Off Axis Guiding

[gnomus]

I had a bit more success last night.  Here is a screenshot of an almost perfectly square star:

I 'super-stretched' one of my subs in PI - is this level of prism shadow acceptable - I can also see it a bit in the normal PI STF.

 

Numerous other issues of course.  The Moonlite focuser won't hold my camera without slipping (it was pointed almost straight up).  Maybe this is why the focus is off.  I also noticed that the focuser unit was unscrewing itself from the SCT body (which is of course entirely my fault - I should have checked).  Finally, my collimation is off.  I'm beginning to think that this imaging through SCT malarkey is a little bit too much like actual work.  Perhaps I need a long F/L refractor with a R&P focuser.    

[Davey-T]

Did you try raising the prism gradually ?

Dave

[ollypenrice]

That should 'flat out' like a charm. You could drop the cursor onto the image inside and outside the shadow to see the difference in ADU. If the difference is les than about 4000 ADU it should be fine.

Moving the prism towards the scope means a lower photon density in my thinking. The FOV won't change, I don't think, but the entire image will be brighter the closer you come to the camera end with the prism.

You shouldn't mess with medication, Steve. Stick with the stuff you brought here last time. 

Olly

[DaveS]

8 hours ago, gnomus said:

I'm beginning to think that this imaging through SCT malarkey is a little bit too much like actual work.  Perhaps I need a long F/L refractor with a R&P focuser.    

Something like this perhaps

http://www.teleskop-express.de/shop/product_info.php/info/p6537_CFF-Telescopes-185mm-f-6-8-Triplet-APO---3-5--Starlight-Focuser.html

[gnomus]

17 minutes ago, DaveS said:

Something like this perhaps

http://www.teleskop-express.de/shop/product_info.php/info/p6537_CFF-Telescopes-185mm-f-6-8-Triplet-APO---3-5--Starlight-Focuser.html

Exactly like that.  I'd want a pair for a dual rig, of course.

[gnomus]

4 hours ago, ollypenrice said:

That should 'flat out' like a charm. You could drop the cursor onto the image inside and outside the shadow to see the difference in ADU. If the difference is les than about 4000 ADU it should be fine.

Moving the prism towards the scope means a lower photon density in my thinking. The FOV won't change, I don't think, but the entire image will be brighter the closer you come to the camera end with the prism.

You shouldn't mess with medication, Steve. Stick with the stuff you brought here last time. 

Olly

I'm not entirely sure that I know how to measure ADU in an image.  I moved the cursor around the image in Astro Art.  It reports a value for X and Y along with a third value.  Is this the ADU?  If so, then the brightest point I could find in my image was around the 35,000 mark.  The value for the prism shadow was ~1500 and the background seemed to be around 1830 or so.  Does that seem OK?

[ollypenrice]

1 hour ago, gnomus said:

I'm not entirely sure that I know how to measure ADU in an image.  I moved the cursor around the image in Astro Art.  It reports a value for X and Y along with a third value.  Is this the ADU?  If so, then the brightest point I could find in my image was around the 35,000 mark.  The value for the prism shadow was ~1500 and the background seemed to be around 1830 or so.  Does that seem OK?

It's the third value which shows the ADU. The difference between 1500 and 1830 is zilch. Don't worry about it if you are using flats.

Olly