New 8" Skywatcher Dobsonian

[Jonny]

Hi,

I recently bought an 8" Skywatcher Dobsonian, upgraded from a 3" newtonian. I'm pleased with the results but a bit dissapointed at the same time. The moon is great and I can see all sorts of detail... but the problem is that when viewing Jupiter through the 25mm lense, all is fine (although i could have just kept my 3" telescope for that), when I move to the 10mm lense the picture appears sort of good but blurred and with slight chromatic abberation... when I move to the 6mm (I think) lense, it's all blurred and i can only make out a faint brown line from the planets atmosphere.

I leave the telescope outside for 45 minutes to cool down, I do not view through a window, there was no haze in the sky and there is only mild light pollution.

I honestly thought an upgrade from 3" to 8" would have brought a lot more detail out in planets. I just got my first view of Mars and anything past the 10mm zoom is just blurry and filled with red/blue, no surface details except you can just make out the polar ice cap and orange colour.

The andromeda galaxy doesn't show up at all... and I've idea why.... maybe I am underestimating the problem of light pollution :/

Any help would be appreciated. Thanks.

[perrin6]

I've seen on this forum it's recommended to replace the 10mm with something better eg a BST explorer (from eg 'skies the limit' ).

The main problem is probably atmospheric distortion AKA 'bad seeing'. Keep looking with the 10mm every few minutes - the seeing varies often throughout the night. Each viewing, keep looking for a few minutes as you may get glimpses of clarity even during bad seeing conditions.

Other than that check the collimation with a cheshire eyepiece and learn how to collimate.

[brantuk]

There's been a bit of thin high cloud spoiling the seeing these past few nights even though it appears clear. Also your dob is f-6 which is getting towards the lower less tollerant end of the scale - so you definitely need a better high power eyepiece.

Barlowing the 10mm only makes matters worse cos your trying to magnify at 240x - again a little over the limit of 200x for average UK seeing conditions. You need to try a few different makes of eyepiece to find what suits you, something like an 8mm Hyperion for around £60-£65 s/h would make a marked difference at 150x. Or BST and TMB produce some reputable eyepieces at around £40'ish(though I haven't used these).

[sunshine185]

I would first make sure the scope is collimated, then get something like a bst 12mm ep. Also, even though you view outside, make sure you have a clear view to the object, don't view it if its above a house our building if you can help it.there its something not quite right,I have the same scope so I know what out is capable of. Good luck and keep us informed.

[Bart]

I would definitely check collimation as it would need to be pretty good for going to high magnification . I have the 5 and 8 mm BST giving me x 250 and 150 and can see great details with both. I have observed a GRS transit with the 5 mm but i would need good seeing for that.... I can recommend them for your scope . Check collimation though.

Bart

[twotter]

I've got the same telescope and they are very tolerant when it comes to collimation, and they hold it well too. A quick star test should show a circular blurry disc which reassures you all is well in that department.

The supplied 10mm EP is the weaker of the two but it's by no means useless, and should still show detail on Jupiter. I would agree with the comments about seeing conditions as I've not been able to push quite that far recently. A 2x barlow would turn the supplied 25mm into a 12.5mm which would prove more useful. Otherwise there are a range of different eyepieces you could upgrade to. I have a collection of GSO Revelation and Meade 4000 plossls which I used for a year or so and they work well. I've since upgraded to Baader Hyperions and their wider field of view really suits my telescope

Good luck!

[Jonny]

Thanks for the replies.

I haven't touched the primary mirror since I bought it. It's simply too complex with too many screws and I figure I'd break something if I tried. I have, however, collimated the secondary mirror to be in line with the Primary.

There's still the problem of chromatic abberation.

Also, I didn't think the lense was much of a problem as much as the primary mirror, and spending £60 on a new lense is way too much.

Hope I can sort it some other way. Cheers.

[John]

You will need to collimate the primary mirror - it tends to need adjustment more than the secondary (smaller) one. It's part and parcel of owning a newtonian scope. Once you get the hang of it it will soon become 2nd nature but if the collimation is out then your scope will not perform as well as it could.

Your 8" should completely outperform your 3" in every department by a big margin. If it's not then something is wrong and collimation would be the prime suspect.

[sunshine185]

It is usually the primary that needs adjusting, the secondary is very light so once it is in the right place and secure you shouldn't have to adjust that for a long while. the primary is so simple, the grub screws are the locking nuts and the Philips head screws are for adjusting, put your Cheshire or collimating cap in the 1.25 holder, then loosen the grub screws, have a look into the ep and see if the little circle on the primary id centred, if not then turn one of the Philips head screws clockwise a little, have another look down the Cheshire and see if it has improved, if not, just turn the screw back to where it was and try another one, you will soon see which screws to adjust.once the circle is centred then its time to tighten the grub screws, do this one at a time and check in the Cheshire each time you tighten a screw, if it moves then turn it back and tighten another, just doing little quarter turns each time. You should be good to go once all the grub screws have a good hold, just be sure not to overtighten.

[callisto]

Thanks for the replies.

I haven't touched the primary mirror since I bought it. It's simply too complex with too many screws and I figure I'd break something if I tried. I have, however, collimated the secondary mirror to be in line with the Primary.

