Eta with weird stars

[peeb61]

Hi All,

I have attached one of the images I took of Eta Carina.
It's a quick 2min exposure using my modded Canon 350D.


There appears to be coma? Or is it field rotation? At both ends of the image.

Taken with my 10" LX200gps using a celestron f6.3 focal reducer and guided with a skywatcher 102.

The focal train is….

MeadeMotofocuser >Deep Sky filter>Orion Adaptor tube>f/6.3 focal reducer>T-adapter>Canon 350D



My questions are ...do I have the focal reducer in the right place in the focal train or do I need to move it to another point in order to reduce this error?
Thanks in advance

Paul

[multiweb]

Looks like massive field curvature to me because the center is beautifuly sharp and in focus.

[AlexN]

Yep, field curvature mixed with a touch of mis-collimation is how that looks to me...

I had similar results once with my C11..

[peeb61]

Thanks Marc and Alex for your comments however it still brings me back to my original questions:

Do I have the focal reducer in the right place in the focal train or do I need to move it to another point in order to reduce this error?

Many thanks
Paul

[TrevorW]

Hi Paul when I use my focal reducer I'd put the filter to that then to an adapter tube no issues

Cheers

[AlexN]

The focal reducer should be approximatly 100 ~ 110mm from the imaging sensor for best results according to Celestron.

Is your LX200GPS an ACF OTA? If it is, the celestron corrector will actually curve the field, rather than correct it... In the case of an ACF ota, you want to use a dedicated reducer like the AP .67xCCD reducer..

If your OTA is not ACF, then adhering to the 100~110mm backfocus requirement of the corrector should do the job.

[peeb61]

Thanks Trevor and Alex,
My LX200 is around 7 years old so it's the plain run of the mill OTA.
I will play around with these measurements and see how we go. I have moved the Focal Reducer to the from of the queue, butted up against the scope tube, then the focuser, then the T-Adapter and finally the camera.

The distance measured is roughly just less than 115mm.

I will test this out at the next opportune and see how it goes.

Many thanks
Paul

[peeb61]

Well I tried this combination out last night and failed miserably!

The focal train was:

Meade LX200 > f6.3 focal reducer > Meade motofocuser > T-adapter > Canon 350D

This put the distance between the focal reducer and the camera chip at roughly 115mm.

Looking through the camera viewfinder all's I could see was the secondary mirror reflection. Could not bring anything to focus.

Stumped.....

Paul

[AlexN]

You are taking into account that the camera sensor is 44mm deep within the 350D body?

That seems very strange.. That is the exact distance I had between the corrector and the sensor with my C11 and it worked very well.

I'll see what I can dig up mate..

[peeb61]

Thanks Alex,
Much appreciated, the measurement was taken from the notch on the Canon 350D supposedly depicting were the sensor lies within the camera to the start of the focal reducer.

Paul

[peeb61]

Alex,
Here's an image of the focal train showing clearly the notch on the Canon 350D to the focal reducer.

Paul