Starting focus for SGP session: ideas?

[KathyNS]

I know that SGP's auto-focus routine needs you to be fairly close to focus before it can begin. How do people arrange this on an unattended session?

In the old days, I used to to a manual Bahtinov mask focus. But I don't want to do that, especially when we get into the colder months. I'd rather run 100% unattended.

I thought that leaving the focus where it was at the end of the last session would be close enough, in spite of the temperature change during a session. However, a Crawford can slip, and apparently mine did since the last session. I happened to be monitoring the start of the session, so I noticed that the platesolve for the target failed. When I checked the platesolve image, I realized that it was badly out of focus. I was able to get it close to focus manually, and resume the sequence.

So my question is how do others ensure that the session is starting with the camera close to focus? My options seem to be:-

1. Go out to the observatory after dark and use a Bahtinov mask. I'd rather make just one trip out there, in daylight, to remove the covers, and then do the rest remotely. A Bahtinov mask focus seems to be unnecessary, based on last night's results.

2. Monitor the start of the session remotely. Do a manual focus to get it close. This precludes a delayed start. If the sky is forecast to clear at 3:00 am, I don't want to have to be awake for that.

3. Use the focuser lock when I install the covers after the end of a session. That should prevent slipping between sessions. I just have to remember to unlock it when I get set for the next one.

4. Bite the bullet (another one!) and replace the Crayford focuser with a rack-and-pinion that won't slip.

Any other ideas?

[Star Dad]

Hi Kathy - My Crawford has a screw you can lock it in place with. I take a picture (5 seconds usually) and manually focus the scope/camera. I use Nebulosity and I try to get the focus as tight as possible on 100% zoom. It usually takes me about 5-10 tries to get it as tight as possible. I have to take down my entire scope/mount and set up each session, but if I haven't done any visual with it, it is usually spot on next imaging session. I don't have an auto-focuser, though I've been thinking of getting one... but then I've gotten really good at manually focusing. I just can't get complacent about doing a good focus - I screwed up my last imaging session by forgetting to get a tight focus. I got beautiful donuts on my first set of images before I noticed them. Murphy won that night!

[KathyNS]

Thanks, Star Dad. I am good at manual focusing, too, but I am trying to get away from that. I want to be warm and asleep when running an AP session.

I am leaning towards option 3, using the focuser lock between sessions, as the most practical solution for now. I just have to remember to unlock it before the next session. You can't use the lock with auto-focus.

[Gordon]

If you have a computer in your observatory and a computer in the house you could always use remote desktop and dial in the focus before you go to bed. I run my entire setups this way. Plus it's a good way to keep an eye on things as it's progressing.

[KathyNS]

That's pretty much what I did last night. I guess that will work most of the year, if it's not cloudy when I do that. My last session was a treat, because I scheduled it to start at 3 am after the clouds had cleared. I don't want to have to get up to focus in that case. And It means staying up late in June.

[Gordon]

With mine I have noted a 'focal point' by number (example 2456 steps) based on the filter I'm using. If I go back to that count it gets me really close as long as I haven't messed with the focuser 'manually'. I'm using a stepper motor drive with a coupler to automate the focusing.

[KathyNS]

With the slippage in my focuser, I can't use the step count. I had been working around 18000 steps in previous sessions. I had to reduce it to about 14000 to get it focused last night. That is 4000 steps that the stepper motor didn't know about, so definitely mechanical slippage.

What I can use is the focuser scale. I know that the in-focus position is about 39.7mm on the scale. So I can check that when I go out in the evening to turn on power and remove the covers. If it's cloud-free before bedtime, I can do a manual focus to get it closer.

Until I can replace the focuser.

[Gordon]

I had the same problem and changed to a rack and pinion. No more slippage! With the weight of the camera / filter setup it was just too much weight for the crayford type. I have just a touch of backlash but can compensate for it enough so it pretty much negates the problem.

[KathyNS]

I think the problems with my starting focus could have been from having the Crayford tension too loose. There is only about 1/4 turn between super-tight and too loose. If it's anywhere close to loose, it can slip catastrophically with no warning and no apparent cause. I have been experimenting with the tension lately, so I might have left it on the loose side.

But cranked super-tight, it still works well manually, moving the focuser with no apparent slippage. The motor can drive it when it is tight - sometimes. Where I am finding slippage when it is tight is not in the Crayford mechanism, but between the motor's drive gear and the focuser shaft. The gear is a friction fit onto the left-hand knob of the focuser, and that friction fit is what is slipping. The gear turns but the knob doesn't. I may have to slip some glue into the joint to prevent slippage. With glue to prevent the gear slipping and the Crawford tightened to prevent focuser slippage, it might work.

[yobbo89]

I know that SGP's auto-focus routine needs you to be fairly close to focus before it can begin. How do people arrange this on an unattended session?

In the old days, I used to to a manual Bahtinov mask focus. But I don't want to do that, especially when we get into the colder months. I'd rather run 100% unattended.

I thought that leaving the focus where it was at the end of the last session would be close enough, in spite of the temperature change during a session. However, a Crawford can slip, and apparently mine did since the last session. I happened to be monitoring the start of the session, so I noticed that the platesolve for the target failed. When I checked the platesolve image, I realized that it was badly out of focus. I was able to get it close to focus manually, and resume the sequence.

So my question is how do others ensure that the session is starting with the camera close to focus? My options seem to be:-

1. Go out to the observatory after dark and use a Bahtinov mask. I'd rather make just one trip out there, in daylight, to remove the covers, and then do the rest remotely. A Bahtinov mask focus seems to be unnecessary, based on last night's results.

2. Monitor the start of the session remotely. Do a manual focus to get it close. This precludes a delayed start. If the sky is forecast to clear at 3:00 am, I don't want to have to be awake for that.

3. Use the focuser lock when I install the covers after the end of a session. That should prevent slipping between sessions. I just have to remember to unlock it when I get set for the next one.

4. Bite the bullet (another one!) and replace the Crayford focuser with a rack-and-pinion that won't slip.

Any other ideas?

number 4! highly recommend a quality focuser for automation

The different temperatures on different nights outside on the optics is enough to shift the focus position way out to a point where sgp takes so long to focus or just fails, depending on what scope and how fast it is of course . ( I auto re focus every 0.5 - 0.75 deg drop and it can drop for 16 deg at night to 6 deg here)

i find that i have to at least manually focus 1 filter before each season then let it run auto for the rest of the night, sgp has a setting that will shift all the filters values over together , say move every filter over 100 position on the focuser together .

anything that's not a donut defocused star sgp will auto focus,it's impractical to use a bahtinov ,frame and focus should give a star profile report which is good enough to start a auto focus routine .

[yobbo89]

The gear is a friction fit onto the left-hand knob of the focuser, and that friction fit is what is slipping. The gear turns but the knob doesn't. I may have to slip some glue into the joint to prevent slippage. With glue to prevent the gear slipping and the Crawford tightened to prevent focuser slippage, it might work.

This is what i did for my sct for planetary work on my rigel n step and works a charm!