ZWO OAG

[thomehm]

I have a ZWO ASI 1600MM, a ZWO ASI120MM (which I want to use for guiding) and a ZWO OAG.  I'm having a problem getting the 1600MM and the 120MM to focus together.  Actually, I've never been able to get my 120MM to focus at all.  I've used the ZWO provided app ASICAP to confirm that the 120 can detect light, so it seems to be working.  But when I try to focus the 120, by itself, through the OAG, the best I can get, is what seems to be a faint even field of stars, all same apparent magnitude, and doesn't change when I point the telescope somewhere else.  It would seem that what I have is some kind of reflection or other optical effect, rather that seeing actual stars.  I've checked the obvious things, yes my prism is pointing the correct direction, I've moved the prism assembly in and out to various settings.  But I'm still not getting an image.

Any ideas as to what could be going wrong?

[michael8554]

Try the 120 in the imager port and adjust focus exposure etc until you get stars.

Then measure the distance of the chip from a convenient surface, say the front of the oag.

Then put 120 in guide port and adjust the distance from that point via the mirror  to be the same.

If you're not then getting stars then look for mirror being vigneted.

Michael 

 

[thomehm]

I don't see how I could measure the distance from the guidescope chip to a convenient point, or from chip in guidescope port to the same distance with anything close to the accuracy that I would need.  The procedure that you are describing does not sound straightforward.  If there is a way to do it I would be happy to hear how it is done.

thanks,

Dana

[michael8554]

Hi Dana

My suggestion was just to get the 120 working, you will need to juggle things to get both in focus.

The distance from the front of the oag to the top of the guider periscope, via the prism, appears to be 40.5mm.

The T2 adapter that comes with the oag adds 5mm

The imaging chip on a 120 is 12.5mm from the front surface of the camera.

So the 120 chip is 40.5 + 5 + 12.5 = 58mm from the front of the oag.

(Note I dont know if the periscope length to the prism can be changed, as well as moving the prism in and out of the light path, the above assumes not)

So to focus the 1600 too:

 The 1600 chip is 6.5 mm from its front surface

So the 1600 front face needs to be 58 - 6.5 = 51.5mm from the front of the oag.

The oag is 16.5 mm thick

So you need a 51.5 - 16.5 = 35mm spacer, minus the thickness of the M48 and M42 1600 to oag adapters.

I don't have this oag so how did I get these figures?

Simply by looking at the specs on the ZWO site, suggest you do the same !

Michael 

[Laurin Dave]

Hi   I have an ASI1600mm with filter wheel and OAG with asi120 .  For me the following arrangement achieves focus in the asi120..  EFW (21mm thick) screwed direct to the camera (ie. Remove the 10mm black spacer  on the ASI1600) then male to male connector(1.5mm thick)  10 mm spacer, then OAG..  I have the camera holder hard up against the OAG body and have a 7mm spacer and about 1mm of delrin rings to get focus ..  as in the photo below.  I also have a thin piece of paper wedged between the camera holder and the OAG stalk to stop it wobbling..  Get the imaging camera in focus then focus the guide camera and adjust its spacing to suit.. Use the Milky way and high gain on the ASi120 to get lots of stars in the ASI120

.. also the 5mm T2 adapter comes screwed on the top of the OAG camera holder...  it took me and many others, so I am told, a while to find it.  To get focus with it on (assuming you have an EFW) you'll need to move the camera holder away from the OAG body by 2-3mm.

hope this helps

Dave

 

 

 

[RolandKol]

Hi guys, 

what about coma? I also use ZWO OAG and ASI1600MM, on 130PDS so I use SW Coma Corrector.

it looks like my OAG is able to achieve a proper focus only in around 33% of the image area... The rest area is covered with stars suffering from Intense Coma...

any ideas?