OAG with a Tak FS60CB - is it possible?

[UBoat]

I'm putting together an ultraportable imaging kit and considering the purchase of a Tak FS60CB. I thought it might be possible to use a 2 inch ocular adaptor to fit an Orion thin OAG to an FS60CB? Can anyone tell me if this would work or if there's another way to use an OAG with a Tak FS60CB?

Thanks Peter

[DJT]

I use an FS60 with a flattener, an Orion "thin" OAG and a canon d60a. Works fine. Spacing to get the flattener right can be an issue but it's better than without the flattener.

Havnt used the 2" ocular though.

[UBoat]

Thanks for your message and I found that I don't need the ocular adapter if I use the flattener as it comes with a T connection. With a back focus of 56mm for the TS-FS60CB + flattener, 13.5mm for the camera and the TOAG thickness of 13mm I'm assuming the imaging camera spacer will need to be about 30mm and the guide camera spacer about 10mm to give the equal distance for guide and imaging cameras. Does that sound about right?

Peter (also based in Sydney)

[DJT]

Hi Peter

Unfortunately I can't remember the exact details as its been a while since I imaged with the DSLR and FS60. I continued to have issues with the correct spacing take up backfocus so set it aside though having learnt more about what's going on I may go and look again.

This is a link to the MMOAG guide to measuring the various distances. It's hardware agnostic and certainly a bit clearer than the "manual" you get with the Orion slim OAG.
http://astrodon.com/mmoag-optical-configuration.html


Where did you get the back focus figures for the Tak from by the way?

[UBoat]

Thanks for that excellent link David and regarding the issue of getting proper spacing would it be worth trying something like the Orion variable T spacer that can be set from 12mm to 17mm?

Re the back focus value I found it at http://www.takahashi-europe.com/en/FS-60CB.specifications.php . Normally I would take back focus to mean the maximum extent that can be taken up by gear before running out of inward travel on the focuser. However, I assumed that when a company selling a flattener quotes a back focus value but not a spacer value they intend the back focus value to represent the spacer value.

This could be wrong. Takahashi also provide a diagram with recommendations on items to be used in various image trains and they may be relying on this instead of a specification. I couldn’t find dimensions for any of the items.

Be glad of your opinion on my interpretation and on the use of a variable spacer.

All the best, Peter

[DJT]

Hi Peter

You shouldn't need a spacer. The quoted back focus of 56mm with the flattener is the distance between the flattener and the focal plane of the camera, ie, the point at which an object is brought to focus.

I believe that's 44mm for a Canon d60a DSLR. (See link https://en.m.wikipedia.org/wiki/Flange_focal_distance) then add on the thickness of the OAG which I think is 10.5 mm per this link

http://www.bintel.com.au/Astrophotography/Autoguiding/Orion-Thin-Off-Axis-Guider/1688/productview.aspx.

and you are at 55.5mm. In theory close enough not to need a spacer but annoyingly not enough to give me pin point stars at the corners. Close, but...

[UBoat]

Is it possible there are two versions of the Orion TOAG? Mine (bought from Bintel) has a raised annulus rather than the annular depression shown in the Bintel spec. Digital calipers give values of 10.5mm across the flats but 11.1mm when the raised annulus is included. so close! Does the T adaptor also add thickness?

My CCD cameras seem all to have backfocus between 12.5mm and 13.5mm so hopefully won't have the issue that interferes with the DSLR camera spacing. Touch wood!