Field Flattener for Skywatcher Equinox Ed 66

[Stub Mandrel]

When I got hold of an ED 66 lens cell, I realised it would need a field flattener. It took me a lot of digging before I found some posts here that suggested the OVL flattener would work. The ED66 is f6.06 and the OVL flattener is for f5-f6 'or telescopes just outside this range'. It does not change the focal length, which mattered to me as I specifically wanted a 400mm set up as I already had a 400mm tele lens and find this this suits lots of targets really well used with an APS-C sensor.

I'm posting this so if anyone else fancies using the Equinox  ED66 for astrophotography, they can know the OVL flattener does work.

Initially I had bad stars in the corners with the appearance of coma. I added a 1mm spacer between the flattener and the canon t-mount adaptor and although DDC inspector said there was curvature present, it was reduced to the point where it's very hard to see in the image.

I will experiment with a slightly thicker spacer, but for the record:

The OVL flattener does correct the ED66 right across the field.

You may need to increase the spacing to get better results and a very small change (about 2%) in spacing can make a dramatic difference.

I got my flattener from @FLO

https://www.firstlightoptics.com/reducersflatteners/ovl-field-flattener.html

[happy-kat]

How much length did it take out of the available back focus length for reaching focus please?

[happy-kat]

It's the amount of back focus distance that field flattener takes up that I'm after please?

[Stub Mandrel]

Oh, sorry, just noticed this.

Technically I didn't lose any focal distance as I designed the scope around the lens/flattener combo.

The field flattener doesn't affect FL noticeably (I checked). The body length is pretty much dead on 2" (50.4mm). I assume they expect you not to use it for visual, and use it in the place of a 2" diagonal for photography.

You may need to add a couple of mm for spacers (still experimenting).

Edited October 14, 2018 by Stub Mandrel

[happy-kat]

Thank you. Perhaps you could see this and comment there please as I don't think it's an option then. Link here

[Stub Mandrel]

Did a session in Ha a few days ago. The results were pretty poor, mostly because I had the scope pointing up and an intrusive dust bunny in a different place on each frame so flats wouldn't take it out. Also, very noisy - thin cloud? A shame as my guiding was rock solid at 0.65" for about three hours!

But... I made a point of focusing at the 2/3 point, I have always focused at the centre before. Result was round stars across the whole APSC frame, so that IS a good piece of advice to follow in the future.

[62Marc]

Thanks for the info. Are you able to tell me what other adapters you used to make this all work please.

I purchase the scope but I'm struggling to find the correct adapters.

 

Thanks Marc

[Stub Mandrel]

7 minutes ago, 62Marc said:

Thanks for the info. Are you able to tell me what other adapters you used to make this all work please.

I purchase the scope but I'm struggling to find the correct adapters.

 

Thanks Marc

Hi Marc, I'm afraid it's a home-made scope that uses the Skywatcher lens cell. I just made it with a  2" eyepiece fitting and the tube length to suit the OVO field flattener which adds about 55mm (from memory) and doesn't change the focal length. The T-canon adaptor screws straight on to the Ovo flattener.

I also made a low-profile 2" to 1.25" adaptor to allow me to use my eyepieces with either a diagonal or a straight 2" extension tube.

It was a bit of a nightmare working out what overall tube length, focus tube length and focuser travel would allow me to use all my kit without vignetting!

These photos may help:

Visual setup without 2" extension - focuses on terrestrial targets but not stars!

Imaging setup, I do need to add a finder !

Edited October 8, 2019 by Stub Mandrel