Inverse Lighthouse Beam Effect on Bright Stars with Tak FSQ 85-EDX

Bruce Donzanti:
I've been enjoying my Baby Q but I have noticed on some bright stars (example, Alnitak on the image below), the so-called "inverse lighthouse beam" pattern which has been attributed to vignetting as a result of the Petzval design.  My setup is the Tak FSQ 85-EDX with the 1.01X field flattener designed for this scope, along with the ZWO ASI6200MM Pro.  The flattener is said to support sensors of size APS-C or smaller, so maybe it is the larger 6200 sensor causing this???  Backfocus is correct and tilt minimized.  I have seen this effect with two different Ha filters.  I have seen this effect on a number of images on AB by others, but other images seem to have tempered its appearance.   Any suggestions on how to reduce this look on bright stars would be appreciated.  

I could be wrong but I would guess that is a diffraction pattern from a lens cell nut. I see the same effect on my camera lenses. 
The only way I have dealt with the issue is in photoshop. I make a circular selection centered around the flare. I then 'layer via copy'. Set the blending mode to lighten. Slowly rotate the selection till issue is resolved.
Hope that helps.
Dan

p.s. Nice image.

Hello, the effect is typical for the FSQ. My dealer told it is a 'signature' of the scope. It appears irrespective of filter. Here

https://www.astrobin.com/full/sha3ff/C/?mod=&real=

is a FF of mine on an OSC camera. The effect is very prominent on both sides of the frame. Anyway, I never remove it.
CS, John

Die Launische Diva:
This effect is also apparent with the Samyang/Rokinon 135mm f/2 lens when used wide-open. I've also seen the same effect in high-end Vixen refractors and other camera lenses (Canon and Sigma). I also don't bother with it. Some Samyang owners either stop-down the lens by using the diaphragm or a step-down ring.

Correct. Seen this with a few of my Canon lenses. Always attributed it to diffraction caused by some kind of obstruction. Just learned to live with it. Not particularly bothersome.

Hi Bruce,

first, I would like to briefly explain (very simplified) where the effect comes from. Everybody knows the diffraction pattern of some typical apertures in astrophotography. So, the diffraction pattern of a round aperture is the Airy disk. The diffraction pattern of a Newtonian telescope is a superposition of the Airy disk with the typical spikes by the spider od the secondary. Also pretty much everybody knows the diffraction pattern of the Bahtinov mask. I have shown all three in the first figure. The diffraction pattern (below) is the 2D Fourier transform of the aperture shape (above).



Refractors should actually always show each star as an Airy disk. However, the telescope does not consist of only one lens, but usually of several lenses (front lens and reducer for example). In addition there are apertures like the dew shield (like Jay said before). If a star is not imaged on the optical axis, but further out at the edge of the image, then the projection of the telescope aperture is no longer a circle, but a superposition of two circles. I have tried to show this schematically.



The Fourier transformation of this aperture shape then results in the inverse lighthouse, as in your image.



The whole thing might be handeled in the image post-processing somehow (I can't do that) or it can be avoided from the beginning, if the star is not too far away from the optical axis or if the outermost projection of the aperture is circular by a clever arrangement of apertures. But the latter means: Either you use small camera chips or increase the f-stop number. But nobody really wants that. I hope I was able to help out a little.

Clear skies
Thorsten

Thank you, Thorsten, for the education.

Bruce, Yes - that is a very fine image.

I still love my Baby-Q ….

Bruce, when all else fails, the Healing Brush in Photoshop can do wonders ....

Parag Batavia:
I'm just about to pull the trigger on buying an FSQ-85EDX and 6200MM - so great to find this thread.

Anyone here with any experience with PixInsight's StarNet++ or StarExterminator on stars that have this lighthouse effect? Are they properly recognized and removed? 

Thanks!

Parag

Yes- StarXterminator does recognize and remove it almost perfectly when used on the linear image.

Die Launische Diva:

Parag Batavia:
I'm just about to pull the trigger on buying an FSQ-85EDX and 6200MM - so great to find this thread.

Anyone here with any experience with PixInsight's StarNet++ or StarExterminator on stars that have this lighthouse effect? Are they properly recognized and removed? 

Thanks!

Parag

I wouldn't make such a serious decision based on the performance of some software which are under constant development/improvement. Both the telescope and the CMOS camera you consider buying are well-established performers in the astrophotography community. Some astrophotographers may even consider a sacrilege having such a high-end hardware combo and rely on StarX/Starnet for obtaining quality images

I agree.  The scope with a decent CMOS camera (as mentioned previously in this thread) are a wonderful combination.  No need to judge its use based on any pre- or post-processing software.  In fact, I have been very pleased with my images even before minimal processing under my Bortle 7, LP skies.

Parag Batavia:
looking on here to find examples of that combo, to confirm that you really can use the FSQ-85 with a full frame sensor.

Well- there are many of examples on AB to help you decide.  Best of luck in your choice.