Askar FMA180 Pro focusing issues and coma problems

Equipment Focuser Refractors Backyard Astrophotography Coma Help Needed Troubleshooting
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Filip Záruba avatar
Filip Záruba avatar

Hello everyone,

I am currently building a wide-angle setup, but I am facing serious issues with my Askar FMA180 Pro. Firstly, there is a problem with focusing: I am unable to reach focus at the manufacturer’s specified 55 mm backfocus distance. Instead, I need to increase the backfocus by approximately 6 mm to achieve focus at the very end of the focuser ring. At this point, there is absolutely no focusing travel left, and I have no margin for fine adjustments. This shift of the optical plane leads to severe coma and elongated star shapes, starting from roughly half of the image and becoming even worse towards the corners. This is especially problematic given that I am using an APS-H sensor, with 2 mm filters, so the scope is not even at full-frame coverage. I have attached some pictures showing the issue.

Am I expecting too much from this scope, or is it likely that I am simply unlucky with this particular copy? Should I contact my dealer to request a replacement unit, since the backfocus offset prevents proper use of the telescope according to the manufacturer’s specifications?

Thank you so much for your advice.

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andrea tasselli avatar
andrea tasselli avatar
Get a replacement unit ASAP. That much (inverted) coma in a refractor is rather unusual to say the least.
Filip Záruba avatar
Filip Záruba avatar

Thank you so much, Andrea. Unfortunately, I am already past the 14 day return period because it has been cloudy for many weeks and I was only able to collect at least five minutes of data in each RGB channel yesterday. The coma is visible in every filter, and I even quickly swapped filters in my Moravian to make 100% sure it wasn’t caused by them, so I am confident that this is a telescope fault. I will have to proceed under the 2-year warranty, pointing out this specific defect.

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andrea tasselli avatar
andrea tasselli avatar
Hi Filip,

You have a very strong case for making the return as it doesn't comply with either their BF distance specification nor with the spot diagrams they provide.
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Filip Záruba avatar
Filip Záruba avatar

I gave it some more thought and decided not to give up so easily. Before returning the telescope, I wanted to run a few more tests. I bought a few extra spacers and slightly different adapters, and I think I’ve now reached an acceptable result. At the moment, the backfocus is exactly 58.5 mm measured from the end of the M54–M48 adapter (this adapter is not included in the manufacturer’s stated backfocus, which is something anyone dealing with the same issue should keep in mind). Ideally, I would still need about a 0.3 mm spacer to get the stars perfect in the corners.

My current imaging train looks like this: 29 mm camera, 7.5 mm low-profile Moravian M48 adapter, 12 mm M48 ring, 10 mm M48 ring, and the stock M54–M48 adapter that came with the flattener.

So I won’t be returning the telescope. That said, the issue wasn’t entirely on my side. I believe the manufacturer should update the technical drawings and clearly mention that the backfocus is not exactly 55 mm and may vary slightly, so users can take that into account from the start.

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LeePhillips avatar
LeePhillips avatar

I believe your scope is out of spec, I tested my friends one over year ago and the back focus was spot on as were perfect round stars into the corners. Then based on this excellent performance I punched one for myself a few months ago, but was very disappointed to find the back focus was way out , and like yours around 59mm but then there is no room for any adjustment, and even then the stars were still slightly curved in the corners exactly like your final set of images. I decided to return mine as I would never be happy with it.

I have tried my friends copy and its still perfect, so I would still like to get one, but not sure I want to go through potential return again.

I believe the issue is that the focal length is not correct on the one I had, hopefully it was just a bad batch.

I would still suggest you contact sharpstar directly as they are a very helpful company and if you send them example image they will be able to figure out the issue and I suspect[ect they would be happy to replace it for you.

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Tony Gondola avatar
Tony Gondola avatar

I’m afraid this is a fact of life in production optics. You see it in telescopes and you see it in camera lenses. In any production situation it’s always a matter of tolerances verses cost. Could they make every scope hit the numbers exactly to within a mm? Sure, but not at the price you paid for it. All you can do if you get a scope that’s not meeting specs is to return it and hope the next one is better. Then about the only thing you’d be guarantied is that it won’t be the same.

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