's hard to tell whether it's coma and/or misaligned optics/imaging setup. He's got 100's of little pacmen there, which is indicative of coma as much as it is of misaligned extension tubes etc.