Image for collimation testing

[davidpretorius]

Hi All,

I have centre spotted my primary, and aligned it with a cheshire, but I would interested to know what every one thinks of this image i have stacked of canopus.

I am assuming the flaring to the bottom right would indicate collimation issues.

Thanks

Dave P

[slice of heaven]

How did you take the image Dave ? Just the Toucam , no ep ?
The double horizontal diffraction spikes, is that from stacking several images ?
Have you checked for pinched mirrors, both primary and secondary ?

[davidpretorius]

This was an early version before collimation basically out of the box, I have since centre spotted, and done up the screws holding the primary only finger tight .

Image was taken with a toucam and stacked in registax,

I will retake an image and have the "after" image.

[[1ponders]]

The double spikes indicate that the image is still not quite in focus. Why only the horizontal one are double I don't know???

[westsky]

Hi David try taking an out of focus image, on both sides of focus would be best, it will tell you much more about your collimation.

cheers
David.

[astro_south]

The double defraction spikes indicate that the spider vanes in one direction aren't quite aligned together (I have the same with my 12.5" dob). The below diagram (loose use of the word:) ) may help you understand. It is a top view of your tube looking down on the spider vanes:

ie
..............|
..............|
....(------O______) <- notice the offset of opposing vanes :(
..............|
..............|

..............^ -Fine this way :)

If you can't understand the diagram I get back to me. To see if this is the thing that is creating your double diffraction spike you can just look along the line of the spider vane and you should be able to pick the miss-alignment. If it is...how do you fix it? If you spider is completely adjustable you should be able to "twist it out" through adjustments, however if the spider vanes are fixed (like my dob :( ) then you just have to live with them or replace your spider (as I eventually will....one day :) )

[davidpretorius]

thanks andrew, i will give it a go tonight, weather permitting

[astro_south]

David

You can do the check anytime - remember you don't have to look through the telescope, just "eyeball" the alignment of your spider vanes from the top of your scope. I have attached a picture I just took of my upper cage assembly that shows the offset in the oposing spider vanes (in the directions of the arrows). It is subtle in the photo, but is more noticable in person. If you extend one of the spider vanes past the centre with an imaginary line you can see that the extension doesn't overlay the opposing vane. You can also notice from the picture that I have no flexibility in adjusting these vanes, so eventually I will replace them (possibly with a curved vane).

[asimov]

This tells me the spider is not made correctly (on Daves scope) Theres no way to fix that, that I can think of? Is there a warranty issue raising it's ugly head here?

[astro_south]

John

I think that Dave's scope's spider will be way more adjustable than mine :), but those with GS dobs will be better able to answer that. I only notice the double defraction spikes on the brightest stars so it may not be a big issue.

[asimov]

Apart from 2 90 deg bends close together.....Hmmm, interesting.

[astro_south]

Dave

Have you checked the alignment of the spider vanes yet? If so, how did they look?

[davidpretorius]

looks ok, here is a piccie.

too many clouds last night to get a bright star to try again after collimation.

i will give it a rip tonight, nice bright star , maybe venus, take a video, stack it and compare them with you guys tonight

[astro_south]

Dave - certainly looks OK going by the picture.

How far out was your collimation?
Do the individual frames of your stacking also show the double spider vane?

[iceman]

It doesn't have to be too bright, I wouldn't do it on venus.

Do it on a medium-bright star, and don't have the gain cranked up too high. Some of the flaring around the star can be caused by overexposure.

It can also be caused by astigmatism - have you followed Geoff's how-to and checked your primary and secondary for pinching?

[RAJAH235]

Davo, I'm not sure if it's just the angle/position that you took the pic from, but the top left/right of focuser, spider, looks slightly 'curved' to me. I put a rule against the monitor,(flat), & it lines up with the B/left to T/right, but not with the other 2. Could be just the angle.
I get a similar thing with mine, just out of focus. When in focus it's gone. So I use it as a guide sometimes. Helps to have an electric focuser tho.
HTH. :D L.

