ASI2600MC Pro + SW200dps - Focus problem

[Aramcheck]

I'm afraid I couldn't resist FLO's sale on ZWO cameras & have splashed out on an ASI2600MC Pro, but I'm having problems finding focus. Previously I've used a DSLR with a Baader Coma Corrector...

The ASI2600 manual shows that the sensor to camera front plate distance is 17.5mm, which with the two supplied extenders gives a back focus of 55mm (17.5 + 21 + 16.5mm). Onto that I've fitted the Baader Coma Corrector.

Is this correct? If so, how far should the camera be inserted into the focuser drawtube?

I've tried focusing in the daytime with the scope pointed at the most distant object I can see from the garden, but still can't get it into focus at any distance...

Also - with the DLSR I used to put the camera strap around the OTA, just in case the two fiddly screws didn't hold the weight. Is there anything I can do to hold the ASI2600 more securely?

Many thanks!
Ivor

[Davey-T]

3 minutes ago, Aramcheck said:

I've fitted the Baader Coma Corrector.

Doesn't that alter the back focus ? I think it's designed for DSLR 55mm back focus.

Dave

Edited July 24, 2021 by Davey-T

[Davey-T]

You maths looks OK, curious why it won't focus.

Dave

[Aramcheck]

Thanks @Davey-T - I think the problem was that I didn't fit the "stopping ring"... Hopefully I'll have another go tomorrow!

[alacant]

Hi

The camera needs to be much further away from the telescope tube. The extension of the focuser in your picture shows the approximate focus position for your dslr. The sensor on the 2600 is just 17mm from the camera case. Imagine now moving the 2600's sensor to the same distance as the sensor on your dslr and you'll begin to see why you can't reach focus. You may have enough extension by simply pulling the camera-cc assembly away from the focuser, but as you've already found, the focuser thumbscrew attachment is pitifully inadequate for this job. One of these or one of these may help.

Cheers

Edited July 24, 2021 by alacant

[david_taurus83]

What alacant says above. Only insert it into the drawtube as far as the coma corrector goes. That's the same as if you used your DSLR. Should focus around the same position them.

[Aramcheck]

Thanks folks! I'd had a bad day yesterday trying to get the camera to play nice with Windows 10 etc, so missed fitting the stopping collar to the Baader coma corrector. All is now ok (fingers crossed).

Ta @alacant for the compression ring suggestion - I've just ordered a couple, although with the stopping collar in place, the camera seems to be held a bit better than it was yesterday.

Cheers
Ivor

[alacant]

 

28 minutes ago, Aramcheck said:

All is now ok

Well done.

It's such a pity there isn't a sw200pdsosc 'made for osc camera' tube with a smaller secondary and longer tube, so giving a more sensible focus position. With the camera that far out creating such a turning moment, you're always fighting tilt with the sw issue focuser. If you still have the chance of exchanging for a 200p, that would be a good move. Otherwise, I hope the collars get you there.

cheers 

Edited July 25, 2021 by alacant