BS Astro Flip Mirror Review – Part 2

Part 1 of the review covered the construction and features of the flip mirror and was written before I’d had a chance to use it seriously. Now I’ve had a couple of clear nights to test it out I can finish off the review with some comments on how it performs.

Firstly I had to go through the procedure of collimating the mirror so that objects appearing in the center of the eyepiece are also centered in the camera. If you read part 1 you’ll find my description of the techniques required to adjust the collimation in both the horizontal and vertical directions. I’m glad to say that these techniques work a treat and within about 10 to 15 minutes I had the collimation spot on. You could probably do it quicker if you have a good distant horizon and could focus on some far off object during the day – in the dark the time taken to find where you’ve put down the alan keys and then feel for the bolt heads soon mounts up.

With the mirror nicely collimated I moved on to doing some imaging tests.

The first issue I encountered is that getting the eyepiece and the camera to focus at the same time takes some tinkering. For the camera to focus, the sensor has to be at the focal plane of the telescope – for a typical webcam this position is maybe 1cm or more beyond the back of the rear 1.25″ eyepiece holder. Now, the eyepiece’s forcal point has to be the same distance from the back of the scope (following the reflected light path off the diagonal mirror) – this typically puts the eyepiece so high that it is actually beyond the top of the top 1.25″ eyepiece holder. Fortunately I had a 10mm T thread extension tube lying around, so I used this to raise up the top eyepiece holder. Even so, I have to clamp the eyepiece well up out of the holder to get the right focal point. Fine focus turns out to be easy – you just unscrew the T-threaded eyepiece holder slightly from the top of the body – a poor man’s helical focuser.

So, the lesson from this is that for webcam use, a T-thread extension in the 10-15mm range is essential. An ideal accessory would be a variable length T-Thread extension (say from 15-25mm or so). I can’t see this being any different for DSLR use since the DSLR sensor is typically 40-50mm behind the connection point of a T-adaptor. Dedidcated T-threaded astro CCDs might do better in this regard.

Actually finding objects using the mirror is a treat – just center it in the eyepiece and flip the lever. This is particularly helpful for smaller sensors where the field of view of the sensor is limited and you can otherwise end up with objects lost off the edge of the field of view and no idea which way to go to find them. I’ve found a 25mm eyepiece quite sufficient for centering the object – this is partially personal preference as I find that eyepiece particularly comfortable to use. An 8mm-10mm would get you slightly better centering, but it’s probably only going to be necessary when you are using a barlow with the camera for very high magnification.

Actually using the mirror takes some getting used to – on the first evening I had a couple of occasions where I was scratching my head over getting no image on the camera only to realise that the lever was still in the eyepiece position. Still, the flip side is that it’s dead easy to take dark frames – just flip the lever to the eyepiece position and fire away.

Now to an interesting point – after playing around a little I decided that using a webcam with an 8″ SCT for long exposures was really pushing the limit of my Nexstar SE mount – you could see the stars moving from frame to frame. So I put my f/6.3 focal reducer in to reduce the focal length to an effective 1260mm or so. My first attempt was to make the change as quickly as possible, so I ended up with the following set up :

SCT rear port : Reducer : SCT visual Back : Flip Mirror NosePiece : Flip Mirror Body : Flip Mirror rear 1.25″ holder : Camera

Believe it or not, at this point I couldn’t get the image to focus on the camera – I’d run out of back focus which is a tricky thing to do on an SCT. Fortunately I could reduce the length of the imaging train by taking out the SCT visual back and the flip mirror nosepiece, so I had

SCT rear port : Reducer : SCT to T converter : Flip Mirror Body : Flip Mirror rear 1.25″ holder : Camera

Still, I do wonder if I will reach focus if I were to put a 0.5x reducer onto the camera to get a f/3 (ish) focal length. Only one way to find out…

So sufficient backfocus is something that people should consider with this flip mirror – obviously this is more critical for newtonian scopes which tend to have more limited back focus available. The mirror body is considerably longer than a standard diagonal because it has to accomodate the pivot mechanism and the mirror length when it is horizontal.

Finally a few words on the visual views using the mirror – I’m not a big visual observer and I haven’t really tried any high power views using it, but I can’t say that I’ve noticed it to be any worse or better than my ordinary diagonals (I have a celestron 1.25″ and 2″). To be honest, I’m sure the mirror is fine but my visual skills aren’t up to confirming it!


Overall I’m really pleased with this – it makes small sensor imaging – at least the framing part of it – much less of a challenge. It’s also fine as a diagonal, so there’s no need to swap it out for a normal diagonal unless you need a 2″ one – just leave the rear port stopped up.

Pros : Solid construction, fully collimatable in two directions, holds alignment accurately.

Cons : Long backfocus requirements, needs T thread extension to balance eyepiece/camera focus, no proper helical fine focuser.

*** Update ***
Bern at Modern Astronomy has contacted me to let me know that my mirror was one of only two in his stocks missing the helical focuser and instructions – they are on their way to me now and I will update properly once they are here and I have tested them out. Likely some packing defect in early stock and nothing to worry about.
*** End Update ***

Based on my experience, I’d say that the collimation feature is probably key to a decent flip mirror – mine arrived mis-aligned somewhat both vertically and horizontally, but I could put that right. Without use adjustable collimation you could well end up with a mirror that was misaligned and couldn’t be fixed.