5th diffraction spike explained! (well sort of)

I talked to the previous owner of my new setup who has been helping me get it working, he said that the 5th diffraction spike is an oddity of my specific telescope. he had it at multiple telescope shops and asked experts about it. every person could not find anything in the optical path.

i'm just going to think of it as the thumbprint of my telescope, i actually kind of like it because it is unique to me.

also here's a 15x600 sec image of the horsehead from last night. i'm adding more subs tonight.

Lyle Aldridge wrote:

It's difficult to believe that experts examining the scope could not identify the cause of such an artifact. There may be a clue in the fact that your four "normal" spikes are also doubled. That usually means that the opposite spider legs are not exactly opposite from each other.

I agree with Lyle's comments above. Not every telescope owner or sales person is necessarily an expert. But did those "experts" assessing your telescope really try the telescope at night, examining the diffraction pattern of an out-of-focus, bright star? That's really the best way to determine what is causing the artifact. Whether or not these experts can determine the cause, there is something causing the spike. If you can find that cause, then you'll be an expert in that area. Your other photo showed the 5th spike only on one side of the field of view. What that means is only from that position in the FOV does a star see the obstruction in the light path. From other positions in the FOV the obstruction is not visible, which is a clue to the cause.

The doubling of the 4 spider diffraction spikes is easier to remedy. Just adjust one or the other of each offending pair of supports so that their diffraction spikes overlap. There is usually enough extra diameter in the hole in the tube to move the support studs a bit to the side. If not, make the hole larger.

Another "solution" is to just ignore the effect. Overall your photos are quite nice, showing great progress. But in your obsession if these spikes bother you enough, get a curved vane support spider. One of these will distribute the diffraction effects of the supports evenly around the entire 360° perimeter of the central spot. The effect is similar to that produced by any of the refractor, Schmidt-Cassegrain or Maksutov telescopes, which have no spider support diffraction issues. Here is a nice discussion of curved spider supports:

Newtonian Curved Secondary Mount

A 3-vane spider is another option. This will give less total diffraction spread around in 6 spikes at 60° intervals. Some people may prefer that pattern to the usual 4 spikes. With the 3-vane spider there is no problem with double spikes since the 6 spikes don't overlap. The following graphic shows the effects of various methods of supporting the secondary mirror.

Diffraction Spike Comparison

Hi,

Nice data on the Flame & Horsehead - it seems to be shaping up well.

I shall be following this out of curiosity when you find out the reason for the 5th spike and post it.

I think there is some other artifact probably caused by internal reflection on the right.

Regards,

Vishwanath

RustierOne wrote:

Lyle Aldridge wrote:

It's difficult to believe that experts examining the scope could not identify the cause of such an artifact. There may be a clue in the fact that your four "normal" spikes are also doubled. That usually means that the opposite spider legs are not exactly opposite from each other.

I agree with Lyle's comments above. Not every telescope owner or sales person is necessarily an expert. But did those "experts" assessing your telescope really try the telescope at night, examining the diffraction pattern of an out-of-focus, bright star? That's really the best way to determine what is causing the artifact. Whether or not these experts can determine the cause, there is something causing the spike. If you can find that cause, then you'll be an expert in that area. Your other photo showed the 5th spike only on one side of the field of view. What that means is only from that position in the FOV does a star see the obstruction in the light path. From other positions in the FOV the obstruction is not visible, which is a clue to the cause.

The doubling of the 4 spider diffraction spikes is easier to remedy. Just adjust one or the other of each offending pair of supports so that their diffraction spikes overlap. There is usually enough extra diameter in the hole in the tube to move the support studs a bit to the side. If not, make the hole larger.

Another "solution" is to just ignore the effect. Overall your photos are quite nice, showing great progress. But in your obsession if these spikes bother you enough, get a curved vane support spider. One of these will distribute the diffraction effects of the supports evenly around the entire 360° perimeter of the central spot. The effect is similar to that produced by any of the refractor, Schmidt-Cassegrain or Maksutov telescopes, which have no spider support diffraction issues. Here is a nice discussion of curved spider supports:

Newtonian Curved Secondary Mount

A 3-vane spider is another option. This will give less total diffraction spread around in 6 spikes at 60° intervals. Some people may prefer that pattern to the usual 4 spikes. With the 3-vane spider there is no problem with double spikes since the 6 spikes don't overlap. The following graphic shows the effects of various methods of supporting the secondary mirror.

Diffraction Spike Comparison

I actually got a 3 vane spader with curved veins with the scope....

im going to look everywhere on the scope so wee whats wrong. however they really dont bug me that terribly much

jammeymc wrote:

I'll be able to verify if it's a signature of the scope. I had been eyeing that Newt for over a year and as planned, ordered it last week. I'm setting it up now and should have it ready to image sometime next week if the weather is good.

That is a fantastic image. I do have a question - what is that green area on the right?

no idea... i reprocessed to get it out. overall this is a better edit.

Lyle Aldridge wrote:

Vishy_b wrote:

Hi,

Nice data on the Flame & Horsehead - it seems to be shaping up well.

I shall be following this out of curiosity when you find out the reason for the 5th spike and post it.

I think there is some other artifact probably caused by internal reflection on the right.

