GSO 10" Dob questions and concerns

[209herschel]

Hi everyone, I bought my GS Dob from Andrews a week ago. I've started by trying it on the planets and moon to start. I saw the band around Saturn but it wasn't very clear, even with the 9mm plossl. I didn't really see the Cassini division. I'm trying to find out what I should be able to see. How do you know if a scope is working correctly? I looked at the primary using my phone torch. The surface is pristine but I noticed some blemishes around the cell. I assumed this couldn't affect the scopes performance because the mirror looks great. If you have any thoughts, checks, techniques to verify a scope is working properly, I'd be very keen to hear what I can do confirm the scope is fine. Thanks very much.

[sn1987a]

Your scope is probably fine Herschel, what you need to do is go along to your nearest Astronomy group, club or gathering, take your scope and let some of the more experienced members show you how to tweak it to get the best results. Put the word out and find if there are any IISers near you with the same or similar gear that wouldn't mind showing you the ropes.
Once you start to understand how your scope works, no worries.:thumbsup:

Cheers
Barry

[Allan_L]

Hi Herschel,
congratulations on your new scope.
I agree with Barry, it is a good idea to find an observing session near you.
Where in Sydney are you?

[209herschel]

Hi Barry, thanks very much for that. I actually agree - it's just that I'm really new to all of this and I need to be patient and there's a learning curve involved. I originally thought I was seeing Saturn as a tiny point and after hearing that I should get some detail, I decided to look at any bright spot in the area. And suddenly Saturn showed and I saw the rings. So I was looking at something else entirely. So there's a great deal to learn. I was just thinking that I should be able to get pretty clear views of Saturn - the Cassini division, some bands, etc - but I couldn't really make that out with the 9mm. I think going along to a club and getting some tips from experienced people is the best thing right now. Thanks again.

[209herschel]

Hi Allan, thanks very much. I was thinking of joining the Western Sydney Astronomical society. They've got a viewing area about 90min from me. I'm in Marrickville, Sydney so I'll try find a place to take the scope. I think I'll learn a great deal that way. One quick question: If the moon looks pretty clear, does that mean collimation should be okay? If collimation was out, wouldn't everything show a soft focus? I'm getting pretty clear images of the moon, starting with the 25mm, then moving to the 15mm and finally the 9mm. Thanks again for the help. This forum is an incredible means of assistance for someone learning the ropes.

[AstralTraveller]

Start by checking collimation. Look at some stars out of focus. You should see a central disk and probably an outer ring. They should be round and symmetrical. If not look up how to collimate a newtonian. There are plenty of sites.

However, to me it sounds like a seeing problem. Either the atmosphere was very poor and/or Saturn was low in the sky and/or the scope wasn't properly cooled. Oh, and is your eyepiece clean? Just because it's new doesn't mean it's pristine.

[209herschel]

Hi, thanks for your reply. I originally collimated with a cheshire. I watched a bunch of youtube videos and I was pretty confident. But I bought an orion laser collimator a couple of days ago and adjusted the primary slightly. I bought the laser so I could check the primary mirror when outside in the dark after the tube had cooled. When going outside, I have to sit the ota on a chair, carry the base outside, then carry to ota to it. So perhaps the primary might shift slightly in the transfer. I'll try the laser outside tonight. On the star test, I definitely get what look like good concentric circles, the only difference being the circles don't look like solid lines. It's more like a circle of close dots when defocused. Does that sound right. When you say it sounds like seeing conditions, how clear should Saturn be? And how big in the ep of a 9mm GSO plossl. I'm just trying to get an idea of what to expect. Thanks again.

[astro744]

Current data for Saturn courtesy of Stellarium:

Apparent diameter = 18.2 arc seconds. with rings = 42.5 arc seconds.

Our Moon is typically 1800 arc seconds (1/2 degree). Have a good look at the Moon unaided, i.e. no telescope or binoculars and see what detail you can pick up.

Now your telescope is 254mm, f5 so focal length = 1270mm.

Magnification with 9mm eyepiece = 1270/9 = 141x.

Multiply 18.2 x 141 = 2,566 arc seconds. (1.4x bigger than lunar disk)
Multiply 42.5 x 141 = 5,992 arc seconds. (3.3x bigger than lunar disk)

As you can see a 9mm eyepiece in your telescope will give you an image of Saturn bigger than the Moon unaided. You should easily be able to see banding on the planet (much more subtle than Jupiter though) and of course the Cassini division should be evident as a think dark 'gap' in the ring plane about 1/3 from the outer edge. In fact your telescope should also reveal the much thinner Encke division (close to the outer edge), but only under very good seeing conditions.

As for collimation, have a look at the bottom of this web site for some diffraction ring images: http://www.willbell.com/tm/tm5.htm

As long as everything is concentric you are collimated. Note start with a medium power eyepiece and then go higher. The higher the power you can get concentric rings, the better the overall collimation. Check both inside and outside of focus.

