Help with Edge 14 SCT On Axis Guiding

Hi Pete, I should read more carefully, I thought it is about OFF axis guiding. So, please ignore my post below ...

Hi Pete, I do not know the Innovations Foresight On Axis Guider, I own an EHD 11 in combination with the Celestron large off axis guider and operate that with an ASI 174 MM. In general terms I understood from my own research a year ago that a large OAG prism and a large sensitive sensor with large pixels is what you should be aiming for. At the focal length of the C11 or C14 you will find that in some regions of the sky, guide stars with become scarce. The Celestron OAG has a quite a generous diameter so the large prism will not be shadowing the sensor (a ASI 2600 in my case). Your ASI 183 sensor is quite small so is the pixel size. I fear that you may find it difficult to find a suitable guide star(s).

Working distance remains at 146.5 mm from the SCT or from the end of the reducer.

Hope this helps

Volker

Pete,
I used ONAG for years on an Edge 14 with great success.  The first challenge is fitting it all within the 5.75” (146.05 mm) back working distance.  I’m not familiar with the Moonlight SCT focuser that you mentioned but if it does more than just turn the focus knob, it almost certainly won’t work.  The number one, best way to focus your scope with ONAG is the Optec SMFS.  That system moves the secondary to focus and it takes up zero space on the back of the telescope.  It also has zero backlash, which makes things really simple.  Computing the spacings is really easy if you simply make a mechanical drawing.  Just remember that the 6mm thick beam splitter will move the focus point for the guide camera back by 2 mm.  Gaston has also provided detailed instructions for configuring the ONAG in the instructions.  I personally find his instructions a bit obtuse so I just make a drawing.  Here's an example that was for my 20" scope (I couldn't find the one I made for the 14"):




My recommendation is to use an imaging camera with the largest possible sensor and to ditch the reducer.  The reducer is most likely to produce soft images and it will likely cause all kinds of trouble.  On it’s own, the Edge 14 will produce pinpoint stars out to a 42 mm diameter field.  I used mine out to 52 mm (with a 16803 sensor) and it worked really well.  You can see the images I took with it in my gallery.

John

I am not sure this combination will work at all. ONAG eats up quite a bit of back focus (67mm if I recollect correctly), and you need 37.5 for camera + filter week, ie you are left with ~40 mm for the focuser+Adapter for the 3.29 SCT thread. AFAIK the Moonlite focuser needs more than that.

Hi Matthias; thanks for the info - I'm looking into the secondary focus option now as suggested by John so that I can stick with ONAG. I'm thinking that I should shoot for maximum field by using ONAG and less interference from a typical OAG within the needed 42 mm area that I need for the ASI6200

Pete

John Hayes:

John Hayes:
The number one, best way to focus your scope with ONAG is the Optec SMFS.  That system moves the secondary to focus and it takes up zero space on the back of the telescope.

Many thanks for the advise John; I'll definitely research this option further as well as the collimation know how that I'll need! Not that I'll ever achieve image quality at your level (stunning to say the least!), but having the right "stuff" lets me know the limits!

BTW, the Moonlight SCT focuser goes onto the visual back of the SCT but, as Matthias noted above, it may not be possible to fit that SCT focuser, an EFW and the ONAG into the 146mm allotted BF.  I do currently have a Celestron EF on the regular focus knob that I would likely remove, especially if I do decide to work at F11 without the reducer (thanks for that advise too; re. the reducer's potential ill effects).

Pete

Pete,
Don't be too quick to dismiss the possibility of achieving the same kinds of results that I do.  The Edge14 with an ONAG and SMFS is a really good start.  If you can get everything set up and properly configured, you get precise auto-focus capability.  At that point, you just need good guiding and good seeing and you'll be getting good raw data.  

I'm familiar with the Moonlight focuser.  I wasn't sure if the "Moonlight SCT focuser" was something different.  The basic Moonlight focuser simply won't work for anything on an Edge14--it's just too big.  Some folks have made the Celestron internal focuser work but that's tricky--and a bit hit or miss.  The Optec secondary mirror focuser system is really the best way to go.  You've got to run a wire through the pupil but that's not hard to manage.  With a solid mount, shooting at F/11 isn't all that hard and it allows a really nice image scale for most galaxies and a lot of nebula.  You just don't gain enough field with the reducer and it is VERY unlikely that you'll get pinpoint star images out of it.  They are also plagued with subtle stray light problems.  You might get lucky but very few of those things work very well.  I even had Celestron hand pick one for me when I was consulting for them and it was terrible.  My advice is to not waste your time with it.

John

Thanks again John; I have both components on order ! I'm looking forward to expanding my set up (and my linguistics per my original message!)

Pete