200P - colimation or poor optics

[gary1968]

Yeah, good luck mate.

[malc-c]

Found the problem.... it's the spider, or more correctly it's positioning in relation to the colimation.

Basically, 1st images showed the same issue, only juts a single additional spike, so I started moving the spider screwed ever so slightly in one direction, and noted that the angle of the spike to the nearest defraction spike reduced. So to prove a point I moved it around another 1/20th of a turn and then re-collimated with the laser and took a further picture (having checked focus again) and now the spike is on the opposite side of the defraction spike. The hard part is trying to work out which spider bolt is needed to move in relation to the angle of the camera, especially as the scope did a pier flip between images !

The attached screen shot gives shows the difference - both were 20 sec at 800ISO (just one was in twilight !)

[Naemeth]

Found the problem.... it's the spider, or more correctly it's positioning in relation to the colimation.

Basically, 1st images showed the same issue, only juts a single additional spike, so I started moving the spider screwed ever so slightly in one direction, and noted that the angle of the spike to the nearest defraction spike reduced. So to prove a point I moved it around another 1/20th of a turn and then re-collimated with the laser and took a further picture (having checked focus again) and now the spike is on the opposite side of the defraction spike. The hard part is trying to work out which spider bolt is needed to move in relation to the angle of the camera, especially as the scope did a pier flip between images !

The attached screen shot gives shows the difference - both were 20 sec at 800ISO (just one was in twilight !)

I've been watching this thread with interest, as I was sure it wasn't the collimation that was out. It's good to see you finally know what the problem is, hopefully now you can solve it, and only have the 4 diffraction spikes, as opposed to 5 or more.

[Freff]

Nearly there then Malcolm. It's great when a plan comes together. Keep us posted.

[gary1968]

I am glad you have your head round the problem now....... its only a matter of time before you nail it.

Gaz

[gary1968]

I am glad you have your head round the problem now....... its only a matter of time before you nail it.

Gaz

[malc-c]

I'm giving up now.... migraine coming on and I'm getting fed up of re-colimating over and over again. I've just re-centred the spider using the laser and a length of 5mm rod through the hole with washers and boldt either side. The Laser struck the bar and then used the plasitcard and a pencil to mark off the distance from the edge of the tube and centre of the bar that stick out, constantly adjusting and re-marking until the pencil mark hit the centre from each opposite direction. Re-fitted the secondary and re-colimated.... here's the result (with LP and coma corrector fitted)

Seems familiar

[malc-c]

Looking at the image, and compare it with the test images without the coma and LP filter, I don't care much for the way they soften the detail compared to images takeb with just the camera !

[malc-c]

OK I'm all colliated out !

Removed the focuser and checked the calculations to find it's centre spot, which was re-marked on the inside of the OTA. Focuser re-attached and laser used to square it up so it hit the centre mark. Spider installed and centred using the plasticard and pencil method as per Dions tutorial. Whilst the laser was still fitted in the focuser the position of the spider was checked using a length of threaded bar (again as per Dion's tutorials) and it was spot on. The secondary was then fitted and the scope collimated using site cap, cheshire and hotech.

If it's still producing the rogue diffraction spikes then I'll be taking a hammer to it or listing this lot on e-bay and converting the observatory to a workshop !

[Black Knight]

I feel your pain, I really do. I've been trying to sort out eggy-shaped stars for 9 months, and although I made a little progress, nothing I do any more makes any difference. I've lost count of the number of times I've stripped my Quattro down and rebuilt it. Each time I feel confident that it's all done right and look forward to testing it just to find the same problem is still there. Like you, I'm just about ready to chuck it in the bin

[Moonshane]

could you make a mask to cover the whole aperture other than one quarter (aligned with your secondary vanes)? moved and photographed sequentially around the aperture, this might allow you to narrow down the area causing the issue.

[Freff]

could you make a mask to cover the whole aperture other than one quarter (aligned with your secondary vanes)? moved and photographed sequentially around the aperture, this might allow you to narrow down the area causing the issue.

I was thinking something similar. Attaching a thin blade to each of the vanes in turn, and noting if it appears on the same as the artefact.

[Moonshane]

personally I think the vanes are a red herring. you could make them 5mm thick and I doubt the diffraction spikes would be much different. I am convinced this is an artifact of something else in the way (such as the focuser).

[malc-c]

personally I think the vanes are a red herring. you could make them 5mm thick and I doubt the diffraction spikes would be much different. I am convinced this is an artifact of something else in the way (such as the focuser).

Uhmmm - looking at the focuser, when in focus with the camera attached the draw tube protrudes approx 7mm into the OTA. There is nothing else that is obstructing the optical path. If I get the same spikes after all this faffing around then I'll try the 3/4 obstruction suggestion, otherwise I'm juts going to have to live with it

[Mr Flibble]

Been doing a bit of googling on diffraction spike problems and found this article :

Other telescope types show similar (but usually brighter) Airy Rings, which in reflectors may be accompanied by diffraction spikes radiating from the star. The spikes are caused by the spider arms. Each arm generates TWO diffraction spikes 180 degrees apart, so 3 arms will generate six spikes and 4 arms will generate eight spikes. However with 4 arms exactly 90 degrees apart, four of the spikes will overlap the other four spikes and be "hidden" from view.

