Off-axis guider woes

[lukebl]

Folks,

Can anyone help me regarding the spacings required for my off-axis guider. Everyone talks about getting spacers to get the required back focus for the guide cam, but my problem is getting sufficient in-focus, as this drawing shows.

The critical spacing for the coma corrector is 55mm, and I can't get the OAG prism any closer than 10mm from the CC due to the depth of the thread of the Corrector (a Baader one). The light tube of the the OAG is 40mm long, so that only allows 5mm left for the guide cam sensor. However, depending on which guide cam I want, the sensor is set in by around 17.5mm so the guide cam housing stops the sensor getting any closer. There simply isn't enough space for in-focus, and I risk damaging  the sensor by pushing the light tube on it. I don't this this is resolvable with my system!

The OAG is a TS Off-Axis Guider TSOAG9.

 

[michael8554]

Hi Luke

 

The dimension on the "light tube" you have marked as 40mm is to the top of the light tube, but the circular flange the guidecam attaches to can move up and down the "light tube", and is held by a lock screw?

The excess "light tube" can be sawn off it it fouls the guidecam sensor when the flange is at its lowest point on the tube.

Or did you mean 40mm to the guidecam flange?

Does your Coma Corrector focal reduce too?

If not, can you place it between the OAG and the imaging camera?

Michael

 

[Tim]

Hi Luke.

I've been exactly here with this same issue

I think your solution lies in getting a thinner adapter between the OAG and the coma corrector (Baader MPCC have a 48mm thread which goes right into the TS OAG, or get a reducing ring from 48mm to T-thread), and adding the extra space then required behind the OAG.

Failing that, carefully reducing the length of the stalk.

Tim

Like this one perhaps? I'm not sure what you have between the CC and the OAG?

http://www.ebay.co.uk/itm/T-thread-to-2-adapter-42mm-48mm-adapter-ring-T2-M48/112076856953?_trksid=p2141725.c100338.m3726&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D20150313114020%26meid%3Da2f12af966e9453dbc159dd8d640dc1d%26pid%3D100338%26rk%3D3%26rkt%3D17%26sd%3D221785150923 

[peter shah]

Yes The Baader MPCC is very lean with the amount of spacing required to get it to correct well. The reason is that designing the corrector with a 55mm or less distancing means you get a better correction and spot size, unfortunately it doesn't leave very much room for additional kit like OAG's or filter wheels etc. You might be able to bring the focus out with a cheapo x2 convertor on the guide camera. This will increase the focal length on the guide camera and limit the amount of available guide stars. Or depending on your guide camera type you may be able to  get a machine shop to mill down the end of the guide camera  to bring it in closer....there is always a element of shoehorning things to fit in this game.

I've used an OAG for years It does take some getting used to but IMO the best way to guide But my corrector allows 79mm spacing loads of room for almost any camera and additional kit.

[Zakalwe]

I had this very same problem. I ended up hacksawing a few mm off the OAG light tube and removing the glass window off the guide camera. It might be worth trying to get the camera closer to the FW too.

 

OAGs are a pain in the posterior when you have to squeeze a FW in there.

[Gina]

I sawed a few mm off the OAG light tube too.

[lukebl]

Thanks for the input, all. The spacers are all as thin as they can possibly be, so I can't get anything closer. More tinkering needed!

[Zakalwe]

6 hours ago, lukebl said:

Thanks for the input, all. The spacers are all as thin as they can possibly be, so I can't get anything closer. More tinkering needed!

You can get a T2 threaded barrel from Teleskop-Express (at a suitably expensive price!) which means you can get the camera flush with the FW.

[lukebl]

Thanks folks. I suddenly noticed that the prism was out of alignment, which was the main reason I couldn't find any stars! A bit of adjustment, and I can finally find guide stars. The spacing is very tight, but focus is there at last. 

The remaining problem is the perennial one of egg shaped stars. It's not the guiding, as it occurs in short exposures so it's something in the optical train. The distortion stays in the same direction relative to the stars if I rotate the camera. Can't remember what that points to!