Collimation- grateful for some advice

[Jason D]

So there it is guys thats got to be the problem after all. What to do now? A new one or get it recoated if thats even possible.

Any experienced observer or imager will tell you that a minor scratch on the secondary mirror will NOT cause image breakdown. If you have a large number of major scratches then the impact is light scatter – still no image breakdown. That is, with a large number of major scratches, your image contrast will reduce a bit and the background will glow a tiny bit – still your image will be sharp. But don’t take my word for it. Ask other experienced observers and/or imagers.

Also keep in mind that every pixel in your CCD uses most of the secondary mirror; therefore, shifting the secondary mirror about my a small amount will not make a difference. The impact of the scratch will still be seen by all pixels.

Jason

[Doc]

I agree with Jason here, I cannot see how a scratch or even a few scratches would cause your image to break down and not be sharp.

[astromerlin]

Well I'm relieved to hear that.

Someone pop over and whak me with a piece of wood, I still cannot see why my images are so poor when it's setup correctly and better when it's not.

Images below (excuse the orange dots) first was taken on the 11th March after a whole day spent collimating correctly with a cheshire. To date this was the best image I've had of any planet. The second was taken on the 15th when I decided to get rid of the "cresent" feature.

This image blew my socks off until I put the camera down the focuser to check collimation. I'm sure the difference is more than just the "seeing".

[Astro_Baby]

Well I am out of ideas - been watching this thread but I cant see any logic to this one at all.

If you were nearby I could whizz ocer and take a look - I know when I se my SW200 the views are superb and thats from other peple who have used it to test their own EPs and so forth.

Have you actually star tested this scope in either of the mirror configurations because I would have thought a star test would settle it one way or the other.

ps forget the laser - that wont help at all becaue what seems to be happening here is some kind of weird secondary issue and a laser wont help much with that. The only thing that would settle this once and for all would most likley be a catseye collimator because that would test all of the optical path out and would show all alignment errors.

[TopHouse]

I know what I'd do in this position, I would strip the whole blumming thing and start at the beginning!

I'm unsure what I'm talking about now but it may create a spark somewhere.

OK, thinking out of the box, when you pull the secondary back, what else does it do?

It changes it's position with regards the focuser.

It changes the distance between primary and secondary.

It changes distance between the secondary and the eye.

I'm still unsure what the 'crescent is you refer to, I was under the impression it was the blackened part of your secondary you were referring to. is it in the image above with the orange dots? if it is, can you put an arrow pointing to it.

Looking at the two planet pictures, I can't help thinking that the 'bad one' is very very slightly out of focus.

could it be a focuser issue? For instance, moving the mirror changes the point on the focuser where you would be bang on focus, now don't laugh but could there be something in the focuser that occurs at the focus point when the bad pic thing happens, something as stupid as a bit of 'gunk' on the focuser tube that causes it to shift or jump, like a car left on a speed bump with the handbrake off, you get it in focuse, start your image and it shifts off the 'bump'.

[Astro_Baby]

Tophouse he is talking about the shoulder of the secondary/edge of focuser. There was a drawing in post 17 on this thread (first page) of the thread.

I cant explain it but my guess would be focsuer misalignment which is what I said up front BUT I would defer to Jason on this as he has more solid know how and accept it may be normal for the scope.

[astromerlin]

Hi, I'm at a loss also. To be honest I've had problems doing a visual star test due to the seeing. I have checked though and before I started on the recent collimation frenzy through the flairing wobbly image all the circles seemed to be close to perfect. I can remember that one day last year when I did a star test the seeing was the best I've ever seen, no flairing and the collimation looked text book. Still I had poor images though.

It is so difficult to say that I should include last years images as I had only started imaging then and knew very little, since then I have patiently perservered, read all the info on the forums and believed that my poor images was due to technique with the webcam settings. I purchased a cheshire collimation tool and set about collimation, funnily enough your guide was the first one I read. Cross referencing with other info and guides I armed myself with as much knowledge as possible and set about it. You know what it's like the first time, you muck it up get it right out, get it back in again etc.

Assured I had it spot on I went out for a few nights and did some imaging, still the same poor images, what can it be.. I looked again at the collimation which was then I noticed the cresent shape in the reflection. I looked at collimation images on the internet to compare what I saw. Non mentioned any cresent in the reflection.