Is it just a simple case of the angle between the four spider arms are not 90 degrees when tightened up in the OTA and so the spikes aren't overlapping correctly?

[Jason D]

There is nothing else that is obstructing the optical path.

Well, there is something that has been bothering me about one of your photos. Check my attachment. It has a photo from your focuser versus one from my focuser. Note how the spider vanes clips are showing in yours. These are in the light path and will have an impact on spikes. I do not know if these are the reason but I wanted to point them out. Of course your focuser drawtube protrusion will have an impact though it is hard to quantify it. Someone has already suggested using a mask. Make an O-ring mask covering these clips and the focuser drawtube protrusion and see what happens.

Jason

[malc-c]

David I think you are right.

Having spent ages doing the daylight setting up I took the 1st test image and whilst it was nice, it still had the same spike (see 2nd image). I then placed a triangular fold of paper over each of the spider arms, and found one that caused the spike to vanish ( 1st image below). I then tried adjusting the arms at 90 degree to it in order to try and reduce the affect, and whilst I managed at one point to get a nice fine line with no interference pattern on one axis, trying to do the same on the other without upsetting the other was difficult. I once came close to getting it to an acceptable image, but then checked the colimation and made the small adjustment to get the laser re-centred and the primary squared which resulted in the double spiked image as per previous posts. So whilst it is related to the positioning and angle of the spider arms, it must also be something wrong with the complete colimation ?

[malc-c]

Well, there is something that has been bothering me about one of your photos. Check my attachment. It has a photo from your focuser versus one from my focuser. Note how the spider vanes clips are showing in yours. These are in the light path and will have an impact on spikes. I do not know if these are the reason but I wanted to point them out. Of course your focuser drawtube protrusion will have an impact though it is hard to quantify it. Someone has already suggested using a mask. Make an O-ring mask covering these clips and the focuser drawtube protrusion and see what happens.

Jason

Jason,

I think that's because to take that image I had nothing in the focuser draw tube. When viewing with a cap or cheshire, I'm sure they are not there. How would I remove them from the optical path if they are having an impact ? Pushing / pulling the secondary near / further away from the primary will mean the secondary will not be central under the focuser, so that won't help.

[Jason D]

Can you measure the distance between opposite clip edges as shown in the attachment? That distance needs to be greater than the diameter of your primary mirror by few millimeters.

[gordonewen]

Hello Malcolm, good to hear from you. David was here on Weds and told me about your problem. The only thing I can think of has already been suggested I think.

I am no optics expert but th the following would seem to make sense. As you have a secondary spike which originates at the centre of the image it suggests to me that you may have a seconday vane which is slightly twisted. If you were to look straght down the tube you would see the that where the vane attaches to the secondary it appears as a very thin knife edge, or blade. As you look to the point where the vane attaches to the tube it would, if the vane is twisted, appear broader, or less blade like. this would mean that light at the tube end of the vane would see 2 edges to diffract on. I assume this would create 2 diffraction spikes, presumably of the same intensity. Other than that I have no other suggestions!

Btw, observatory almost complete; needed a winch to move the roof, but it works a treat!

Cheers

Gordon

[malc-c]

Hi Gordon, and thanks for the input.

Looks like I'm going to have to continue with the investigation as it would appear that items such as replacement spiders for 200P's are to order only - from China, so could take upto 12 weeks ! - Logically thinking about what you's said it seems right, in that the defraction spike is radiating from the centre, rather than being parallel to the "original" so to speak. So in the left image in last nights post, I placed the obstruction over one arm and the additional spike wasn't present, so does this indicate it's that arm that has the twist, or the one opposite given David's (Mr Fibble) explanation above.

Glad to hear the observatory is coming along - will have to arrange a visit when its complete

[squeaky]

How about taping a light plastic ruler securely to the suspect spider vane so that it points out of the OTA. It should (obviously) look to be standing out square at 90 degrees. You just might be able to spot which one is twisted. Especially if you have two of them and can do opposite pairs at the same time. That way you have an immediate and clear comparison.

Maybe

[malc-c]

Well I can live with this.... it's not ideal, but I'm getting fed up pithing about and not taking advantage of these skies. I've been twisting and tightening spider arms, and collimating the life out of the scope, and I still have not got to the bottom of what's causing this... but it's not as bad as it has been

Thanks to everyone for their input... I might still try and tweak the thing some time in the future, but for now I want to try and get some subs in

[JamesF]

Perhaps an artificial star might help for tweaking? You'd not need to spend "clear sky time" on it then.

James

[Teslar]

Just a thought. What are you using to take the images. Could the fault lay there. If the imaging device isn't square to the scope or maybe play in the focuser putting it out if line?