Electric Autofocus & Focus Travel C11 Edge HD

[AstroAndy]

Hi folks

On a C11 Edge HD, how does one know when one has reached the end of focus travel (both directions) with an EAF. Will it just stop, will there be a warning? Can I unwittingly damage the primary mirror if I go too much in one (either) direction?

What is the focus travel range (both directions) of a C11 EHD?

What mirror travel will one (360 degree) turn of the focuser give me?

Once you reach focal plane, it shouldn't matter, for focal purposes, which camera is used. Disregarding pixel size, FOV, if camera A reaches focal plane, then so should camera b at the same distance. So why does my EAF position vary from about 1600 to 30,000 with 2 different cameras (using the .7 reducer).

I reach focus consistently with the reducer, however, am having a very hard time finding it without. Why is that? I know I have to bring the focus in (ie. towards the telescope by going clockwise on the focuser by a lot as it gets pushed out without the reducer.

Is there a sort of focus emulator (lasers?) that lets me do this ahead of my observing session?

Living not only in a geographically unfortunate zone, but also an unfortunate location, I have to lug about 150 pounds of equipment, up and down 4 flights of stairs to dark sites,hours away, and would like to make that process as easy on me as possible. Focusing in daytime is impractical (my main cams won't go shorter than 0.2 secs., and even if I reach some sort of focus during daytime, it doesn't work at night).

Not reaching focus during the few times I go imaging far away from home is very frustrating.

- Andy -

 

 

Edited October 18, 2021 by AstroAndy

[michael8554]

Not sure I have understood your descriptions of your focus problems.

All your adjustments should be to place each camera sensor 146.05mm back from the rear of the OTA, that's Celestrons' recommended back focus for the C11 Edge HD. 

Which cameras are you trying to make "parfocal" ?

How are you attaching these cameras with the FR,  without the FR  ?

Some images of the setups would help.

Camera A and Camera B might have different flange distances, so just swapping cameras without adjusting spacing won't work. 

If daytime tests won't work, try getting a feel of the focusing on a distant streetlight, or the moon, at night.

Michael

 

 

[AstroAndy]

Hi Michael

Thanks for your reply.

Indeed, the 146.05 back focus distance has been measured out, that is not the problem. Using the reducer also isn't the problem.

My problem is getting to focus without it. In itself, that is not a great problem, however, at night, with the reducer, cam., and cables connected, this becomes a little fiddly.

Where it gets a bit problematic is finding focus with a cam. that has a long download time (Atik 16200). I know all the theory behind it, down to the mathematics of how to calculate focus travel.

The cams in question are mainly: Atiks 383L+ (backfocus 17.5 mm, w/ 3 mm inset), and Atik 16200 (19.5mm w/ 3mm inset), and secondarily ZWO ASI 120 s, ZWO ASI 204 (bf also 17.5mm).

I'm making allowances for camera swapping (eg. 1/2 turn of the focuser corresponds to a mirror movement of .0375mm, which in turn corresponds to a focus travel of 9.375mm. The formula used for this is: secondary mirror f/l ^2 x primary mirror movement = focus travel length, in my case 5^2 x 0.375 = 9.375. So, moving my focuser 1 turn, I'd move my focal plane 18.75mm. So I can cover a large amount focus travel with minimal adjustments.

Of course, once I have the focuser positions for the individual components, then I'm good.

I'm now resigned to lose part of a night, and focus on Polaris.

- Andy -

 

[michael8554]

Hi Andy

I refocus every night with my permanent pier-mounted system, I don't see any way of avoiding that, it takes very little time.

Once you have worked out what spacers are required for each camera, to make them close to parfocal at the same focuser position, all you do is swap and tweak focus.

I appreciate your desire to "automate" your swaps, I'm afraid how to do that is more than my pay grade 😆

Michael

Edited October 20, 2021 by michael8554

[AstroAndy]

Hi Michael

Lol, np, the sad thing is, this should be a relatively simple operation, my problem is probably mostly down to impatience, funny thing is, with the reducer I got the swap down to a t, without, I've ever only found focus once (with an Atik 120 s, no less). That is, of course, the only time I did not note down the focuser position, otherwise, cake.

I appreciate the difference in spacing, just one mm off, and you're dealing with a focus travel of almost 190mm.

- Andy -

[michael8554]

Hi Andy

Is there room in your image train for a motorised zero-shift focuser ?

Moving the mirror 190mm changes the focal length.

Michael

[AstroAndy]

Hi Michael

I go to town with a Pegasus Focus Cube, which is attached to where the focus knob would be on an SCT, as that is the way to go with SCTs. So basically, w/ the reducer, as the fl is shortened, I'm at one position, w/out the reducer, the FL changes, and I have to bring it back (inwards) to it's optimal 146.05 backfocus.

- Andy -

[michael8554]

19 hours ago, AstroAndy said:

I go to town with a Pegasus Focus Cube, which is attached to where the focus knob would be on an SCT, as that is the way to go with SCTs.

Many would recommend to roughly hand-focus the mirror, lock it down to prevent "flop", and use a motorised Crawford-style focuser to fine focus.

Michael.

[AstroAndy]

Hmm, Michael, you just gave me an idea..while I'm loath to disconnect the autofocuser (it's fiddly, and I have 3 left hands, plus I know my focus positions for with reducer use), I could disconnect the motor only, and work the fouser on the SCT manually.

- Andy -