Problem with 200/800 newtonian

Not super familiar with the Tecnosky scopes but they seem to be the usual GSO clones everybody has. Which means they will have the same design and quality issues. In this case it looks like the usual primary mirror surface roughness. Every one of these mirrors I’ve tested have varying amounts of texture to them.

Are you able to take an image with no coma corrector to rule out that possibly causing issues?

Collimation looks good, assuming you haven’t applied blur-x to the image.

Remove the primary mask, assuming this is what you call a “deflector”, and see what happens. I shan’t think its surface roughness.

This flare on stars is tipical GSO produced problem. I tried all possible solutions and in the end I asked Wolfgang Promper what could be cause of this. His reply was correct. Secondary mirror was defected, I replaced it with Lacerta one, about 50 EUR and since then my stars are perfect.

Thanks guys for the replies, you're making me a little worried about the quality of the secondary lens!

I shot my first M13 without a mask on the primary, and the result wasn't much different; the diffractions were more evident in my opinion.

Unfortunately, I no longer have the original image (I'm deleting the tests right away), and I only have this crop.

Since I had made the mask on the primary lens a bit too tight to "waste" less light, I've now tried again, making it 1 mm tighter, and I'll have to test how it behaves. If the diffractions don't disappear, there's obviously a problem with the mirrors, and I'll discuss it with the seller.

I'll update you as soon as a clearing appears!📷 686942266_942319945370100_3362990366791496928_n.jpg

Michele

In these telescopes (GSO N6 f/4 clone) a primary mirror mask will not solve the problem. It helps but not to much. Primary edge is rather good shaped and do not make significant diffraction. Holders to.

However secondary is not. Try to place a small circle at the front of the telescope. Centered on the spider vain. The circle should be large enough to cover the secondary mirror but do not worry if it is too large. You can make a circle from thick paper for testing but the shape must be very precise. See if there is any change.

If not cover a little bit focuser hole from insight of the tube. Just make a diaphragm and ‘glue’ it inside tube to reduce focuser hole aperture. Again, it's important to get the circle shape right. And check the effect again. Share the results.

For now, don't worry about collimation.

Best regards, m.

Marcin Cikała · May 10, 2026, 08:30 PM

Michele

In these telescopes (GSO N6 f/4 clone) a primary mirror mask will not solve the problem. It helps but not to much. Primary edge is rather good shaped and do not make significant diffraction. Holders to.

However secondary is not. Try to place a small circle at the front of the telescope. Centered on the spider vain. The circle should be large enough to cover the secondary mirror but do not worry if it is too large. You can make a circle from thick paper for testing but the shape must be very precise. See if there is any change.

If not cover a little bit focuser hole from insight of the tube. Just make a diaphragm and ‘glue’ it inside tube to reduce focuser hole aperture. Again, it's important to get the circle shape right. And check the effect again. Share the results.

For now, don't worry about collimation.

Best regards, m.

Thanks for the suggestion. I have a 3D printer, and it's not difficult to custom-design what you're suggesting to mount on the front. I obviously need to consider the diameter.

I don't understand why I should "close" the focuser hole, considering I have a coma corrector that reaches almost to the bottom of the focuser tube.

Hi.

I don't want to be stubborn, but in my opinion, we are dealing with a diffraction effect. This is what the star images currently look like. As I mentioned, diffraction won't appear on a low-quality mirror. The cause of diffraction should be sought in sharp edges.

Replacing the secondary mirror, as @Astro Hopper mentioned, may confirm my theory about a bad secondary mirror side (not surface). The new mirror is probably smoother on side and qute good darkened. Sorr again but I don't believe a $50/50Eur flat mirror is much better quality than the one originally installed one. A good flat mirror costs ten times more.

@Michele Campini sorry, I may have misspelled it. It's not about closing the inlet to the focuser, but rather limiting its diameter with a diaphragm slightly smaller than the focuser inside diameter. Sometimes, diffraction can appear in a very unexpected place. Here is the edge of the focuser.

This is only to eliminate the possibility of diffraction occurring on this element. You can also make the diaphragm slightly smaller than the corrector lens just to see what happen.

Also remember that 3D-printed circles aren't circles. They are polygons. This can do more harm than good. You'll need to sand the edge.

You have time to buy new mirror (if this does not help).

All the best, m.

Marcin Cikała · May 11, 2026, 01:26 PM

Hi.

I don't want to be stubborn, but in my opinion, we are dealing with a diffraction effect. This is what the star images currently look like. As I mentioned, diffraction won't appear on a low-quality mirror. The cause of diffraction should be sought in sharp edges.

