Askar FMA180 f/4.5 Astrograph - my Takumar's End?

Introduction

Most objects in the night sky are small and require a long focal length. Telescopes are available from about 300mm to several meters of focal length. But what if you want to image a really large object? For a long time Borg was the only brand that offered short focal length scopes. Given the price of these instruments prime photo lenses have been a good alternative and still are. If premium image quality is expected the price is in the premium range as well. Not too long ago William Optics came up with the Redcat. Now there is something new again. In October 2019 Jiaxing RuiXing optical instrument from China, also known as Sharpstar, founded the new brand 'Askar'. Among others they launched a modular mini astrograph comprising a 40mm f/5.5 triplet APO lens with two ED glasses, a helical focuser, a flattening 0.8x three element reducer and several extension tubes. With the reducer it becomes a 180mm focal length f/4.5 astrograph. The question arose, is it time to retire my 200mm Ashai Pentax Super Multi Coated Takumar?

Daylight Test

50 years lie between these two lenses: left Askar FMA180 from 2020, right SMC Takumar from the early 1970

A look into the front lens tells a story. While the Askar shows the green reflections we are used to these days the Takumar obviously has less effective coatings. As it seems there was some progress in the last 50 years.

Let's see how the two lenses compare at daylight. There is a cell phone antenna mast nearby, an ideal object for high contrast test images. Here is a 100% 400x400 pixel crop of the antennas cables and mast. As the Takumar comes with 10% more focal length the object is slightly larger but there is another difference between the two. Do you see the pink fringes at the shiny clamps?

Get ready for the night

Being the third astrograph in my collection the new newcomer may stand in the middle on the family photo.

Little Asky between my GSO RC8 and my TS 65 Quad

Right after posing for the photo the TS 65 Quad took off it's rings to help the newcomer to carry a guide scope that is larger than the main lens. I have drilled threaded hols in the rings and and added thumb screws some time ago to hold the Takumar in place.

Not a problem at all, securely locked in the rings the 50mm guidescope goes on top. The dew strap is long enough to wrap it two times round the lens. The camera is connected via a Baader protective T-ring that holds 2" (48mm) filters inside. The Askar comes with a T-thread (42x0.75) for which the T-ring comes with a zero length 42mm to 48mm adapter.

Askar FMA180, Canon T7i, TSL50D guide scope and lacerta MGEN-II camera

The small lens comes with a rich accessory kit. Part of it are two red anodized rings with some felt inside. Using two thumb screws each the can be used to mount the tube to a small bar. I used one of them without the screws as a slider. While the large front ring holds the objective lens in place the other large ring only holds the little red ring. When turning the helical focuser the rear part can slide back and forth. Once in focus a small screw at the focuser is locked.

Star Test

Let's start with my status quo. The popular PixInsight script AberrationInspector automatically creates a mosaic of an images center, corners and border. Here is one from an image of the Veil nebula taken with the Takumar in 2019. The image is perfectly flat, not a trace of aberration to be seen. Only the fringes are a bit disturbing and I feel like we have seen them before.

Let's see how the new family member performs. The conditions were far from optimal and between the clouds and the rising moon I did not even get two hours worth of data. This is not meant be an award winning image, it is just the first light that I was able to get the day the lens arrived. I also have to add that it was a hazy night with dim stars and a very bright sky. If the stars look a bit blurry it is very likely not caused by the lens. The object was low in the sky that is why a lot of the blue is missing.

Askar FMA180, unmodded Canon T7i, 59x2min at ISO 400, 200 bias, 30 flats, no darks, guided and dithered, mount: Skywatcher EQ6R
click the image for full resolution (21MB)

During the better part of the night the average star diameter of the single subs was constantly below 1.8 pixels. Hard to say if this is the resolution limit of such a small aperture or if this is the spatial low pass filter in the camera. The T7i's pixel pitch is 3.7µm. The image scale is 4.27 arc seconds per pixel so that the star diameter is very likely not determined by the seeing. Here is how the star diameter is distributed over the frame. Please not that this is an APS-C sized sensor. The lens supports a 44mm full frame image circle. Alas I do not own a full frame camera.

Star diameter map, full width at half maximum, unit is pixels, based on 9988 stars in the image

Like in the daylight image there are no annoying color fringes. Pixel peeping reveals minimal traces of blue but they are all at the same side. This is very likely caused by the atmosphere. When I image at low altitude with my longer focal length scopes I often see this when an object is low in the sky. This is not an effect cause by the lens.

Let's have a look at a more challenging image. Some days later I had a chance to capture 5 hours of the Sadr region. The good news is that there are no reflections caused by the bright star Sadr.

full image (APS-C sensor) scaled to 800pix width

There are a lot of bright blue stars in the Milky Way and alas they have some fringes. This is a 3x magnified view of a tiny little part of the image not too far away from the center. It contains a few blue stars as well as a deep red one.

Splitting it into channels shows that the image is in focus in green but out of focus in red and blue.

When the whole image is scaled down so that it fits a full HD display the crop is a mere 49 x 28 pixels small and the fringes just make the blue stars a bit larger.

Of course this can be corrected in post processing. The links below are a naive beginners processing without corrections and a more professional processing. In the latter an artificial L layer was extracted from 70% green and 30% red without any blue. The artificial L was deconvoluted and than re-combined with the color data. The blues have been desaturated. Of coursed I would like to have a telescope that does not require correction of optical flaws in post processing. Looking at the price tag of a Borg astrograph I can live with the extra work.
simple processing w/o correction
professional processing

The flattner is doing a good job. Here is the Aberration Inspector mosaic:

Aberration Inspector mosaic: Askar, Sadr region, professional processing

Conclusion

The field is reasonably flat and the lens and the reducer seem to be well collimated. The objective lens is almost certainly not an FPL53 triplet because whenever FPL53 is used in a lens is is advertised in large letters. The telescope shows an amount of color fringing mainly in blue that needs correction but it is not severe as long as the image is scaled down to a reasonable size. I do not know if I want to recommend it because I have never used the obvious alternative, Willam Optic's Red Cat. What I do know is this: it easily outperformed it's vintage predecessor. Sorry, Takumar! You are retired now.

Thanks for reading!

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