This is when I decided to get rid of it and make what I see down the focuser exactly like the other photos on the web. I then went out and did a nights imaging. The good image of Saturn was a fluke, I didn't even worry about the webcam settings, brightness @ 50% gain @60% gamma always @0 off we go.

I've been using Metaguide in the hope I can get the collimation sorted and I do have star images Metaguide produced on the same nights as the images posted of Saturn.

I cant remember which stars (I must remember to keep a log of these things).

I dont know if these will help, I dont as yet understand them.

cheers

John

[stev74]

I've also been following the thread and few things come to mind -

The Skywatcher's are great scopes (I have one) but they are mass produced in China.

So my point would be maybe your secondary was just machined slightly off. So if I was to look at my secondary lying flat, face up on a table, I'd only see the outline of one oval. You would see a crescent at one side because your secondary is cut slightly wrong ie. cut at an angel at one side so the back is wider than the front. This probably wouldn't affect your collimation just look a bit different to other images of collimation in that you would see a crescent.

Also have you tried another web-cam as maybe there's an intermittent fault with it, which would also cause the differance in your pics.

Lastly are there any astro society's, observing groups or even an astro shop where you could check your scope to another 250p. There maybe an anomaly which might stand out more if you can see it in the flesh next to another scope rather than in a photo on a website.

DISCLAIMER - Just for the record I'm no expert so please ignore any or all of my reply as you see fit. I'm just thinking out-loud a bit and maybe none of the above is relevant but I have heard about problems with the early 300p's. Apparently some of the OTA's were cut to the wrong length and people were having problems focusing without some sort of extender added to the focuser.

Cheers

[astromerlin]

Hi, I have a couple of webcams, 2 Vestas an SPC900nc and a Toucam pro. So no differences in the images with any of them (apart from the Spc900 which is far superior in quality, no dead pixels and less noise) my focus point for all webcams is about 10mm out from the focuser being fully in.

Although FLO are just down the road from me (15miles) I was hoping I could solve the issue myself as part of the learning curve. I think I am now at the point of requiring professional help though.

What I plan to do is re-collimate the secondary correctly and then when this latest bout of rain ceases and I get a clear night I will take it out and try to do a star test and get images of it, use Metaguide to get a profile of the star test and then image Saturn again. If I am lucky and get a couple of successive clear nights I will repeat the tests and then compare them.

If I still have poor images then I'll pop down to FLO and see if they can help.

As to the focuser, This was the first thing that I had to get sorted last year as the slop was really bad when I bought the scope. I have stripped and cleaned and then rebuilt it and now it is the smoothest you could ever get an R&P focuser with very little image shift.

As a matter of course, before I image anything I focus on a bright star and set the webcam to 1-2 secs exposure to really get the diffraction spikes showing well and adjust the focuser until they are as sharp as i can get them and then lock the focuser recheck and then slew to the subject. One thing I did notice is that when I setup for the good image I took of Saturn the diffraction spikes from the star I setup on were a lot thinner than usual.

Well crossed fingers for good weather soon.

Cheers

John

[TopHouse]

I'm praying for the weather too, dying to check if my flocking made a difference.

Have you not flashed your Toucam to a 900? They are the same gubbins and you can flash them to 900 firmware.

[Jason D]

Tophouse he is talking about the shoulder of the secondary/edge of focuser. There was a drawing in post 17 on this thread (first page) of the thread.

I cant explain it but my guess would be focsuer misalignment which is what I said up front BUT I would defer to Jason on this as he has more solid know how and accept it may be normal for the scope.

I provided the explanation in post#35 of this thread. I will explain it differently.

First photo is off my collimated scope via the cheshire pupil.

Second photo is the same as the first but I edited it to simulate a shiny drawtube barrel.

Third photo is the same as the second but I edited it to simulate a smaller secondary mirror. Note how the end result is quite similar to John's photo.

Bottom line: John's photo is different because his scope has a secondary mirror that is about the same size as the drawtube. Most scopes have a larger secondary mirror than the focuser drawtube size. But John's image breakdown has nothing to do with the secondary size or its placement under the focuser.

