Focusing hassles

thegableguy (Chris) · #1 04-10-2016, 03:09 PM

Okay focusing is driving me nuts.

I'm shooting with a crop sensor DSLR into an 8" f/5 Newt. It's a nice new GSO with a nice new Crayford focuser, which does have a locking screw on it. For some reason I focus it perfectly before alignment then an hour later it's significantly out of focus. I've had two wasted nights of imaging because of this problem.

I know that ytou're supposed to let them cool a while before using them and I kinda neglect to do that; also drift aligning / 3-star alignment makes the thing slew around a fair bit... but once I start guiding I can't go messing around with the focus as touching the camera/OTA makes the guiding fail.

My question is what order should I be doing the following:
- focus DSLR
- drift align
- 3-star align
- select & slew to target
- put guide camera in guidescope
- begin guiding

That's the order I've been doing things but there clearly needs to be another DSLR focus in there somewhere. Suggestions??

Atmos (Colin) · #2 04-10-2016, 03:18 PM

I refocus every hour most nights, have been doing that manually for a long time but once I get my PDF up and running it'll automate that too and have it focus every hour

Nath2099 · #3 04-10-2016, 03:30 PM

Pdf?

thegableguy (Chris) · #4 04-10-2016, 03:34 PM

Ah okay, thanks for that.

I'm happy to do that if required but the tricky thing is that I can't touch the OTA because it makes the guiding stop. What's the process with stopping & starting PHD2? Do you leave it struggling on, or do you pause it and start it again when you've finished adjusting focus? Does it matter if it chooses a different guide star?

Atmos (Colin) · #5 04-10-2016, 03:57 PM

Quote:

Originally Posted by Nath2099

Pdf?

Precision Digital Focuser made by Fingerlake Instruments.

Quote:

Originally Posted by thegableguy

Ah okay, thanks for that.

I'm happy to do that if required but the tricky thing is that I can't touch the OTA because it makes the guiding stop. What's the process with stopping & starting PHD2? Do you leave it struggling on, or do you pause it and start it again when you've finished adjusting focus? Does it matter if it chooses a different guide star?

I would stop the guiding, slew to a nearby bright star, put on the focusing mask, focus, slew back to what I am imaging, start guiding again and leave it for another hour or two.

Camelopardalis (Dunk) · #6 04-10-2016, 04:14 PM

Dare I venture to add that if the temperature is dropping, and your scope tube is metallic, it'll be the scope tube contracting that is throwing the focus out.

With my imaging 'frac (short focal length) it's quite noticeable on a winters' night in QLD, while my SCT doesn't budge. There's a formula for it apparently.

doppler (Rick) · #7 04-10-2016, 05:05 PM

If you have spent a bit of time to frame your target nicely, before you slew to a bright star to refocus; if you hold down the escape button for a couple of seconds it will start flashing.. re-centering object, then click enter and when you slew back the object should be nicely framed to start shooting again. To pause guiding just click stop, then to start guiding again click on looping, pick a guide star and click on the guiding button. PHD wont recalibrate unless you restart the program.

thegableguy (Chris) · #8 04-10-2016, 09:35 PM

Thanks everyone for the tips.

I was given the wonderful suggestion of simply doing a one-star alignment on the star nearest my intended target to avoid slewing all over the place. Never would've thought of that but it makes huge sense. So I've done that tonight.

I put the OTA out for a few hours before starting, and I used the fan whilst getting aligned and focused, so temperature shouldn't be an issue.

I've also gotten rid of the coma corrector, which I think is actually the main problem. Despite what the various forums say, I'm pretty sure the spacing I'm using isn't right. Even dead centre I just can't get focus as crisp as I can without it.

And finally I'm using a vastly better, full-frame camera for a change. The live view is far superior and made the process a whole lot easier. It's happily churning out subs now.

Now the only real problem is the friggin' cyclonic winds that are doing their best to ruin the seeing..... if it's not one thing it's another!!!

thegableguy (Chris) · #9 04-10-2016, 09:38 PM

Quote:

Originally Posted by doppler

If you have spent a bit of time to frame your target nicely, before you slew to a bright star to refocus; if you hold down the escape button for a couple of seconds it will start flashing.. re-centering object, then click enter and when you slew back the object should be nicely framed to start shooting again. To pause guiding just click stop, then to start guiding again click on looping, pick a guide star and click on the guiding button. PHD wont recalibrate unless you restart the program.

