Diffraction Spikes in Apo scopes due to the effect of the CCD Microlensing

[lensman57]

Hi,

Has anyone had an experience with the above problem ( diffraction spikes due to microlensing ) and is there a way of sorting this problem out?

Many Thanks and Regards,

A.G

[Daniel-K]

looks like dew on the front element

[lensman57]

looks like dew on the front element

Hi Dan,

No this is not dew, the image is a straight stretch of the simple stacked subs, no darks, flats or anything else. The dew zappers for both the scope and the guider were working fine and the lens was as clear as a whistle, I have not corrected for gradients either . So far my research points to the title or perhaps some light bouncing back and forth between the imager and the rear element of the WO 0.8 FF/FR.

Regards,

A.G

[Daniel-K]

have you tried stretching a single sub?

[lensman57]

have you tried stretching a single sub?

Hi Dan,

Yes I have, same result, there may also be an incompatibility issue between the FF/FR and the scope.Going to do a bit more reading on this. Apparently the KAF 8300 series sensors were notorious for this problem but I did not expect it from  an state the art series 674 Sony chip. Early days though, it might be something totally different, at least they are not objectionable to me.

Regards,

A.G

[jnc71106]

i have the samething on my M45 image using and APO frac and atik 314 in fact its something i've seen quite often on m45 images, they don't bother me

how about moisture in the atmosphere ??

[lensman57]

i have the samething on my M45 image using and APO frac and atik 314 in fact its something i've seen quite often on m45 images, they don't bother me

how about moisture in the atmosphere ??

Hi,

That is a possibility but unless I do another image without the FF/FR then I wouldn't know. The microlensing issue was discussed in the Cloudynights forum in some detail. As you have stated these diffraction spikes are not objectionable. I was almost temted to blame them on the kinfe edge baffeling of the tube but I don't think that correct.

Regards,

A.G

[Daniel-K]

very interesting would it show visually? i have had something similar with my old ed80 when imaging m45 also i just put it down to seeing and dew. id like to know what the outcome is though. good luck

[lensman57]

very interesting would it show visually? i have had something similar with my old ed80 when imaging m45 also i just put it down to seeing and dew. id like to know what the outcome is though. good luck

Hi Daniel,

This of image of M31 was taken on the same night, same set up, same camera, scope FF/FR , temperature, exposure, and number of subs as the M45, no darks or flats . But I have processed the subs in Startools 1.35.258 Beta the best of my ability. The stars are clean and no sign of any diffraction spikes anywhere to be seen even on the bright dwarf companion galaxy. Nothing special as  it was a test and my first of M31, 14 subs 240s each.

Regards,

A.G

[davew]

 Apparently the KAF 8300 series sensors were notorious for this problem but I did not expect it from  an state the art series 674 Sony chip.

Regards,

A.G

Hi AG,

I'm not sure what to make of that.

Anyway...... I have an 8300 chip and it does show what looks to be very small diffraction spikes through just about anything if the star is bright enough. They appear as very faint vertical / horizontal spikes. On an APO scope it shows flaring that does look similar to the spikes you have but not nearly so well defined. The same thing seems to happen on many scope / chip combos so long as the star is bright. Even DSLRs with scopes. The fact that they always follow the chip edge direction would perhaps indicate it's either the line of pixels or may be line of lensing. I read of one theory that they acted like diffraction gratings at certain wave lengths !

Why they look so pronounced on your scope I don't know but it's not a totally abnormal occurrence,

Dave

[ollypenrice]

Refractors with lens cell clips intruding into the light path can produce this kind of thing. Even Takahashi are not exempt. It can also come, as you imply, from camera chip microlenses. I found that the Bayer matrix on my Atik 4000 One Shot Colour could generate them when the mono equivalent, which I also had, didn't. I can't say that I found this too upsetting, though. My own M45 has both mono and OSC data combined, and the microlensing effects are there but I'm OK about this. http://ollypenrice.smugmug.com/Other/Best-of-Les-Granges/i-WQXKB6Z/0/X3/M45%20COMPOSITE%20FL-X3.jpg

Olly

[Uranium235]

Might seem like a daft question, but how good was your focus?

With an 80ED, when the focus is perfect I get almost newtonian diffraction spikes on brighter stars - as Olly has already mentioned, this comes from the cell clips. If your focus is either side of perfect, then it can cause additional spikes around bright stars (eg: those around M45).

Here is an example of those diffraction spikes:

[lensman57]

Thank you all for your replies and advice, I am not unduly worried about the spikes, I have applied heavy decon and sharpening to show the spikes at their worst so that is why they are so well presented. I read that the microlensing will bounce the light back and forth to and from the FF/ FR rear lens element and apparently the faster the scope the more pronounced the effect, this is not helped by the fact that the WO 0.8 FF/FR was not really designed for a triplret.   I am not sure if all this makes sense as I am not an optocal engineer but I guess that is just a part of this delightful and complex hobby of our's.

Regards,

A.G