Jump to content

Stargazers Lounge Uses Cookies

Like most websites, SGL uses cookies in order to deliver a secure, personalised service, to provide social media functions and to analyse our traffic. Continued use of SGL indicates your acceptance of our cookie policy.

malc-c

200P - colimation or poor optics

Recommended Posts

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?

Thanks for the comments. I get the same results using two different cameras, my 400D and Davids 450D were used to rule out a defect in the camera...

Share this post


Link to post
Share on other sites

Interesting... I've been trolling through my archives and came across the attached image of Sirus taken around a month after I first purchased the scope in February 2011. Ok, ignore the poor tracking / PA, hot pixels and lack of focus, but you can still make out the same spike.... so it would seem that this issue has been with the scope ever since purchase.

Maybe it's simply a case of mass produced optics and I was unlucky ?

post-10726-0-84843900-1347200838_thumb.j

Share this post


Link to post
Share on other sites

Right now I've got the resistance heater on my secondary "secured" with bits of gaffa tape, most edges of which are lifting, so my secondary's profile looks more like a sunflower, and the cable for it is taped to a spider arm and some of those bits are lifting and my diffraction spikes are still cleaner than yours. (As in my recent Pleiades post).

It's certainly a very ODD problem. Ask your retailer to check back with Skywatcher to see what they say????

I know you're well out of guarantee but the problem has demonstrably been present from day one and has not been amenable to a solution available to the user (you) - maybe they can tell you what causes it even if they won't offer a free fix?

Share this post


Link to post
Share on other sites

I've held off fitting power resistors to the rear of the secondary for now, giving all the advice about having obstructions in he optical path, especially as the wires have to come out via the spider arms. If you've done this mod and have wires attached to the spider with tape, and still get a nice clean diffraction spike pattern then that really blows that theory out of the water.

More to think about !!

Share this post


Link to post
Share on other sites

Take a look at the Pleiades image and if you think the diffraction spikes are OK you might as well try it.

What you could do is just tape a bit of cable along the top of a spider arm as if the heater was there and see if it makes any difference to your images. Yes/no? If it makes no difference you can go for it; otherwise don't.

Share this post


Link to post
Share on other sites

Forgot to say - I have such a frequent and heavy dew fall here that in any event the benefits of a dew free secondary far outweigh any possible oddities on a diffraction spike. That's just my own situation and my take on it. Yours may vary.

  • Like 1

Share this post


Link to post
Share on other sites

Take a look at the Pleiades image and if you think the diffraction spikes are OK you might as well try it.

Can you provide a link to the image ?

Share this post


Link to post
Share on other sites

Malcolm,

I haven't followed all the comments on this thread, and this might have been mentioned before, but are you sure that the fifth spike is due to diffraction? If it is diffraction then there should be a second spike associated with it on the opposite side (180 degrees) of the source (imaged) star. Any diffraction spike should be paired about the source (in this case the star you've been using for your test, Vega) and there should always be an even number of spikes. An SLR camera IRIS, with five petals, will have a pentagonal aperture and each straight edge of the aperture should produce a pair of radial spikes giving a total of ten spikes evenly spaced in angle. For the secondary spider there are effectively two orthogonal straight edges (the vanes opposite one another through the centre are effectively one straight edge) and there should be four spikes. If one vane isn't at 90 degrees to the others (say the vanes are at 0, 175 and 90, 270 degrees) then the correctly aligned vanes will produce the four "classic" diffraction spikes at 90, 270, 0 and 180 degrees and the misaligned vane will produce additional spikes at 85 and and 265 degrees. I'm not sure this is what you're seeing as the additional spike is unpaired. The same effect should also be seen if the vane is twisted as the width of the vane is tapered. The rear edge of the vane (facing the primary) is angled and any rotation of the vane will place the rear edge of the vane at a slight angle compared to the upper edge. The rear edge will still produce a pair of diffraction spikes at a slight angle to the main quartet - giving six spikes.

I also wonder if the additional spike you're seeing is due to an artifact in the readout from the CCD sensor. We occasionally see an effect with our X-ray CCD detectors when a very strong Bragg reflection can cause a very bright spot on the image with a tail in the direction that the pixels are being read out. The charge accumulated in each pixel during the exposure is shuffled along in rows to be counted at the end of the row during read out. If the intensity is strong enough then charge will accumulate in what were adjacent pixels as they are shuffled under the spacial position of the Bragg spot during read out. To minimise this effect we synchronise the detector read-out with a fast shutter, which blocks the X-ray beam. It could be the case that for very strong intensties on the CCD sensor of your camera there are additional counts being registered on the pixels in the milliseconds it takes for the image to be read out. If the CCD read out direction is aligned with one of the diffraction spikes you'd never see this effect. If there is a very slight misalignment then the result would be a single spike radiating out from the bright spot (or the star on your image). It might be worth rotating the camera very slightly to see if this has any effect.