Is that when using ASCOM, connecting the mount to the laptop? I'm just using the ST4 cable from the guide camera to the mount.

Atmos (Colin) · #10 04-10-2016, 09:38 PM

Moving over to the Baader MPCC? Being unable to get that real "snap" into focus does sound like a spacing issue.

The seeing down in Melbourne hasn't been good since summer this year, even then it wasn't fantastic.

thegableguy (Chris) · #11 04-10-2016, 09:53 PM

Quote:

Originally Posted by Atmos

Moving over to the Baader MPCC? Being unable to get that real "snap" into focus does sound like a spacing issue.

The seeing down in Melbourne hasn't been good since summer this year, even then it wasn't fantastic.

Yep for the Baader. I've found someone who has a very generous asking price for his unused one, so very much yep! Should've gone that way in the first place. Serves me right for trying to do things cheaply. It's a shame because the GSO is supposed to be this optically amazing thing from a top optics designer. Unbelievable stupid how it comes with no instructions and no spacers - literally nothing at all. Just a hunk of glass and metal in an unmarked cardboard box. Ridiculous.

I've always been curious - how do you know how good the seeing is in your area on any given night? Is there some metric used?

raymo · #12 04-10-2016, 10:54 PM

I don't mean to be irritating or anything, but why did you dump the
SW 8" f/5? The SW coma corrector is made for it, and needs no spacers,
works properly out of the box, and is reasonably priced. GSO stuff is nothing special, neither better nor worse than any other mass produced equipment.
raymo

doppler (Rick) · #13 04-10-2016, 11:33 PM

Quote:

Originally Posted by doppler

If you have spent a bit of time to frame your target nicely, before you slew to a bright star to refocus; if you hold down the escape button for a couple of seconds it will start flashing.. re-centering object, then click enter and when you slew back the object should be nicely framed to start shooting again.

Quote:

Originally Posted by thegableguy

Is that when using ASCOM, connecting the mount to the laptop? I'm just using the ST4 cable from the guide camera to the mount.

I meant the SynScan hand set buttons, I find it useful to get back to my target when the goto is not behaving

thegableguy (Chris) · #14 04-10-2016, 11:41 PM

Quote:

Originally Posted by doppler

I meant the SynScan hand set buttons, I find it useful to get back to my target when the goto is not behaving

Oooh. Didn't know it could do that. Good stuff, that will be very useful - thanks. Reminder that I should probably track down a manual for it and read it sometime.

thegableguy (Chris) · #15 04-10-2016, 11:53 PM

Quote:

Originally Posted by raymo

I don't mean to be irritating or anything, but why did you dump the
SW 8" f/5? The SW coma corrector is made for it, and needs no spacers,
works properly out of the box, and is reasonably priced. GSO stuff is nothing special, neither better nor worse than any other mass produced equipment.
raymo

I didn't dump it, I've still got it... but I bought the GSO for a bunch of reasons. I think the guy who owned it between you and I wasn't particularly nice to it.

The focuser itself was knackered - one of the bearing rollers snapped one night when trying to tighten it enough to not slip with a full-frame DSLR on it. I'm still deciding whether to get a nice new 10:1 Crayford for it or just repair this one.

The whole tube itself was more than a little dented, particularly around the focuser which wasn't anywhere near straight - I adjusted it as far as it would go but the secondary simply didn't line up with the focus tube, even when the focuser was tilted. My father and I have just panel-beaten and resprayed it.

The spider vanes were twisted and a little bent in places. They're now straightened.

The collimation screws were horrible. They've been replaced with Bob's Knobs.

All these things are fixable - indeed, everything except the focuser is now fixed - but I had a pretty dim view of it when I got it. The GSO happened to be on special that week at $422.

Also, the only coma correctors I could find were either the Baader or the GSO, and the GSO is $100 cheaper than the Skywatcher.

Also I had no idea it would be this much of a pain to get it working properly.

Does that explain it?

raymo · #16 05-10-2016, 01:27 AM

Fair enough Chris; the SW coma corrector specifically for the f/5 is $149.00.
I've often wondered why scope makers[even budget level ones] don't
charge a few dollars more for their products and fit decent primary and
secondary collimating knobs, although I suppose it is lucky for Bobs Knobs
that they don't.
raymo