Celestron C8

[Virgoman]

Hi all,

I hope some kind soul(s) can help me. I have a Celestron C8 which is less than a month old. After initial observations with the scope yielded disappointing images, I was lad to believe that the scope needed collimation. The sky here was gin clear last night and I took the chance to carry out collimation of my scope. I ended up with nice concentric rings just in and out of focus of a bright star near the zenith exactly and as per the Celestron manual instructions. I used this and information from various websites recommended by users as my guide.

The scope was allowed to cool for 2-3 hours before I carried this out. I then spent well into the small hours trying to get decent images. Star images are ragged far from sharp and like streaks near the edge of the field and Mars appeared as a pinkish blob with no surface features visible at all. This despite me looking longingly at it for ages almost willing some detail to appear. Nothing. I used both 25mm and 10mm eyepieces, focussed and refocussed - absolutely nothing! I should also mention here that the corrector plate was covered by a dewshield and kept clear of any dew by using a small hairdryer on cool - as recommended by Celestron. Dew was kept at bay.

Like I said I have had this scope less than a month and have yet to have a satisfying nights viewing with it. I have also noticed that the inside of the corrector plate has in places all I can describe as a layer of scum - is this normal?

I am now beginning to wonder if I made a mistake in getting the C8. I did a lot of research before I purchased and most opinion was positive. I also was attracted to the flexibility of the system.

I hope someone can help as I am at my wits end and I am now regretting selling my ETX105 (to partly finance this) which always gave wonderfully sharp views. Mars` markings were easily visible and in worse viewing conditions than I experienced last night. Even my inexpensive Skywatcher Startravel 102ST gives much better views.

Many Thanks & Kind Regards,

Ralph:cry:

[johninderby]

You say the scope is a month old, is it new or second hand?

While it could just have been poor seeing conditions the mention of scum on the inside of the corrector is a bit of a concern. How has the OTA been stored? Horizontal or facing corrector end down. If stored with corrector end facing down it is possible that grease can drip onto the inside of the corrector.

John

[Virgoman]

Thank very much, John, for the reply. The scope is brand new from D. Hinds and is stored on its mount pointing upwards. Regarding the seeing conditions, last night the stars were very steady with very little twinkling even relatively far from the Zenith. This was the main reason why I seized last night to carry out any collimation required.

Kind Regards,

Ralph

[brianb]

it could just have been poor seeing conditions

That's my bet.

I've had a reasonably sharp image with a 9mm eyepiece in my CPC1100 exactly twice in 10 months ...

Most people find that the collimation of Celestron SCTs "as delivered" is pretty good.

The corrector plate can be absolutely filthy without significantly affecting the sharpness of the image, dirt will however reduce the contrast to some extent.

[dweller25]

Ralph,

I was out last night with my refractor.

The stars as you say were steady BUT seeing was poor. I suspect there was a lot of water vapor at high altitude.

If the 'scum' you see on the inside of the corrector is slightly blue then it is outgassing from the paint used on the inside of the OTA. It can easily be cleaned off but is not a major problem.

HTH

[johninderby]

Now that you've eliminated what else I thought it could be, I'd have to agree with the last couple of posts, just poor seeing.

Very frustrating when you look up and see what appears to be a clear steady sky, but observing turns out to be like looking trough running water.

Hopefully you'll get some good observing conditions soon. I took delivery of a new scope on Xmas eve and have used it just once since then.

John

[Virgoman]

Thanks for the replies all. Poor seeing seems to be the recurrent theme. I am still not convinced that this is the case. I don`t recall having as many problems with the ETX. As I say the sky was wonderfully clear and I expected more, at least a modicum of detail on Mars not a pinkish featureless disk. Incidentally as a footnote there were times when Mars appeared to show a ghost image with the two combining to give a figure of eight-like effect. Is it possible that the OTA is defective?

[ollypenrice]

Let's recap; a star on the zenith gave clean circular diffraction rings? But what did it look like in focus? And what did it look like as you swung the scope to put the star in the edge of field? (Maybe you didn't try this?) Did the star images degrade as you observed nearer the horizon? If so, this would be a good sign for the health of your scope.

