Focusing Question

[kookoo_gr]

The following question about focusing is bugging me from when i started imaging with a newton and followed me at my rc scope with a dslr and a ccd camera but i can't find any answers, olny some spequlations i made.

When starting imaging i focus with the help of a bahtinov mask. After i finish the setup procedure i go to my imaging target and check focus again with a nearby star right before imaging. While imaging after a period of about 15 minutes i can see that the focusing is getting a bit off but enough to see it at my stars. This problem will bug me at some nights but not at other nights, also i may have it at an imaging run but when i notice the focus is off and focus again then there is a chance of 90% that i won't have this problem for the duration of the night. The theories i have made are the following

1. It could be focuser sloop but when i installed a moonlite focuser with a dc motor the problem is still there, not as frequent but it is still there.

2. Is it possible that the temperature drop during the night will cause the focus to get off? Or could the atmosphere affect the focus during the night? Note that the time from setup to imaging takes about an hour so the scope has reached ambient temperature and the focus problem will appear randomly. I image at nearby field from my house (http://www.astrobin.com/15154/) and i pick my targets away from any rooftops or the horizon.

How can i deal with this issue because checking focus after every frame is kind of tirring and my frames usually are from 5 mins and above.

[swag72]

Temperature changes can absolutely have an effect on focus. I saw some presentation on the net a while ago about focusing and image capture and I was amazed at the difference in focus with every 1 degree change in temperature.

[kookoo_gr]

i thought so, is there a chance you have the link to the presentation?

[ncjunk]

If you are getting changes of focus that are significant over 10/15 minutes I would look at the mechanical side first.

I can usually set up the ed80 and it will image the whole night BUT you can notice the focus drift as it cools down.

This drift doesnt usually occur in 15 minutes, it gets steadily worse over a couple of hours.

What I do when my setup is working, its a bit broken, is re-focus automatically if the stars are out by X amount or refocus at the start of each new target (but my system is automated)

So check for anything slack in the mechanical side.

[kookoo_gr]

on a number of occasions i have seen the focusing gradually going worse the way Neil mention it, so there shouldn't be any mechanical issue, as i said this problem is bugging me from when i started imaging with a newton scope and followed me at my new setup (i can't be so unlucky to have a crayford focuser and a moonlite focuser with mechanical issues). Neil how have you solved your problem?

[ncjunk]

on a number of occasions i have seen the focusing gradually going worse the way Neil mention it, so there shouldn't be any mechanical issue, as i said this problem is bugging me from when i started imaging with a newton scope and followed me at my new setup (i can't be so unlucky to have a crayford focuser and a moonlite focuser with mechanical issues). Neil how have you solved your problem?

I built a homemade focuser using steppers and paid quite a bit of money for some control software. so as soon as it sees an error in the focus or at preprogrammed intervals it refocuses.

It's probably not a very good answer for you.

With the stepper powered I am assured of no slip in the focuser and it holds it in place. This could be another problem in your setup. If the focuser isnt sufficiently stiff (adjusted usually by tightening some screws on the back of the crayford) it may be slipping during the night with the weight of the dslr.

I would guess you may have slight mechanical slippage (only a may as you have stated the same problem on two scopes).

So-

1. Check every thing is as tight as possible on the focuser.

2. If you have no focuser motor control then are you locking the focuser down with the screw after achieving focus? - when you lock it down slight movement can occur so it can be difficult to lock down and keep it focused (trial and error)

3. you will probably need to refocus a couple of times depends on the night. this time of year is bad as the temp change for me can be 10-20 deg c, in winter i can get away with not refocusing during the night. If you have a manual set up it's going to have to be you i'm afraid. if you want to automate it then stepper, stepper controller, ascom driver and sonething like ccd commander or acp to control when refocus is done but that will start costing. (possibly add maxim to that list)

[swag72]

i thought so, is there a chance you have the link to the presentation?

Here you go http://www.ewellobservatory.com/bestpractices/player.htm Very interesting to watch.

[ncjunk]

I got a page not found error on that link. any keywords i could put in google to find it?

[kookoo_gr]

go to http://www.ewellobservatory.com/ then click ccd imaging and watch Image Acquisition Best Practices - Richard A. Bennion

[ollypenrice]

To see if it's slip just try a test image on a low star when the tube will be realatively horizontal. Even a bad focuser should hold there.

Do you get the scope to ambient before imaging? Or do you take it out from a warm living room? Clearly that would not be a good idea.

Olly

[kookoo_gr]

at my house the temperature is a bit lower from outside and while setting up takes me at about an hour i believe it is enough for the scope to reach ambient temperature (caps from the scope are off while setting up). as for the star i will test it at the weekend, also i am waiting for a usb thermometer to record the temperature drop while imaging. after watching the presentation the spokesman always checks the focusing every half an hour and makes adjustments, he doesn't mention how far off his focusing goes off. The focuser i have is a moonlite with a rigel dc motor and it keeps the weight of my ccd/fw/oag/guide camera firmly at place (no vissible slop with naked eye).

[kookoo_gr]

it appears that rigel provides a temp sensor for the nfocus motor (which i own) along with a usb connector in order to copenstate for temperature focus problems. i think after doing some test this weekend i'm going to purchase such a set