Replacing the secondary mirror, as @Astro Hopper mentioned, may confirm my theory about a bad secondary mirror side (not surface). The new mirror is probably smoother on side and qute good darkened. Sorr again but I don't believe a $50/50Eur flat mirror is much better quality than the one originally installed one. A good flat mirror costs ten times more.

@Michele Campini sorry, I may have misspelled it. It's not about closing the inlet to the focuser, but rather limiting its diameter with a diaphragm slightly smaller than the focuser inside diameter. Sometimes, diffraction can appear in a very unexpected place. Here is the edge of the focuser.

This is only to eliminate the possibility of diffraction occurring on this element. You can also make the diaphragm slightly smaller than the corrector lens just to see what happen.

Also remember that 3D-printed circles aren't circles. They are polygons. This can do more harm than good. You'll need to sand the edge.

You have time to buy new mirror (if this does not help).

All the best, m.

As for 3D prints, to find polygons in a circle, believe me, you need a very good microscope, because it's difficult with the naked eye. But even to the touch, you don't feel anything. It's all smooth and round. Maybe with 3D printers from 15 years ago.

Anyway, I tried what you said, trying to cover the secondary mirror well; that circle in front is 72 mm in diameter.

I understand what you mean about the focuser; I'll add it to the list of tests.📷 Senza titolo-1.jpg

Michele Campini · May 11, 2026, 04:15 PM

By the way, I forgot to write that I have this little mark in my secondary, I wonder if it could have some impact on my problem.
I don't think so because it remains right behind the cross support of the secondary and if that were the problem I would always have the diffraction in the same place.

📷 PXL_20260511_155249164.jpg

That’s the thing that still puzzles me. The fact that the position of the flair isn’t stationary with respect to the optics and its supports.

Michele Campini · May 11, 2026, 04:11 PM

Marcin Cikała · May 11, 2026, 01:26 PM

Hi.

I don't want to be stubborn, but in my opinion, we are dealing with a diffraction effect. This is what the star images currently look like. As I mentioned, diffraction won't appear on a low-quality mirror. The cause of diffraction should be sought in sharp edges.

Replacing the secondary mirror, as @Astro Hopper mentioned, may confirm my theory about a bad secondary mirror side (not surface). The new mirror is probably smoother on side and qute good darkened. Sorr again but I don't believe a $50/50Eur flat mirror is much better quality than the one originally installed one. A good flat mirror costs ten times more.

@Michele Campini sorry, I may have misspelled it. It's not about closing the inlet to the focuser, but rather limiting its diameter with a diaphragm slightly smaller than the focuser inside diameter. Sometimes, diffraction can appear in a very unexpected place. Here is the edge of the focuser.

This is only to eliminate the possibility of diffraction occurring on this element. You can also make the diaphragm slightly smaller than the corrector lens just to see what happen.

Also remember that 3D-printed circles aren't circles. They are polygons. This can do more harm than good. You'll need to sand the edge.

You have time to buy new mirror (if this does not help).

All the best, m.

As for 3D prints, to find polygons in a circle, believe me, you need a very good microscope, because it's difficult with the naked eye. But even to the touch, you don't feel anything. It's all smooth and round. Maybe with 3D printers from 15 years ago.

Anyway, I tried what you said, trying to cover the secondary mirror well; that circle in front is 72 mm in diameter.

I understand what you mean about the focuser; I'll add it to the list of tests.📷 Senza titolo-1.jpg

I really like the idea of a mask like this. Not many people pay attention to the edge profile of the secondary and support but they should as any edge irregularity there will show up the same way a rough mirror edge will.

This evening I did a quick test on Arcturus.

It's nine 5-second shots stitched together by luminance.

Basically, as soon as I exit a "narrow" central circle, those ugly reflections start to grow.

I'm using an ASI 2600mm.

My question is: could the coma corrector (ED for full-frame) be the culprit?

The photos were taken with the front mask mounted, but I also tried without and it made no difference.

📷 Senza titolo-1.jpg

You can now clearly see that the spikes are symmetrical about the center of the field of view. Most probably you have diffraction close do the detector and very close do optical axis. Now perform a very important test. Rotate the camera and corrector. If the spikes rotate too the problem is with the corrector or the adapters. If the spikes do not rotate then check the focuser. Photos of your telescope show that the focuser is extended into the OTA. If this is the working position, use a diaphragm to block the inlet to the focuser.

You may also VERY slighty rotate the secondary mirror and observe the spikes.

m.