Anyway, sorry for the ramblings.

all the best and good luck

Dave

  • Like 1

Share this post


Link to post
Share on other sites

Well the other night I tried tweaking the spider a bit and ended up making it worse, so today I started from scratch (again), only this time I've removed the veins from the spider and checked them for squareness and flatness. I aligned them using squared paper to ensure they were flat and at 90 degrees to each other. I then used a pencil to extend the line of the veins which intersected the center bolt. The spider was then carefully placed in the OTA and cented using a length of plasticard and marking it to the centre lines on the spider from the edge of the tube, repeating the process over and over until they were equidistant. I then followed the normal procedure of siting the secondary in the focuser and using a combination of cheshire site tube and Hoteh laser to get the collimation right.

One thing I noticed that often after doing so the secondary never looked centred correctly, as if the collimation process had upset the original positioning and it was lower in the view (the OTA in its parked position is upside down). I had originally centred the focuser as per Dion's website, so in theory it shouldn't be off axis, however I search the net and found one site where the author suggest centering the secondary in the sight tube by using the spider adjustments, obviously this is just in one plane, so I used this and after doing so the collimation appears to be as close to the drawings / photos on the websites. So now I just need a gap in the clouds tonight to see what difference (if any) this has made.

I must admit that whilst this has helped me learn the collimation process, I was at one point this afternoon seriously considering throwing in the towel and either sell up completely and convert the observatory into a workshop or PX the 200P for a refractor...both have their plus and minus points. It is certainly one of those "I wish I hadn't flocked the tube" moments !

Share this post


Link to post
Share on other sites

Well the other night I tried tweaking the spider a bit and ended up making it worse, so today I started from scratch (again), only this time I've removed the veins from the spider and checked them for squareness and flatness. I aligned them using squared paper to ensure they were flat and at 90 degrees to each other. I then used a pencil to extend the line of the veins which intersected the center bolt. The spider was then carefully placed in the OTA and cented using a length of plasticard and marking it to the centre lines on the spider from the edge of the tube, repeating the process over and over until they were equidistant. I then followed the normal procedure of siting the secondary in the focuser and using a combination of cheshire site tube and Hoteh laser to get the collimation right.

One thing I noticed that often after doing so the secondary never looked centred correctly, as if the collimation process had upset the original positioning and it was lower in the view (the OTA in its parked position is upside down). I had originally centred the focuser as per Dion's website, so in theory it shouldn't be off axis, however I search the net and found one site where the author suggest centering the secondary in the sight tube by using the spider adjustments, obviously this is just in one plane, so I used this and after doing so the collimation appears to be as close to the drawings / photos on the websites. So now I just need a gap in the clouds tonight to see what difference (if any) this has made.

I must admit that whilst this has helped me learn the collimation process, I was at one point this afternoon seriously considering throwing in the towel and either sell up completely and convert the observatory into a workshop or PX the 200P for a refractor...both have their plus and minus points. It is certainly one of those "I wish I hadn't flocked the tube" moments !

Have you got any local shops around you that can do repairs (assuming this try doesn't work)? This would probably sort it out.

Or, I suppose you could contact Skywatcher for a replacement secondary with spider vanes, and ask them how far the secondary needs to be from the primary.

I really hope you can get this sorted, wouldn't want you to sell up.

Share this post


Link to post
Share on other sites

Have you got any local shops around you that can do repairs (assuming this try doesn't work)? This would probably sort it out.

Or, I suppose you could contact Skywatcher for a replacement secondary with spider vanes, and ask them how far the secondary needs to be from the primary.

I really hope you can get this sorted, wouldn't want you to sell up.

There are two places that are around an hour or so's drive, Greenwich and David Hinds, so I might try and contact them if all else fails and let them see if they can sort this out. The problem with this hobby is that it's difficult to find someone who really knows what they are doing to teach others. I would love to know how the SW 200p is assembled and collimated at the factory

Share this post


Link to post
Share on other sites

There are two places that are around an hour or so's drive, Greenwich and David Hinds, so I might try and contact them if all else fails and let them see if they can sort this out. The problem with this hobby is that it's difficult to find someone who really knows what they are doing to teach others. I would love to know how the SW 200p is assembled and collimated at the factory

I can imagine David Hinds would certainly know what he's doing. Have you tried contacting Skywatcher with your problem though, they just might be able to help.

