funny stars in my images.

[redfox1971]

Can anybody tell me what is causing the stars in the corners of my images to all drift inwards while the center of the image is ok,is this whats called coma?

[Daniel-K]

sounds like you need a filed flattner if its for your cpc800

[Owmuchonomy]

Sounds like field curvature which can be solved with a field flattener in the imaging train. I'm not familiar with your scope design though. My 9.25 SCT doesn't suffer from field curvature.

[sharkmelley]

Classic SCTs suffer from both field curvature and coma.

Mark

[redfox1971]

I have the celestron 6.3 focal reducer,corrector on the back of the cpc800 scope,i have been imageing with 2 inch adapters so will down size and see if i get any change,thanks all.

[perfrej]

The documentation for the flattener will have a figure on the distance between the flattener and the imaging plane. That needs to be closely observed.

/p

[sharkmelley]

The standard Celestron 0.63x reducer/corrector comes with no documentation.  There are endless threads on forums discussing what the "correct" distance should be. Experimentation showed me that distance to imaging plane matters very little - changing the distance only seems to affect the amount of reduction.  So the correct distance is probably the distance that gives the advertised 0.63x reduction.

For imaging, this particular reducer/corrector does very little in the way of correcting the distorted stars at the edges and corners of the image.  At least that was my experience when using a DSLR on the Celestron C11. 

[jambouk]

Sharkmelley, what solution did you reach then to flatten the image from the C11?

Jd

[sharkmelley]

Sharkmelley, what solution did you reach then to flatten the image from the C11?

After much frustration with the Celestron 0.63 reducer I have ended up using the Teleskop Service corrector:

http://www.teleskop-express.de/shop/product_info.php/language/en/info/p3291_Image-Corrector---0-8x-Focal-Reducer-for-Schmidt-Cassegrains.html

It reduces by only 0.8x instead of 0.63x but it does an excellent job of flattening the field and reducing aberrations - take a look at the spot diagrams and example images on the TS page.  I also found the tilt adjuster that TS recommend to be essential - the flattener produced an obviously tilted image field without it.

Another possibility is the Starizona corrector but I don't have direct experience of it:

http://starizona.com/acb/Starizona-SCT-Corrector-P3230C0.aspx

However, take a look at the spot diagrams on the Starizona page - the comparison with the Celestron 0.63 reducer is most enlightening.

Mark

[jambouk]

Mark, they are impressive results! What camera are you using and what are do you do about spacing from the flattener to the sensor? Did you have to get any of the extra spacers advertised on that webpage?

James

[Davey-T]

I've got the same problem 

Dave

[sharkmelley]

What camera are you using and what are do you do about spacing from the flattener to the sensor? Did you have to get any of the extra spacers advertised on that webpage?

I'm using a DSLR so I bought a combinaton of their T2 extension rings to make the optical distance from the corrector to the sensor 97.5mm.    One of them was a fine tuning adjusting ring so I could make any necessary adjustments to allow for the internal filter I removed when modding the camera and any extra filter (e.g. EOS clip in) I add to the path.

Mark

[jambouk]

How do you know you've got the required 97.5mm gap?

I'm tempted to just buy one of these flatteners and a selection of rings on your advice.

JD

[ollypenrice]

How bad the distortions are depends on how big the chip is. They get worse away from the centre. Edge distortions in non ACF or Edge SCTs are quite severe and the 6.3 reducer cannot produce a large flat field, though it is a step in the right direction.

Unlike most reducer flatteners the 6.3 is, indeed, very tolerant of chip distance variation.

Olly

[jambouk]

Does a 0.8x reducer result in better correction or worse compared to a 0.63x on a full frame DSLR sensor?

Jd

[redfox1971]

I set the distance from my Qhy8l to the front of my  focal reducer glass to about 108 mm to 110 mm that's what seems to be recommended by people,that was i think the same for a dslr camera i did have it set at 75 mm before that and the stars from the edge seemed to be more out of focus,it seems i may have a big ccd chip but when useing with the focal reducer you have to crop the image down witch is a shame.

[ollypenrice]

Does a 0.8x reducer result in better correction or worse compared to a 0.63x on a full frame DSLR sensor?

Jd

I don't know about the 0.8 but you won't cover a full frame chip (by which I guess you mean 35mm on the long side) with the 6.3. Remarkably few optics will cover this format, especially when focally reduced.

Olly

[jambouk]

By that do you mean there will be even more vignetting than without a focal reducer? I was expecting that, which is another reason i thought the 0.8x might be better.

But what proportion, roughly, or the non-vignetted (does that term really exist) image would be "flatter" with, compared to without, a reducer/flattener?

Jd

[jambouk]

My sensor is 35.8 x 23.9 mm.

Jd

[sharkmelley]

Sorry for the late reply - I've just come back from an imaging weekend.

Does a 0.8x reducer result in better correction or worse compared to a 0.63x on a full frame DSLR sensor?

Jd

The TS website (and spot diagrams) show that the 0.8 reducer works well for a full frame DSLR sensor but I haven't tried it.  There will be some vignetting though.  It is an order of magnitude better than the 0.63x reducer.

How do you know you've got the required 97.5mm gap?

The sensor to flange distance on a Canon is 44mm I think.  You just need to make up the rest with extension tubes and Canon adaptor ring.  I'll post a picture of my setup later.

Mark

[sharkmelley]

These 2 pictures show my optical setup. 

I've marked the flattener so I can always attach the tilt and flattener in the same orientation with respect to the C11.  If I wish to rotate the camera I do it with the fine tuning ring - the extra half millimetre either way in optical path length doesn't seem to be too critical.

Somewhere, I may have an exact list of the adapters and extension tubes I ordered.  I'll take a look.

Mark

[jambouk]

Blmey, that is a lot of adapters and stuff! I'd have a focuser on there too as i don't have the dual speed focuser on the scope.

It might be easier just to buy a C11 Edge lol

Thanks for posting the images, i would be interested to see the list.

Jd