The tiniest refractor setup

To say the Askar FMA135 telescope is small would be an understatement. This APO triplet with an ED lens for color correction is the smallest APO i’ve ever seen. Askar has released this new line of small refractors called the FMA series. Right now they consist of the FMA135, FMA180, and FMA230, each name is an indicator of their total focal length. So the FMA135 is an F4.5 135mm focal length telescope. The FMA135 supports up to an APSc sized imaging sensor.

Technical Specifications:

Aperture: 30mm
Focal Length: 135mm
Focal Ratio: F4.5
Objective Type: Air-spaced APO triplet with ED glass
Adapter Type: M42x0.75 thread for imaging, or 1.25” visual adapter

The Askar FMA135 is multipurpose

Out of the box, the FMA135 comes with a mounting ring which holds the scope and has a finder shoe base. This can be bolted to a vixen or losmandy dovetail for use as a stand alone telescope, or it can be mounted in a guide shoe and used as a very high quality guide scope. The back focus requirements are 55mm, so this handily works with a DSLR, or most astrophotography camera setups. In addition, it comes with a visual extension tube which can hold an eye piece directly or your guide camera when used as a guide scope. There’s a set screw just visible on the left of the scope which allows you to lock down focus. It uses a rotating barrel focus mechanism which is very smooth to operate.

Testing the Askar FMA135

For my initial tests. I paired it with my color camera, the ZWO ASI2600MC-Pro as it is an APSc sized sensor. I wanted to test how well color corrected the lens is, and how the stars looked across the biggest imaging sensor supported with this telescope. So far, I’ve only been able to spend one night imaging with this setup. I have been using it primarily as a guide scope for several months, and it is a flawless companion in that regard.

I spent a full night, with two hours to two different targets. I wanted some wide field views, so I chose NGC 7000 and M31 as my targets. I do live in very light polluted skies (bortle 7/8), so two hours are not enough time to do these targets justice, but there’s enough information here to see how the telescope performs out of the box with a large sensor.

NGC 7000 shot with the Askar FMA135 and ZWO ASI2600MC-Pro camera.

M31 shot with the Askar FMA135 and ZWO ASI2600MC-Pro camera.

How does the FMA135 perform?

Performance of this little scope is pretty good. I was surprised my targets were this bright after two hours of images. The F4.5 speed certainly helps gather light quickly despite the small aperture. Color correction is very good. I can see no visible fringing on any stars all the way to the edge of both images. The main issue I can see with this setup is that flat field correction is not perfect. With such a large sensor, it’s imperative to make sure any sensor tilt is adjusted out of your setup. The results are not forgiving if you have tilt in your system.

Aberration Inspector from PixInsight reveals some sensor tilt.

In the image above you can see some clear diagonal tilt in the system. Coma in the upper left and astigmatism in the lower right. Adjusting tilt will certainly reduce these issues, but can it correct it completely? For this, I don’t currently have an answer. I’m working on a test rig to fix the sensor tilt, and hope to write about this in a future article. I also plan on trying out the ZWO ASI294MM-Pro mono camera setup which uses a much smaller micro 4/3 sized sensor. This will certainly make a difference in the corner stars without any tilt adjustments. The center stars are very tight, and focus was easy to achieve. Over the two hours, focus did not shift, so I’m happy to report that the small system doesn’t suffer from temperature changes in a way that is very visible.

Here’s a reconfiguration of the setup using the ASI294MM-Pro camera and filter wheel.

Accessories for the FMA135

One thing I managed to stumble upon was some very nicely made accessories for the FMA135. Fellow astronomer Naoki (in Japan) has designed and manufactured some very nice mounting brackets to take advantage of the 3 additional finder mounts on the mounting ring of the FMA135. He has created both tall and short mounting brackets. I’ve attached two of the short mounting brackets to the sides, and a tall one on the top of the FMA135. I have the ZWO ASIAir Plus control system on one side, and the ZWO Mini Guide scope on the other side. I plan on adding a ZWO EAF belt driven focuser to the top bracket to turn the focus barrel automatically for perfect focus. Here are some photos of the accessories attached to the FMA135. Total weight of this setup is 4.4 lbs.