11 replies to this topic

[DonR]

I have a new 8" f/4 imaging newtonian, and a new Baader MPCC coma corrector that I use with it.  Last night I acquired some images that contained a relatively bright star (~mag 3) for the first time with this gear.  In the images I see a prominent third diffraction spike, and I am trying to determine what causes it.  I went back and looked at some other images with the same gear from a few weeks ago, and I see a trace of this extra spike on the image of a mag 7 star, but not on the image of a mag 3 star aquuired a few days earlier.  So it appears that something "happened" a few weeks ago.

 

I have examined the equipment - no spider webs in the OTA, no scratches on the primary mirror, no residue I can see on the MPCC.  There is a small (3mm) blob of something stuck to the primary mirror about a half inch from the edge, but nothing else I can see.  The only glass between the imaging sensor (a Canon T2i DSLR) and the secondary is the coma corrector.

 

I am suspecting the MPCC.  Should I be looking at something else?

 

The star is Propus, in Gemini.  All subexposures contain the extra spike. 

 

Thanks,

 

Don

Attached Thumbnails

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Edited by DonR, 08 March 2015 - 12:25 PM.

[Brian Carter]

Have you tried a quick image without the baader? That would at least let you know whether it was the scope or the corrector.

[MitchAlsup]

You are looking for an edge across the FoV that is 1/10 as thick as the spider vanes.

 

To me, this is not enough diffracted energy to worry about.

[DonR]

Have you tried a quick image without the baader? That would at least let you know whether it was the scope or the corrector.

 

No, not yet.  I just discovered this extra spike this morning when examining last night's images.  Looks like the weather won't cooperate tonight, but I will try an image without the MPCC when I can.

[DonR]

You are looking for an edge across the FoV that is 1/10 as thick as the spider vanes.

 

To me, this is not enough diffracted energy to worry about.

 

I'm not worried about the diffracted energy, but about the appearance of the bright star.  This extra spike really sticks out in the integrated result of stacking a dozen of these images.

[csrlice12]

Weird.  I see the extra spikes at the 10 o'clock and 4 o'clock position, but I also see a dimmer spike forming a narrow X pattern with one just above the 10 and under the 4 position.  Have no idea what could be causing it maybe a hair on the objective or mirror?

[Nils Olof Carlin]

How about a focuser slightly intruding into the light path? Try a defocused image and see what might intrude. It ought to be something at 45 deg from the spider vanes.

Nils Olof

[DonR]

Weird.  I see the extra spikes at the 10 o'clock and 4 o'clock position, but I also see a dimmer spike forming a narrow X pattern with one just above the 10 and under the 4 position.  Have no idea what could be causing it maybe a hair on the objective or mirror?

 

Yes, there is a faint fourth pair of spikes close to the third.  I guess there could have been a hair on the mirror during the imaging session - it wasn't there the next day after I brought the telescope inside though. 

 

Don

[DonR]

How about a focuser slightly intruding into the light path? Try a defocused image and see what might intrude. It ought to be something at 45 deg from the spider vanes.

Nils Olof

 

Hi Nils Olof,

 

It's not the focuser drawtube - it doesn't come anywhere near the light path.  I do see a nick in one of the baffles inside the OTA.  It's the front-most baffle, about four inches back from the front of the OTA.  It's just about 45 degrees from the spider vane, and has two "bumps"  that look to be about the right distance apart to make the two extra spikes seen in the photo - sort of like this:

 

 

------/\__/\------

 

Sorry, I can't get a good photo of it.  The baffle is just in front of the spider vanes  and about 1/2 inch wide, and the bumps are perhaps 1mm high.  I think the bumps are just barely in the light path.  Could they be causing the spikes?

 

I will try a defocused star image when weather allows.  I can remove that front baffle if it is the culprit, and try to remove the nick, or just leave the baffle out.

 

Thanks,

 

Don

[precaud]

Is there any sign of astig in the same axis as the extra spike?

[DonR]

Is there any sign of astig in the same axis as the extra spike?

Hi John,

 

Not that I can tell.  I haven't done a star test yet, but images of focused stars at the center of the field look nicely round.  There is a little coma apparent in the corners even with the MPCC, but it's not too bad.  The image I posted above of Propus is cropped from a position about 2/3rd's of the way from the center to the lower right corner of the FOV.

 

Don

[ruachsheavens]

was the cause of this ever determined? Im really curious to know - thanks

 

Patrick