There's still the problem of chromatic abberation.

Also, I didn't think the lense was much of a problem as much as the primary mirror, and spending £60 on a new lense is way too much.

Hope I can sort it some other way. Cheers.

It will deffo be the collimation if you have not touched it since you bought it

[Jonny]

Thanks again for the posts.

I feel better now knowing that it's probably the collimation of the primary mirror. Still a bit hesitant as to whether I should touch anything (I have a fear of cracking such an expensive mirror). I did craft a collimator out of an old camera film holder.... would this be enough to collimate the primary mirror? Or should I invest in one of those laser collimators?

[brantuk]

I knew I missed something out - collimation is a must. I was observing with a brand new 8" dob on Friday night. The views were a little off so I had a look at the collimation and it was out a little even though it was delivered only 3 days before.

Soon as the optice were back in line things were much better. But the 10mm was still naff I hasten to add - and forget barlowing it - that will only magnify atmospheric imperfections. As a rule though - longer focal lengths and higher f-ratios (eg f-10) will be more tollerant of poorer eyepieces. Anything f-6 or less consider better quality glass

[Jonny]

I knew I missed something out - collimation is a must. I was observing with a brand new 8" dob on Friday night. The views were a little off so I had a look at the collimation and it was out a little even though it was delivered only 3 days before.

Soon as the optice were back in line things were much better. But the 10mm was still naff I hasten to add - and forget barlowing it - that will only magnify atmospheric imperfections. As a rule though - longer focal lengths and higher f-ratios (eg f-10) will be more tollerant of poorer eyepieces. Anything f-6 or less consider better quality glass

Problem is, I wouldn't know where to begin with collimation. Do I need to be looking at an out of focus star whilst doing it. Is it worth buying a laser collimator?

[hes not well again]

Thought I might add my pennyworth, from what I have heard the laser collimators can themselves be out of collimation [cheap ones]. I recently bought a cheshire and found it excellent. Cheap, simple to use. Start with that and you will get it very nearly spot on and then when the seeing is good you can do a star test to get it perfect.

[sunshine185]

The cap you made from a film canister will get you close enough, no fear of cracking the mirror as long as you don't go mad when you tighten the grub screws. You will be askew to tell when they are tight enough, just don't put your full weight on them, just enough until they bite. P.s if you do buy a collimator I would recommend a Cheshire, then you know it is properly collimated.

[Zachariah]

Could be seeing conditions as there has been alot of thin high cloud about. The sky doesnt look a dark blue its a palish blue. You cant expect to see anything clearly through thin cloud.

[Bart]

Could be seeing conditions as there has been alot of thin high cloud about. The sky doesnt look a dark blue its a palish blue. You cant expect to see anything clearly through thin cloud.

This is true, there has been a lot of high thin cloud about recently. Certainly more than this time last year, around my neck of the woods at least

Bart

[Jonny]

I've just been out with my telescope to get to the bottom of the issue. I did a star test.... couldn't see anything like what is shown on many guides.... I didn't see any diffraction rings...... what I did see I have drawn below:

http://img716.imageshack.us/img716/5756/telescopedrawing.jpg

The drawing in the top-left is a random star I focused on, but it doesn't seem to focus into a pin-*****.... instead if goes from a blob-shape, to eliptical when it is 'focused' and then back into a blob.

[sunshine185]

Ok, the drawing of the mirror you have done means the secondary looks fine, but when you look down the sight tube or collimation cap, the little circle on the primary mirror should be directly in line with the little hole you made in the canister

[sunshine185]

Do you live anywhere near Oldham? I would gladly bring my Cheshire round if you do.

[Jonny]

Do you live anywhere near Oldham? I would gladly bring my Cheshire round if you do.

I've just tested it in the lounge again, but this time I pointed the tub in such a direction that light from a light bulb was hitting the engraved ring on the primary mirror, making it visible to me through the old-film-canister/collimator.

Here is a drawing of it:

http://img577.imageshack.us/img577/4113/telescopecoll.png

I might have over-exagerated the leftness of the ring, but it is noticeably over to the left. Is this a primary mirror issue or is it the position of the secondary mirror?

[sunshine185]

Primary, inthe canister did you make a very small hole or a big one? It needs to be as small as possible, just big enough so you can look into it and see the reflection of the small hole. Then you line this up with the little circle that's on the primary mirror, just adjust one screw at a time and check down the cap to see which way it moves. Doing it slowly like this you will see which way the mirror needs moving.

[sunshine185]

Jonny, the last drawing you did can you do one which shows the primary ring and the reflection of the little hole that you look down? When it is closely collimated, the little hole should be in the centre of the primary ring.

[Jonny]

Jonny, the last drawing you did can you do one which shows the primary ring and the reflection of the little hole that you look down? When it is closely collimated, the little hole should be in the centre of the primary ring.

http://img835.imageshack.us/img835/4113/telescopecoll.png

The green dot is the eye hole on the film canister. It took ages to see it, because the only way of seeing it was to shine light directly into my eye so as to have it reflected

It isn't much off..... is this a problem?

[sunshine185]

The dot should really be in the centre of the primary ring, if the hole in the canister its just a tint pinprick, try making it a little larger so its easier to see. Have you tried adjusting the primary yet?