Regards,

Vishwanath

I neglected to mention that green object, but if I had to take a really wild guess, it looks to me like a distorted reflection of the secondary mirror, perhaps with some kind of defect - a coating defect, a chip, or some oddity in mounting ? - producing a dark streak in one direction and a flare of light in another. Whatever it is, I strongly suspect it's another product of whatever is causing that fifth spike. I am wildly conjuring such weird possibilities as a secondary that somehow got mounted with the wrong surface forward, or that has some kind of surface or edge defect like a drop of adhesive projecting from one of its edges. Or it could be a reflection off the inner surface of the focuser tube, or of the adapter used to mount the camera. When one starts WAG-ing, possibilities are endless.

That is such a nice image overall that the artifacts would drive me nuts. Had the OP not already decided to live with them, I might advise posting this image at cloudynights in either the reflectors or ATM/optics forum, to see what participants there might think.

i dont love them! its just that right now the are not the most concerning things to me at this point. i want to get more comfortable with everything before i go looking to fix thses minor problems

Sir Canon wrote:

RustierOne wrote:

Lyle Aldridge wrote:

It's difficult to believe that experts examining the scope could not identify the cause of such an artifact. There may be a clue in the fact that your four "normal" spikes are also doubled. That usually means that the opposite spider legs are not exactly opposite from each other.

I agree with Lyle's comments above. Not every telescope owner or sales person is necessarily an expert. But did those "experts" assessing your telescope really try the telescope at night, examining the diffraction pattern of an out-of-focus, bright star? That's really the best way to determine what is causing the artifact. Whether or not these experts can determine the cause, there is something causing the spike. If you can find that cause, then you'll be an expert in that area. Your other photo showed the 5th spike only on one side of the field of view. What that means is only from that position in the FOV does a star see the obstruction in the light path. From other positions in the FOV the obstruction is not visible, which is a clue to the cause.

The doubling of the 4 spider diffraction spikes is easier to remedy. Just adjust one or the other of each offending pair of supports so that their diffraction spikes overlap. There is usually enough extra diameter in the hole in the tube to move the support studs a bit to the side. If not, make the hole larger.

Another "solution" is to just ignore the effect. Overall your photos are quite nice, showing great progress. But in your obsession if these spikes bother you enough, get a curved vane support spider. One of these will distribute the diffraction effects of the supports evenly around the entire 360° perimeter of the central spot. The effect is similar to that produced by any of the refractor, Schmidt-Cassegrain or Maksutov telescopes, which have no spider support diffraction issues. Here is a nice discussion of curved spider supports:

Newtonian Curved Secondary Mount

A 3-vane spider is another option. This will give less total diffraction spread around in 6 spikes at 60° intervals. Some people may prefer that pattern to the usual 4 spikes. With the 3-vane spider there is no problem with double spikes since the 6 spikes don't overlap. The following graphic shows the effects of various methods of supporting the secondary mirror.

Diffraction Spike Comparison

I actually got a 3 vane spader with curved veins with the scope....

im going to look everywhere on the scope so wee whats wrong. however they really dont bug me that terribly much

Yeah, there are plenty of other, bigger issues we have to deal with in astrophotography. It will be interesting to see how the diffraction pattern changes with the curved 3-vane spider installed.

Sir Canon wrote:

I talked to the previous owner of my new setup who has been helping me get it working, he said that the 5th diffraction spike is an oddity of my specific telescope. he had it at multiple telescope shops and asked experts about it. every person could not find anything in the optical path.

i'm just going to think of it as the thumbprint of my telescope, i actually kind of like it because it is unique to me.

also here's a 15x600 sec image of the horsehead from last night. i'm adding more subs tonight.

You have the same colours as jammeymc! I just told him I thought they ware strange, the flame being too red, but IDK.

Lots of detail

With a Newtonian, the focuser often protrudes into the light path when imaging with a DSLR because you have to rack it a long way inwards to achieve focus. That could well be the cause of the diffraction spike.

Mark

sharkmelley wrote:

With a Newtonian, the focuser often protrudes into the light path when imaging with a DSLR because you have to rack it a long way inwards to achieve focus. That could well be the cause of the diffraction spike.

That would explain why the spike is most apparent on just one side of the image. From the viewpoint of the other side, the focuser drawtube would not interfere with the light path as much.

sharkmelley wrote:

With a Newtonian, the focuser often protrudes into the light path when imaging with a DSLR because you have to rack it a long way inwards to achieve focus. That could well be the cause of the diffraction spike.

Mark

i think that is it.

chris gunn wrote:

Sir Canon wrote:

I talked to the previous owner of my new setup who has been helping me get it working, he said that the 5th diffraction spike is an oddity of my specific telescope. he had it at multiple telescope shops and asked experts about it. every person could not find anything in the optical path.

i'm just going to think of it as the thumbprint of my telescope, i actually kind of like it because it is unique to me.

also here's a 15x600 sec image of the horsehead from last night. i'm adding more subs tonight.

You have the same colours as jammeymc! I just told him I thought they ware strange, the flame being too red, but IDK.

Lots of detail

that color is the result od bad settings in my pre processing steps. i ran the untouched raf through dss and got a generally less satureated image but with "normal" color in the flame

sharkmelley wrote:

With a Newtonian, the focuser often protrudes into the light path when imaging with a DSLR because you have to rack it a long way inwards to achieve focus. That could well be the cause of the diffraction spike.

Mark

the focuser is paralell to one of the spider veins... not in between two so maybe that isnt the cause

Sir Canon wrote:

sharkmelley wrote:

With a Newtonian, the focuser often protrudes into the light path when imaging with a DSLR because you have to rack it a long way inwards to achieve focus. That could well be the cause of the diffraction spike.

Mark

the focuser is paralell to one of the spider veins... not in between two so maybe that isnt the cause

You're right, it certainly makes my explanation less likely.

Mark