Jump to content

SkySurveyBanner.jpg.21855908fce40597655603b6c9af720d.jpg

Vixen AX103S Spacing


Recommended Posts

I picked up my new AX103S on the bank holiday, and have been absolutely loving it.  It's one of these:

https://www.firstlightoptics.com/vixen/vixen-ax103s-flat-field-apo-refractor.html

although I got mine second hand, along with the optically matched 37228 focal reducer which turns it into a 578mm f/5.6 scope, and some assorted bits and pieces (carry handle, baader clicklock visual back, etc).  It was over my budget for a triplet, but I asked and negotiated and got it for what I consider to be a very good price indeed (it helped that we were spending the long weekend near where it was being sold, so I could pick it up in person and pay cash!).

When trying to get it all set up and working, I found that there was very little information on the internet regarding the telescope and reducer, in particular regarding the correct backspace to use.  I started doing my own investigations, and I thought I would share what I came up with for posterity, so that if anyone else gets one of these lovely telescopes the internet should show this result in a search for it 🙂

The Vixen AX103S is a quad -- a triplet objective lens and a flattener built into the draw-tube of the focuser.  Screwed on to the focuser is a dedicated 0.7x reducer, which puts the whole thing at f/5.6.  The back thread of the reducer is M60.  From that point, I need to get some distance in mm to my camera sensor.  The reducer unfortunately doesn't provide official numbers to the sensor, only a diagram like this:

image.thumb.png.fbab12939f19ae7441d00600d0753c01.png

The wide photo adapter 60mm is 8mm in length:

image.thumb.png.a2ea1abd24681e7c666ce92955226b58.png

DSLR + T-Ring is typically 55mm (44mm + 11mm), so the expected backfocus is 55+8=63mm.  I ran some tests yesterday and had reasonable results with the following:

image.thumb.png.9d441966e718dc9a8989525aee1fefc6.png

37mm to the filter wheel, my camera is a ZWO ASI1600MM-Cool which has 6.5mm backfocus, and the filter wheel is a 7x36mm ZWO EFW which has 20mm backfocus, so the total is 63.5mm from the end of the reducer to the camera sensor.  This is pretty close to the 63mm from the diagram so I think I'm interpreting it correctly.  This is the result of this spacing:

image.thumb.png.b35487b9729b2e81d9d7d64fd3da978c.png

(raw stack, 6 x 300s Ha through clouds, but good enough to show the stars).  Looking closer it's clearly still not right, but possibly from tilt, e.g. top right seems OK, bottom right in one direction, top left in another -- a mess but possibly from tilt:

image.png.cfbfe526cbf1de0fae9096ed4e7e71a2.png  

I'm probably going to build up the optical train as follows:

- M60 extension - 14mm
- M60 male -> M48 male - 3mm
- M48 CTU (M48 female -> M48 female) - 17.3mm
- M48 male -> M42 male - 2.2mm
- ZWO EFW - 20mm
- ASI1600MM - 6.5mm

Total: 63mm

I will then tune out from 63mm using the CTU.

I hope that's of use to someone 🙂

-simon

  • Like 2
Link to comment
Share on other sites

  • 3 months later...

Well, another Vixen AX103s user. Maybe there aren't many of these telescopes, but I like mine very much. I will tell you that the #37228 reducer does not flat the field perfectly also in the APS-C cameras, also in my QHY 163m (same as your 1600mm) The new VX-37247 0,77 x  works a lot better: perfect stars at the corners due a different optical project.

Thanks a lot for your experience Simon, clear skies 🙂

 

Here a  full res test with the new HD reducer:

https://www.astrobin.com/full/kxnhjy/0/

  • Like 1
Link to comment
Share on other sites

  • 2 months later...
On 10/12/2020 at 09:49, Roberto Coleschi said:

Well, another Vixen AX103s user. Maybe there aren't many of these telescopes, but I like mine very much. I will tell you that the #37228 reducer does not flat the field perfectly also in the APS-C cameras, also in my QHY 163m (same as your 1600mm) The new VX-37247 0,77 x  works a lot better: perfect stars at the corners due a different optical project.

Thanks a lot for your experience Simon, clear skies 🙂

 

Here a  full res test with the new HD reducer:

https://www.astrobin.com/full/kxnhjy/0/

So, with my ASI1600 I did manage to get perfect round stars in the  corners using the #37228 0.7x reducer (using a Gerd Neumann CTU to adjust for tilt).  However I have now moved up to an APS-C IMX571 camera (the Altair 26M) and it is indeed impossible to get perfectly round corner stars with it, no matter what distance I have from sensor to reducer.  Without the reducer at f/8 I get a perfectly flat field, so it’s just the reducer which is not capable of properly handling APS-C — which is disappointing, because it’s sold specifically as an APS-C capable imaging scope!

Do you have any sample images with an IMX571 camera and the new HD reducer by any chance?  It might be worth me investing in it — it’s currently galaxy season and the 825mm f/8 actually works surprisingly well from Light Polluted London (bortle 8, 18.25 SQM on a good day).  However longer term I think I can live with f/6.16 and 635mm if I know for sure that I can get good corner stars 🙂

It’s actually not *that* bad with the 0.7x — I may just live with it as the stars do round off a little in stacking (a couple of my sample images attached — soul with the reducer, bode’s/cigar without) but it’s going to bug me...

68730880-9E23-4B85-A042-AF60E6B82B65.png

56186EF8-6569-4D91-9BA7-6AD3C6AAF112.png

  • Like 1
Link to comment
Share on other sites

  • 2 weeks later...

Update: I've been doing some digging and I now understand things are meant to work with this scope.

Here is what you will get with the scope without any reducer (it's naturally flat, due to the built-in flattener😞

image.png.cd33c3cc7c93c7542eb6cd84e2e34a3f.png

This matches up with my experience -- the stars in the corner at APS-C are nice and round, and it works pretty well for galaxies at f/8.  Yaaay.

Now, with the reducer I have, 0.7x which gives me f/5.6, the stars in the far corner look pretty odd when you go in close up:

image.png.9018232e53a0ada52b825071cf00c7f8.png

and indeed that's been my experience.  However, as evidenced by my soul nebula above, they do round off nicely in stacking (and it's only in the extreme corners).

Finally, there is another reducer available, 0.77x which gives me f/6.16, which should in theory give better results in the corners for APS-C:

image.png.7967647ea63548bc2e47f4bb8e347dd7.png

In fact, this reducer looks like the stars wouldn't be "too bad" in full frame even (based on how my APS-C stars round off with the other reducer).

For now, I think I can live with the 0.7x and deal with the stars in the extreme corners; as mentioned they do seem to 'round off' in processing.  If the 0.77x turns up for sale inexpensively somewhere in future I might get one, but it's not vital imho.

-simon

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.