Focusing problems

[Kenza]

Ok, I have spent the last four hours trying to focus the Atik 4000 Mono and failed. It's attached to an SX filter wheel which is attached to an APO Refractor ( FL is 900mm at f/7.5). I have tried everything but nothing works. Once I managed to make a glimps of stars but that was all. I couldn't get them focused. I have attached an image just so you know what I usually get. I don't know what to do. 

[oldpink]

Ok you have a problem there
first up have you tried it with no filter wheel
if it is a focus issue can you try the moon to see if you can get close

could be both an inward or back focus issue

best I can suggest is test in daytime on a target as far away as you can on the horizon (streetlamp or tree etc) get it centered with an eyepiece
swap to the camera then experiment
try a barlow with out the lens to increase back focus etc

[Kenza]

Yeah, I've tried without the filter wheel and it's the same. I'll test it in daytime.

Could a reducer help?

[ChrisLX200]

You may need an extension tube in order to reach focus. If you don't have a suitable extention tube then temporarily, if you have a star diagonal, insert the camera into that and try to focus again. The diagonal will extend the distance from the focuser to the sensor face. If you manage to achieve focus using the diagonal you will have confirmed you need an extention tube. The setup won't be vary stable using a diagonal so take care, this is just to test focus distance.

ChrisH

[Kenza]

I have tried focusing it in daytime but it's still the same. Well it's not exactly the same. It's not that noisy. There is usually a bigger or smaller white circle on a black background. I have also tried with the diagonal but it's the same. Could it be that the problem is not in the back focus but the inward focus? In that case, a reducer might do the trick ,right?

Also, I don't know if I am doing the imaging right. Maybe that's the issue. I have just started using MaximDL. Are there any specific settings that you have to set before focusing the camera?

[ChrisLX200]

I have tried focusing it in daytime but it's still the same. Well it's not exactly the same. It's not that noisy. There is usually a bigger or smaller white circle on a black background. I have also tried with the diagonal but it's the same. Could it be that the problem is not in the back focus but the inward focus? In that case, a reducer might do the trick ,right?

Also, I don't know if I am doing the imaging right. Maybe that's the issue. I have just started using MaximDL. Are there any specific settings that you have to set before focusing the camera?

Very unlikely for there not to be enough inward focus on a refractor, a reflector (Newt) yes - but refractor no. It is common to need an extention tube though for a straight-through imaging optical train on a refractor, I have to use one with mine. When focusing there is only a couple of mm either side of critical focus where anything will be discerable in the image, outside of that there will be little or nothing visible. With a 2-3sec exposure you should see (if you centre a bright star before you start) a doughnut which will get both brighter and smaller as you move the focuser towards correct focus. You may have to enable 'auto-stretch' in order to see this.

ChrisH

[Kenza]

Ah, so it's much different than with a DSLR, a CCD has a narrower focus range then. It's much easier to focus a DSLR.

I have so far tried with 1 sec exposures. I'll try with 2-3 seconds and see if it makes any difference.

Which auto-stretch mode would you suggest for focusing: Low, Medium or High?

[sharkmelley]

Take the camera and filter wheel off the scope and place a sheet of white paper in it's place.  Move the paper towards or away from the back of the scope until an image of the moon or distant streetlight appears in focus.  That is then your focal plane which is where the plane of the camera sensor needs to be.  It ouight to give a clue as to what extension tube(s) may be necessary.

Mark

[ChrisLX200]

Ah, so it's much different than with a DSLR, a CCD has a narrower focus range then. It's much easier to focus a DSLR.

I have so far tried with 1 sec exposures. I'll try with 2-3 seconds and see if it makes any difference.

Which auto-stretch mode would you suggest for focusing: Low, Medium or High?

Any stretch will do, even low. The signal from an out of focus star is weak and will not appear without a stretch. You can reduce the exposure once you get closer to focus.

For a simple refractor (non Petzval) where you know the focal length, say - 600mm, measure back from the objective lens 600mm and that's where the focal plane will be.

ChrisH

[Kenza]

Done it!

CCD Image 55.fit

[Kenza]

Here is a jpg image.

[ChrisLX200]

Well done that's in focus for sure.

ChrisH