Like the others my first suspect is seeing. Even in very good scopes if the seeing is bad that's your lot on the planets. Nothing doing, just a swirling blob.

I would register your concerns with David Hinds and if the scope does prove faulty you will get a solution. The C8 and other 200mm SCTs are normally perfectly conpetent scopes.

You can try a daytime star test if you can find a long dark room. I used my friend's condom warehouse... Just glue a small ballbearing to a dark matt card and shine a well beamed torch at it to illuminate it. Put the scope as far away as possible and observe the illuminated ball. It makes a passable artificial star. You can see if you get good diffraction rings on and off axis. Don't expect them to be perfect off axis. That's what Takahashis are for.

Olly

[DaveGibbons]

Thanks for the replies all. Poor seeing seems to be the recurrent theme. I am still not convinced that this is the case. I don`t recall having as many problems with the ETX. As I say the sky was wonderfully clear and I expected more, at least a modicum of detail on Mars not a pinkish featureless disk. Incidentally as a footnote there were times when Mars appeared to show a ghost image with the two combining to give a figure of eight-like effect. Is it possible that the OTA is defective?

A double ghost image is indicative of optics not reaching thermal equilibrium. The extremely cold weather we have had of late has ment

getting closed tube sct scopes cooled down very difficult. I have had it in a c11 and 7" maks and often in 10 and 12" newts.

cheers

Dave

[brianb]

Incidentally as a footnote there were times when Mars appeared to show a ghost image with the two combining to give a figure of eight-like effect. Is it possible that the OTA is defective?

Possible, yes, but more likely way out of thermal equibilbrium and/or badly collimated.

[michael.h.f.wilkinson]

I have had my C8 for years (since about 1996), and have rarely had to do anything about collimation. However, there are nights when the seeing can be so bad that it causes ghost images like you describe. What happens is that the scope's line of sight intersect two convection cells, causing a temporary double image. Bad thermal equilibrium can cause similar problems. A collimation problem would show a persistent double image.

[Virgoman]

Thanks guys all for the replies, much appreciated. In response to Olly, the star image on edge of field looks elongated and when out of focus the diffraction pattern is skewed. Also, BrianB, two sharp images in 10 months - even with a 9mm is not terribly encouraging, do you consider that acceptable? I`m having problems with a 25mm. I have spent a not inconsiderable amount of money, to me, on this scope and am just very disappointed and apprehensive and that I`ll never get it sorted. i`ve never had problems like this with other scopes I`ve had. How would I know if the tube was faulty? Dweller25 whereabouts in the UK are you? - perhaps seeing conditions were different?

Kind Regards,

Ralph

[dweller25]

I'm in Lancashire.

[michael.h.f.wilkinson]

Are the skewed diffraction patterns only on the edge of the field, or all over? If the former, this raises the question of whether the eyepiece is OK (if it is the standard Celestron Plossl that comes with many of their scopes I would not expect there to be a problem). If the latter, the collimation may be off. Another problem can be the location. In autumn I can have problems with heat produced by a nearby sugar-processing plant. Our 16" RC scope on top of the university has serious problems with "dome-seeing", i.e., it takes forever for the heat of the roof of the building and within the dome to get down to acceptable levels. Nearby house can similarly seriously harm seeing.

[Virgoman]

Thanks for the reply Michael. The skewed patterns are only at the field edge and the eyepiece I am using is indeed the Celestron 25mm, though I prefer the 25mm Tal Plossl from my 100r. My viewing is from my back garden - in a residential area - probably typical of many users. I was interested to read that you have not had any collimation issues and your images show what can be achieved....I wish! This scope has been a disappointment so far.

Regards,

Ralph

[BlueAstra]

I have a Celestron C8 for about six months and never really had any problems with it. I got a set of Bob's knobs to aid collimation, but I've not fitted them yet since I keep checking the out of focus rings and they're never out!

