Problem with 200/800 newtonian

Tecnosky Newton 200/800 CARBON Series
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Michele Campini avatar
Michele Campini avatar

I recently started using a 200/800 Newtonian, the Tecnosky Carbon Series.

It's my first F4 Newtonian; I previously had an F5 and had fun with it.

I have a 2600mm with an EFW36 and OAG L, and I'm using an FF ED coma corrector with backfocus at 55mm.

I also mounted a deflector on the primary mirror.

I collimated the instrument with an OCAL2, and although I'm waiting for the OCAL4 to arrive, I think I've achieved "decent" collimation.

The problem is that on the brightest stars, I get a "smear" of light pointing outwards, as you can see in this luminance image of M13.

Hi-res image here:
https://drive.google.com/file/d/1PrOUGvy5KW31tJuW0CLxsTlViyh13sij/view?usp=sharing

What do you think could be causing this phenomenon?

Thanks,

Michele

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Dan Watt avatar
Dan Watt avatar

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.

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Dan Watt avatar
Dan Watt avatar

📷 ronchi_1.1.3.jpgronchi_1.1.3.jpg📷 IMG_0348-2.jpgIMG_0348-2.jpgThe top image is from a 6” GSO image near null on the test stand. The 8” and 10” mirrors I’ve tested from Synta and GSO all appear pretty similar. For comparison, the bottom image is a 12.5” mirror from an older Discovery dobsonian that are known for high optical quality.

Think about what the effect of surface roughness does to the reflected light beams. Those high and low spots are sending light all over the place.

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andrea tasselli avatar
andrea tasselli avatar

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

Tony Gondola avatar
Tony Gondola avatar

I don’t think it’s the mirror as well. My best guess is it’s a diffraction effect. The weird thing about it is, it clocks around the center of the field. I’d follow the advice above to try to narrow down the source.

Astro Hopper avatar
Astro Hopper avatar

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.

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Marcin Cikała avatar
Marcin Cikała avatar

Sorry Guys but I do not believe that this is mirror surface quality problem. The spikes are very symmetrical relatively to the image center and are very sharp. Problem with the surface quality should reflect in degradation of image contrast or destroy star shape, not the spikes around bright stars.

As @Tony Gondola said - I also suppose the diffraction problem. I own a GSO 6” Photo Newton f/4 and have exaclty the same problem. I have no time to investigate the problem now but I suppose it is caused by diffraction on the secondary side/edge. In my case is not blackened or even perfectly shaped (I see many thin rows on mirror side/edge). I also do not reject idea that diffraction is caused by focuser. Have plan to examine at summer.

Changing the secondary may solve the problem but I suppose using cover for secondary mirror center support should help.

CS. Marcin.

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