Share this post


Link to post
Share on other sites

Well initial shot showed it was way off (on the left). I've re-collimated 4 times having the secondary from close in to far out from the primary, and each time the spike has been present. The last attempt can be seen on the right, and whilst not ideal, seems to show an even halo around the star..... so I think I'll have to live with it.

What's needed is for someone to invent a colimation camera, so it takes all the tollorances out of the individuals eyesight..... this way you could completely set the optical path up all nice and square without any of the assumption that things "look" right.

Mind you my trials were hampered somewhat as the folks next door waited until i had rolled back the observatory to have a bonfire... and the wind was blowing in my direction. The observatory stinks, and I'm hoping that the wind will clear what's hanging around soon. She had forgot our agreement, that she wouldnot have a bonfire if it was clear... she never looked up !

post-10726-0-39713000-1347741667_thumb.p

Share this post


Link to post
Share on other sites

Well initial shot showed it was way off (on the left). I've re-collimated 4 times having the secondary from close in to far out from the primary, and each time the spike has been present. The last attempt can be seen on the right, and whilst not ideal, seems to show an even halo around the star..... so I think I'll have to live with it.

What's needed is for someone to invent a colimation camera, so it takes all the tollorances out of the individuals eyesight..... this way you could completely set the optical path up all nice and square without any of the assumption that things "look" right.

Mind you my trials were hampered somewhat as the folks next door waited until i had rolled back the observatory to have a bonfire... and the wind was blowing in my direction. The observatory stinks, and I'm hoping that the wind will clear what's hanging around soon. She had forgot our agreement, that she wouldnot have a bonfire if it was clear... she never looked up !

What happens when you defocus on a star, do you get 4 diffraction spikes, or 5? (Visual only)

Edited by Naemeth

Share this post


Link to post
Share on other sites

long defraction spikes !!

Here's a 60 sec exposure..... man those spikes are long !

post-10726-0-10303700-1347743781_thumb.j

Share this post


Link to post
Share on other sites

Well I'm going to leave things as they are.... I would like to thank everyone who has contributed to this thread and for the support given.....

I'll leave you with this 180 second sub of M27. Raw off the camear apart from a rotation and auto-levels on Photoshop... I can live with this

post-10726-0-76142000-1347745570_thumb.j

Share this post


Link to post
Share on other sites

Well I'm going to leave things as they are.... I would like to thank everyone who has contributed to this thread and for the support given.....

I'll leave you with this 180 second sub of M27. Raw off the camear apart from a rotation and auto-levels on Photoshop... I can live with this

After all that worry, it all looks fine on M27, I don't think you need to worry, especially as it doesn't seem to show on what you are trying to image. It looks very blue, but processing will probably make the background darker right?

Share this post


Link to post
Share on other sites

Have to agree, the subs are coming out quite tight, maybe I was being too picky ? The blue is caused by the CLS LP filter, which baiscally blocks nearly all red light... I might end up changing that at some point, plus the camera is un-moddified so that won't help either.....

Share this post


Link to post
Share on other sites

Have to agree, the subs are coming out quite tight, maybe I was being too picky ? The blue is caused by the CLS LP filter, which baiscally blocks nearly all red light... I might end up changing that at some point, plus the camera is un-moddified so that won't help either.....

As long as you don't image a bright star, you should be fine :).

Share this post


Link to post
Share on other sites

LOL - just stacked the subs from last night and the spike comes out in the bright star near M27 :)

But I'm otherwise happy with the results. Here's 35 x 180s @ 800 ISO plus 20 x 180 darks - no flats etc, all stacked in DSS and then tweaked in PS and reduced image size to suit the forum (hopefully the colour balance won't change too much in the upload)

Uhmm - seemed to pull the red out more than what's shown on my monitor......never mind !

post-10726-0-50525700-1347812292_thumb.j

Edited by malc-c

Share this post


Link to post
Share on other sites

Nice image the detail is good, but where is the green?

Share this post


Link to post
Share on other sites

The problem isn't too noticeable to be honest, it requires a good zoom in to see it :). Nice image.

Share this post


Link to post
Share on other sites

Thanks guys. In PS the image is very green, with little red. Adjusting the colours in PS to display them "correctly" on the monitor here results in a saturated Red on the forum upload.... still need to play about with the post processing me thinks :)

I agree that the spike is not that noticeable unless viewed at fill resolutions (zoomed).

Share this post


Link to post
Share on other sites

Here's the raw TIF (cropped so it falls below the 20mb file limit) from DSS - please feel free to post process if you want a bit of fun

m27 crop .tif

  • Like 1

Share this post


Link to post
Share on other sites
Guest
This topic is now closed to further replies.

  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.