Jason

[Jason D]

John, moving the secondary mirror higher towards the OTA opening will get rid of the crescent as you have already observed. What you did is effectively moving the secondary mirror as depicted in the right diagram of the attachment. Though the crescent has been eliminated, unfortunately, the secondary mirror is no longer intercepting the complete light cone for the on-axis star.

Jason

[TopHouse]

Seems to me that being the case, skywatcher are using the same secondaries on their 200P and higher scopes, in the 200P it's fine with no crescent, John's is a 250P.

[astromerlin]

Hi, I do understand your explaination of why I see the "cresent" I am now satisfied that this is not the problem.

As an aside, I have had the secondary out to photo the scratch which is posted on another thread (Wanted section). I understand the scratch isn't the problem either.

I have measured the reflective part of the secondary (81mm X 56.5mm approx) If placed at 45 degree angle, you should have a circle measuring 57mm. The hole in my focuser at the bottom edge is 58mm exactly.

So yes the secondary and focuser sizes are close.

What i'm still not sure of is why my images are so poor when collimated correctly. If I had taken a more scientific approach in the first place I could provide accurate findings. However, now I can discount some of my observations thanks to you guys/gals, I will when weather permits start again and document every process I go through so I can eventually find the problem. It does seem strange that my images from when I collimated correctly from the 11th,12th and 13th are all poor and then when I decided to move the secondary nearer the spider hey presto great image.

Anyway, cant do much about it now as it's peeing down and will do so for the next few days.

TopHouse, I was intending to reflash the Toucam but it died a while back and then came back to life for a while after playing with it and then died again. So now it's in a box somewhere. Most of my images had been done using the Vestas as the SPC kept crashing at the time. This I think was down to my refusal to buy a laptop as I was using my home computer on a couple of 10mtr usb cables. I bit the bullet an purchased an 1.8Ghz athlon laptop cheap, Shortened the usb cables to 5mtrs and the SPC still didn't want to play. 3 weeks later, the screen went on the laptop. The wife took pity on me and we purchased a 1.8Ghz dual processor laptop. I love this thing, never a problem, the Spc works fine on 5mtr usb cables and I can run all the programs I want at the same time and it doesn't break a sweat.

So yes, I'm getting there slowly, if I can sort this problem then perhaps i'll be able to give Damian Peach a run for his money. (I wish, stop dreaming.)

[TopHouse]

I've got a toucam I flashed, got the nosepiece and the IR filter, just don't have a laptop lol. I had two toucams that were used as live streaming webcams on robotic mounts in my pub at one time, it made good press as we were Britains first interactive pub, so since coming out of the pub and getting into astro I sold one of the toucams for the price of a nosepiece and IR filter. Thing is, even if i used USB repeaters I don't think it would reach from my desktop to my observing spot in the garden

[LukeSkywatcher]

Well I'm relieved to hear that.

Someone pop over and whak me with a piece of wood, I still cannot see why my images are so poor when it's setup correctly and better when it's not.

Images below (excuse the orange dots) first was taken on the 11th March after a whole day spent collimating correctly with a cheshire. To date this was the best image I've had of any planet. The second was taken on the 15th when I decided to get rid of the "cresent" feature.

This image blew my socks off until I put the camera down the focuser to check collimation. I'm sure the difference is more than just the "seeing".

That is one hell of a difference. I agree, i think the difference is down to more then just "seeing".

It makes me want to recheck my collimation with something other then a cillimation cap that i made from bits i had.

You say you used a Cheshire collimator in between both images?

If thats the case then i am going to pick one up tonight.

[astromerlin]

It makes me want to recheck my collimation with something other then a cillimation cap that i made from bits i had.

You really do need a cheshire tool to collimate, I use a cap with hole to get the secondary central to the focuser and then put in the cheshire to do the rest.

Next week isn't looking good weather wise. Doh

[Astro_Baby]

With reference to the pics in post #82 - I dont really know how representative they are of the visual aspects of the scope BUT the left hand pic seems to suggest collimation is out of whackl. The bright point of the star appears to be lower left. I assume the fuzzy bits are kind of analogous to an airey disk - if thats so collimation is out for sure.

IF we ever get to the bottom of this I shall rewrite my guide to take this into account.

ps I have a TAL 1 I am restoring (4.2" F7 scope) and its a doddle to collimate

Its so basic you could it by eye and the sweet spot is about 3" wide so you dont exactly need any great accuracy.