[davidpretorius]

thanks for the help guys

yup mike when i spotted my primary, i loosen the screws for pinching.

raining hard tonight, so no stars to try, but first oppotunity, me and the toucam are going to work on the dob!

[asimov]

Hehehe! I did the same thing Laurie! With the ruler I mean...The 2 near vertical vanes in the pic look curved, yeah, but also one looks 1/2 a vane thickness not aligned with the other If you know what I mean to me ?

[ballaratdragons]

Dave,

don't panic too much about the vains being out of alignment.

GS use a very thin vain material (which is good).

I had the same prob and all I did was twisted the 2ndry housing a bit and they lined up. I checked by placing a rule along them. Perfect!

Luckily GS vains are so thin, the thinner the better!!!

[ballaratdragons]

Oh, also, make sure the vains are perpendicular to the light path. They can twist a bit. Just twist them back!

[acropolite]

I would guess that the vanes being not aligned with the light path as BD suggests would be the cause of the double spike.

[ballaratdragons]

Thanks Phil! :thumbsup:

:love:

[[1ponders]]

David, I just had a thought.

Before you go twisting spider vanes have a good look at your avie frame by frame in registax to see if there is any up and down movement in the image. If you don't know how to do that, click and hold on the right hand side of the slider at the bottom (just like a scroll bar) and it'll scroll through all the images. What may have happened is that registax might not have lined all the images up correctly. It's simple enought to check the images and untick some from the frame list and run the prog again. If you don't know how do untick them from the frame list, use the space bar. The unticked frame will show up beside the scroll bar as a red square.

Rather that first than trying structural deformation on your vanes. If you do and accidentally stuff it up you might pop one of your own veins.

[davidpretorius]

i will definately wait for a new vid that i will take first before any structural changes.

i have collimated since, so i hope to have the motorized focussing happening so i can take a vid for collimation for noobies to show them what to look for and sort out if it is registax or the veins

[astro_south]

If the veins are angled to the light path they will only produce a wider spike - not a double spike. I also noticed the curve in the vanes in Dave's photo but assumed it could have been due to the camera lense. On closer inspection I think your right Laurie - certainly something to look at. The clear double spikes on the left of the star in Dave's original image don't exactly match the oposing spikes on the right - in that the ones on the right seem to converge together. Perhaps this is strong support for the curved vane that Laurie interpreted. There is still something not aligned to produce those dominant double spikes on the left though - I don't think Registax could missalign them like that in a stack.

Remember that with a four vane spider there are actually 8 diffraction spikes that eminate from the airy disc - it is just that they pair up giving the illusion of only 4 diffraction spikes. A 3-vane spider will have the impression of 6 diffraction spikes eminating from the airy disc.

[slice of heaven]

Dave,moving away from whats causing the double spikes, have you checked the 'secondary' for pinching??????

[davidpretorius]

Ok, i have taken a video of Rigel Kent. I am wondering if i did not do this before and confuse it with Canopus. Anyway, Rigel is a double star.

So tonight i got some video and have stacked it.

Of course i will get double diffraction if i have two stars????

[[1ponders]]

Quite likely David. Check out this link about focusing with a hartmann mask. It covers a similar issue. http://www.iceinspace.com.au/forum/showthread.php?t=412&highlight=mask

[davidpretorius]

thanks, my next project is a hartmann mask i may get the kids involved today in some scissor work

[acropolite]

Dave, you should be able to easily split Rigel Kent @ around 75x mag and it will appear as two distinct stars with noticeable separation. Whether you can accomodate that mag with your combination of ToUcam and EP's I'm not sure. If you can, bump up the mag to around that value and see if you can image the two; if not call Houston as I would say you have a problem...One other thought, I seem to remember that you're using the ToUcam afocally and the double spikes could be simply reflection between the EP and Toucam lens surfaces....

[pmrid]

Hi Paul. This is an old thread I know but more or less on topic for the question I'm asking.
In the attached image, the diffraction spikes are not double but split. Does this suggest collimation and/or tracking to you?
It's a single 5 min sub at ISO 200 - I was trying to do some short exposures of Alnitak so I could layer them into some brighter images taken a few nights back.

Peter