Is my field flattener working properly?

[TARDIS737 (Lachlan Ellis)]

Hello everyone, I recently purchased my first scope which is a William Optics Z61 and also its dedicated field flattener. The first image I took with it was on the Carina nebula. For a first image I am pretty happy with it, but I have found that there is still a lot of distortion around the corners of the frame. Does anyone know if this is an acceptable amount of distortion, or did I get a dud scope or field flattener?


This image is also only about 15 minutes of exposure time, but I would love to hear any advice on how to make my images better



https://cdn.astrobin.com/thumbs/Jvaf...0_wmhqkGbg.jpg

[brisen (Brian)]

Hi Lachlan

Whilst I am not familiar with the particular scope you have, I have two that use a flattner and have round stars right to the edge of the frame. The key is to get the back focus exact. From your image it looks close but does not look to be exact and this causes the distortion you see. I found using a set of verniers from Bunnings to be the best option to get the distance exact and used these to work out the extension spacers I needed. Remember that you need to measure from a specific point on the flattner as it may not be from the edge of the flattner. This distance and the point where it is measured from should be in the documentation with the scope. Also need to allow for the glass cover over the camera chip and the thickness of any filters used.

It can be an exercise in frustration getting the exact back focus but it is worth it.

Brian

[Paul Haese]

Your flattener does not appear to be at the right back focus distance to the sensor. There is still significant field curvature. The stars should appear round in the corners.

[CeratodusDuck (George)]

Looks like off-spacing and slight tilt to me.

Are you using the WO spacers to get the correct metal-back distance?

[Nikolas (Nik)]

Remember the correct backspace is from the back of the flattener to the sensor, not sure about WO or which camera you use but on my 294mc the backspace is 55mm from the back of the fattener to the sensor. your backspace is imho a little short

[gregbradley]

What camera? If its a full frame camera you may want to check the size of the corrected circle of the flattener. Full frame cameras need 44mm of corrected circle. A 61mm scope may not do that. It looks like its corrected for an APSc sized camera.

Greg.

[TARDIS737 (Lachlan Ellis)]

Hello everyone. Thank you for your help. I have a Canon 200D (crop sensor DSLR), and since you mentioned the back focus I wasn't sure how I would be able to increase the distance between the sensor and the lens, but after taking a look at my field flattener it is actually adjustable to increase the spacing, so I will have to take some time to get it down pat.



Thanks for your help!