spacing when used with a canon lens

alcolmoon

Silly question I know, but can someone tell me what the distance is for the above set up. I have looked at the manual and it says a T2 Extender 11mm plus an EOS T2 Adapter. I have these and when fitted they amount to 37mm to the face of the lens. Is this distance correct?

I ask because I have horrible shaped stars away from the center of the image.

Thanks for any help

martinf

Do you use a filter wheel as well? That adds 20 mm in that case, making proper distance.

alcolmoon

No, no filter wheel, just the two fittings I mentioned, giving a spacing of 37mm. is the correct spacing?

radarboy

If you have horribly shaped stars, its more likely you are using a lens with bad coma. If you are unable to achieve focus, then its likely a spacing issue. Can you post a picture of the stars? Most canon lenses require you to go down a stop or two to get round stars across the frame. You can stop down the lense by having the camera on, setting the f-stop on the camera, and pressing the DOF button. Then, while still pressing the DOF button, take the lense off the camera. This "freezes" the iris at that F Stop.

Support@ZWO

camera with 11mm ring work with EOS-T2 adapter will reach the best focus length.

alcolmoon

Focusing is Ok, stars in the center of the image are fine, only the corners are rubbish. Presumably the bulk standard Canon 50mm lens isn't perfection?

See attached for images of center and corners

alcolmoon

Should have said, I had stopped the lens down as you suggest

martinf

The Canon 50mm is horrible in the corners, even after stopping down to f/4. I gave up on mine a long time ago. Both the f/1.8 and the f/1.4 USM are bad. I bought a Sigma 50/1.4 EX lens that is a lot better.

alcolmoon

Thanks for feedback Martin, had a look at that and looks like serious money, although as they say - you get what you pay for. May have to stick with my TSAPO60 and forget about using a wide r view lens. Thanks for your inputs.

Just a thought, does anyone have recommendations on the best "cheap" canon lens?

Thanks again

martinf

The best cheap lenses (although not that cheap) are the Samyang manual focus lenses. I have the 24mm and the 135mm and they are razor sharp even wide open. Maybe there is a 50mm? I know there is an 85mm.

radarboy

I'll second the Samyang lens option. I have the 14mm 2.8 and its wonderful with almost no coma. Lenstip.com does lens reviews that specifically look at coma and astigmatism, which is ideal for astrophotography.

Definitely coma on the shots you posted.

Support@ZWO

Lens might not perfect, it is much complex than telescope.