[astromerlin]

Hi, AstroBaby, Yes I have only started to use this software recently and there isn't much info on what all the numbers mean and the graph on the left could do with more info too.

The fuzzy bits on the star in the left image is flaring. When using the program you get a red dot that follows the brightest part of the star as seen by your ccd.The program is supposed to take the seeing out of the equation by stacking images real time. If collimation is off the dot usually follows the flaring and it dashes around anywhere but in the middle where you want it. The left image was taken when I set about collimating correctly. This was the worst I've seen the images produced by the software.

Conversly, The right image was taken when I moved the secondary to the "not normal" position. The red dot pretty much stayed in the center of the image and there was little if no flairing. The star shape was very small and circular. This is what I would expect to see when pefectly collimated.

This problem has me totally baffled and whats worse it looks like the rain is set for a week now. I'll be glad to get to it and sort out whether this problem is due to the scope or operator error.

Anyway, I'm suprised that no one seems to be using Metaguide as the seeing in the UK is poor most of the time for star testing, well it seems that way to me, being new to this I really need steady concentric rings to look at instead of the usual disco light show I get when looking at a star through the EP.

Here's a quick question, when working out my f number I get F4.8 for my scope but the Skywatcher website marks it as an F5. Needing to enter the F number into Metaguide, which should I use?

Cheers

John

[Astro_Baby]

At a guess Sky-Watcher probably round the number up. Actually I get 1200/250 = F4.8 but the FLO site lists it as F4.7 !!! Not sure there of whats going on.

[John]

At a guess Sky-Watcher probably round the number up. Actually I get 1200/250 = F4.8 but the FLO site lists it as F4.7 !!! Not sure there of whats going on.

The 250PX that I used to have had a mirror that was 254mm in diameter. That might explain F/4.7 assuming that the focal length is in fact 1200mm. The primary in my Orion Optics 10" is only 247mm:(

[astromerlin]

Hi, This is starting to get depressing. I had the opportunity to get the scope out last night. The sky was reasonably clear and there was very little if no twinkle in the stars.

I spent the afternoon collimating, Took too long and started to lose the light. It looks like I could have moved the secondary a bit more nearer the primary. Set the scope up at 10.30.pm and left the cooling fans running for an hour and a half. I slewed to Denebola and tryed a star test. The off focus looked reasonably concentric but getting near to focus to do a ring test the image was bubbling and boiling. I waited for another hour but still the same. I took several images of the off focus star, didn't look pretty. I then slewed to Saturn, I was expecting the clouds to come over any time so I'm rushing a bit by now. Settled down to image Saturn and did a quick process in Registax. Image looked as my usual disappointing images. I slewed over to Arcturus and waited until 3.00am to see if the boiling would settle down. It didn't. Packed up went to bed.

Images below.

[TopHouse]

By 'boiling' are you saying even after a couple of hours the rings were moving and swaying like you've just taken the scope outside?

[Doc]

Looking at your star test image your collimation is off. Did you try tweaking your primary to centre your secondary and if so did the image improve.

[astromerlin]

By 'boiling' are you saying even after a couple of hours the rings were moving and swaying like you've just taken the scope outside?

Hi, I should have said the image on the screen was boiling, The image when looking thought the EP wasn't so much. To me this seems like alot of flairing of the star around the edges.

Doc, I didn't adjust it at the time. I wanted to take a set of images to compare without fiddling. The secondary needed to go a bit more closer to the primary. I have collimated many times over the past few months and must have moved the secondary around the correct centered location many times. I have not yet found the sweet spot. All the images of the defocused star looked like the one I posted, some more collimated than others, It is easier to see through an EP than to get good images though. I have sometimes had perfectly concentric rings looking through the EP but then imaging was still poor.

I think we should be thinking outside the box here. Questions like "Is my secondary flat, what if the secondary has not been glued on to the mount in the right place. Something is very amiss here.

My observation is that when collimated textbook style the focuser is not concentric with the secondary reflection. I get poor images.

When I center the focuser in the secondary reflection I get good images.

Tonight hopefully cloudless sky permitting, I am going do it all over again this time with the secondary set when I obtained the good image of Saturn to see if I can reproduce it.