alnitak spikes

[alacant]

Hi everyone. Of 4 spikes, two are as I would expect whilst the other two are double. All stars in the frame have the same two spikes double. Attached is a close up of a light frame: Canon 700d. Any ideas? TIA and clear skies. 

[cjdawson]

Was this properly centered in the FoV?   If not, look in the opposite corners of the image and see if the opposite effect is happening.

Is the scope properly collimated?  Both the primary and secondary mirror.

 

[alacant]

Hi and thanks for the pointers. The effect is happening with the same spikes no matter where it is in the image. Yes. I've got the secondary in the centre of the primary and adjusted the return beam of a -correctly aligned- laser. The train is primary-secondary-lp filter-cc-sensor. I'll attach a processed snap. Has anyone seen this effect? TIA.

[richyrich_one]

Usually caused by slightly twisted spider vanes.

I have the same problem since redoing my secondary position and twisting them slightly in the process.

[cjdawson]

Twisted vanes.  Sounds painful, but does explain it.   You can get bent spider vanes for newts that disputed the spikes altogether.

[alacant]

Hi and thanks for the pointers. I checked the vanes with a steel rule held up against them pointing down the tube. It looks for all the world parallel with the tube on all four vanes. Are we talking very small distances here? Are the vanes visibly twisted? Mine really do look perfect. In suppose with the emphasis on look! Is there a way of making them optically straight?

[cjdawson]

They look the same with a steel rule.  As two of them are fine, I'd guess that it's not that issue.

Could is be collimation?   I'm talking secondary collimation rather than primary.

If something is at an angle that shouldn't be, it might explain things.

(From what I head, secondary collimation is a pain in the backside to get right, as you need to do it in x,y,x rotation and y rotation to get everything to line up properly.  Oh and of cource, the primary will need to be adjusted slightly too when the secondary is moved.

I believe that are methods out there to make that easy - and to check it.  As an SCT owner, I don't need to know it, and kinda haven't had to go though that pain.

[richyrich_one]

It should be just visible looking down the tube.

I presume you checked all the way along each vane?

The thing is any kind of minor obstruction can cause this effect. I've had it caused by shooting too low with my obs wall getting in the way, also shooting with a branch in the fov has done it. Off centre dew shield may be? Focuser intrusion can do it, although it would only be one spike and the one where the focuser is.

[alacant]

57 minutes ago, cjdawson said:

haven't had to go though that pain

Yeah, I really hate reflectors. I've a 6" refractor which is great. No spikes, no collimation. Ever. It just works. I got the reflector cos it's half the weight. I'm beginning to think it's more trouble than it's worth. Still thinking... Everything looks dead centre with the collimation cap and checks with the laser... And the photos are good. Give up?

[alacant]

Any tips on dealing with Anitak in general? It makes most snaps look a mess. Mine more than most.

[cjdawson]

@Uranium235 did a processing workshop at the March 2016 which showed a way of processing some images taken from that region.    There was step by step guide which I think would be useful for you, as it did involve making sure that none of the stars went out of control.

May he'd be kind enough to share that with you (it's his work, so I feel that I'm not at liberty to pass on the document without permission) as it could be useful for you.

[Uranium235]

5 hours ago, cjdawson said:

@Uranium235 did a processing workshop at the March 2016 which showed a way of processing some images taken from that region.    There was step by step guide which I think would be useful for you, as it did involve making sure that none of the stars went out of control.

May he'd be kind enough to share that with you (it's his work, so I feel that I'm not at liberty to pass on the document without permission) as it could be useful for you.

I've no problem with that mate, if you want to PM it to the chappie please do. I think you may have both SGL XI and XII documents (the XI instructions should be more suited to the task).

[cjdawson]

2 hours ago, Uranium235 said:

I've no problem with that mate, if you want to PM it to the chappie please do. I think you may have both SGL XI and XII documents (the XI instructions should be more suited to the task).

Yep, I've got both.   Yeah, I'll send a pm.   Wanted to make sure everything was OK before doing it.

[Adam J]

Looking at your spikes they are actually not that bad compared to some I have seen...mine for example. It will be a twisted spider vane but it will only be very very slight to cause the amount of distortion you have. You would be shocked how small an amount would cause that.