Have you tried using a bahtinov mask to aid focus? Use it on a bright star and at least you know the focus is spot on. You can then assess the image quality. Seeing can be bad where I am, but I have managed to get some decent images with this scope. Visually though, I've not seen a good Mars yet but have got some detail with a Neximage.

[Virgoman]

Thanks for the input BlueAstra. Are your rings regular at he edge of field also? Are star images elongated at the field edge? I know this scope is capable of good images as is evident from various sites. Thanks for the advice about the mask. I was also looking at Bob`s knobs - seems they are recommended. Direct from Bob himself apparently...!!!

Regards,

Ralph

[brianb]

Also, BrianB, two sharp images in 10 months - even with a 9mm is not terribly encouraging, do you consider that acceptable?

It's exactly what I expected. Most nights usable magnification tops out at x200, some nights it's even less than that ... one particularly awful night (courtesy of an exceptionally turbulent jet stream) stars were fuzzy at x24 in a 80mm refractor.

[BlueAstra]

Thanks for the input BlueAstra. Are your rings regular at he edge of field also? Are star images elongated at the field edge? I know this scope is capable of good images as is evident from various sites. Thanks for the advice about the mask. I was also looking at Bob`s knobs - seems they are recommended. Direct from Bob himself apparently...!!!

Regards,

Ralph

I'm not an expert but I think you assess the focus 'rings' for collimation only in the centre of the field of view. I think the rings may be asymmetric at the edge of the FOV even when well collimated (?)

The eyepiece can affect the quality of the image at the edge of the FOV. I use the E-Lux 25mm supplied with the scope and don't see any substantial elongation of images.

I got the knobs direct from Bob's website with no problem. Check the website to ensure you order the correct ones.

Here is an interesting site on collimation:

Thierry Legault - The collimation

[michael.h.f.wilkinson]

When using very wide FOV eyepieces (like my 40mm Paragon), even a well-colimated scope can show some asymmetry in the out-of-focus stellar images at the edge, due to vignetting effects. If the eye is not centred at the exit pupil, this effect occurs. One last thing to set up your C8 next to some other C8, and check for differences. There may be a (rare) error in manufacture.

[brianb]

I think you assess the focus 'rings' for collimation only in the centre of the field of view.

Yes.

The "donut" pattern you get just out of focus can be used for rough alignment, and is sensitive enough to get the collimation almost right even when the seeing is bad. With a 10" or above scope, if you actually see stationary diffraction rings at focus, your seeing is unusually steady, observe rather than fiddling with the collimation - you won't get conditions like that often - if you do, don't tell anyone or you'll be invaded by hundreds of manic planetary & lunar imagers!

[michael.h.f.wilkinson]

Yes.

The "donut" pattern you get just out of focus can be used for rough alignment, and is sensitive enough to get the collimation almost right even when the seeing is bad. With a 10" or above scope, if you actually see stationary diffraction rings at focus, your seeing is unusually steady, observe rather than fiddling with the collimation - you won't get conditions like that often - if you do, don't tell anyone or you'll be invaded by hundreds of manic planetary & lunar imagers!

Indeed, though even with the Schmidt corrector plate, diffraction patterns themselves will show asymmetry due to the same vignetting effects described above, because the aperture becomes slightly asymmetric. Seeing the diffraction rings themselves is generally only possible in lab environments

[Virgoman]

Hi all and once again thank you all so much for your advice and understanding. I think I will try and get a few images as soon as possible to see if I am on the right track -even images of diffraction rings? I am just posting a couple of images (already posted elsewhere on this forum) to show albireo taken with the ETX and the C8. The C8 image was taken at first light with the scope and illustrates what I was getting initially. Just chucking in an image of Mars I obtained 4 years ago with the ETX, for old times sake, as I`m having problems getting one at the moment!

All the best,

Ralph

[brianb]

The top image shows gross coma, the primary star is overexposed too much to show detail but the secondary clearly shows the "V" shape caused by the optics being miles out of collimation (or the target being a long, long way from the centre of the field of view).

[Virgoman]

Hi Brian,

that`s how I received it.

Ralph