[209herschel]

Thanks very much for this information. I'll check this tonight. I definitely don't think my image 2 nights ago was anything that large. What could account for my image being what appears to be smaller?

[mental4astro]

If you need further help, you'll find a couple of informal gatherings listed in the Star Parties forum, one at Katoomba Airfield, and one at the Pony Club at Mangrove Mountain to Sydney's north. Both are slated for the 28th of June.

[Renato1]

Don't panic just yet.

Even with perfectly collimated optics, on some nights you won't have much luck.

If the stars are twinkling - you won't have much luck with sharpness.
Even if they aren't twinkling, some times you won't have luck with sharpness.
If you are viewing from suburbia - with neighbours having their heaters on (or air conditioners on in summer), don't expect much sharpness. If you are viewing over your heated house, don't expect the best.

If you aim at a star at high power, and throw it out of focus, and you see a bulls-eye pattern, then you can see whether you are nearly collimated or not (the black bit of bulls-eye is not central, an dyou have to adjust the three screws at the back of the telescope to get it central). Unfortunately, my 14.5" dob is my only telescope that doesn't give a bulls-eye pattern - I have to use an aperture mask to get the bulls-eye to show up. I don't know whether a bulls-eye pattern shows up in your telescope.

If your mirror has a black dot in the centre, a Cheshire eyepiece and a little flashlight will let you collimate the telescope pretty close. Though, as I was advised here a few months back, an Autocollimater gadget is needed for precise collimation to get the absolute best out of the telescope.

Laser collimators are around. I've bought one, but haven't actually used it yet, so can't comment.
Regards,
Renato

[209herschel]

Thanks very much. I was out last night but only briefly and too late because Saturn was too low in the sky to see - behind trees. The moon looked good again and I do think it's pretty clear. I tried to defocus some stars and I think the collimation may be slightly out. Unfortunately, I didn't get the time to try to make adjustments while outside last night. I'm really hoping for a clear night tonight so I've got a few hours to try again on the collimation. I now think seeing conditions are not that good where I am. When I look at the edge of the moon, there is a lot of shimmer, like heat. When defocusing a star, I'm not getting the clear circles I see when I google star collimation images. My defocused star is hazy even when the circles are concentric. Thanks again. I'll take the laser collimator and cheshire out tonight and take my time on the collimation. Cheers.

[astro744]

I was trying to figure what time of night you were out.

The Moon was rising E at about 11 pm and then Saturn was WNW at 60 deg. altitude. At 1:30 am the Moon was NE at 30 deg. altitude and Saturn was W at 30 deg. altitude. The 30 deg. mark is getting on the low side and you will see the effects of the atmosphere more readily. See if you can catch Saturn from 8 pm tonight since it will be 60 deg. and rising. to nearly 70 deg. at 9:15 pm due north.

[209herschel]

That's brilliant information. I was out just before the soccer started, around 1am. Problem is the clouds then came over so it pretty hopeless. I'll be out from 8pm tonight. I was going to buy the GSO ED 2" barlow today from Bintel so I'll try the barlowed laser in my attempts to collimate accurately. Thanks very much.

[Renato1]

In my list below of things that affect seeing things sharply, I forgot to mention tube currents and the mirror cooling down.

Basically, for the first hour of viewing - unless it's a moderate evening in early autumn or late spring or cooler days in summer - you can't really expect much out of your telescope in the sharpness department for the first hour or two. That shimmering on the moon and the hazy circles in the defocused star image is most likely heat in your telescope. Everything should settle down hours later so long as view overhead. But when viewing a planet near the horizon, you'll still see similar shimmering due to all the other houses underneath (unless you are luck enough to be out in the country).

It just means that for the first hour or two, you would be better off looking at deep sky objects at lower power than you would on the planets - the bigger stars will be a bit blobbier, but that doesn't really matter much when looking at galaxies or globular clusters or nebula.

With my 14.5" dob, when viewing out in the country, I would never bother looking at the planets till the end of an observing session three or four hours later - and then the rewards were great.

The reason lots of people also have short tube 80mm or 100mm refractors is so that they can go out and do those quick 20 or 30 minute observing sessions, as the refractors often give decent images straight away.
Cheers,
Renato

[AstralTraveller]

Years ago when I did a lot of public outreach I had a 10" with a full thickness mirror. Often a planet would be the only suitable object and sometimes they were too low ... but the punters wanted to see it so I tried. Several times I've seen a double image of Jupiter, two blobs side by side. Half an hour later things were better (not perfect but tolerable). Tube currents can be that big a problem! If you plan to observe late in the night consider having the scope outside hours early, with the cap off, so that it is cool by the time you want to use it.

[Pinwheel]

I would like to advise that when my 12" dob was delivered, even though the Dob was collminated prior to dispatch it had a bumpy ride up here. The collimation was totally disrupted & one vain on the secondary was twisted. After re doing the colmination everything was perfect. My second point is my brand new laser collimator was not collminated out of the box either & that had to be re aligned before it could be used.

[Renato1]

Might be tricky keeping the cap off.
If aimed too high you can get dew inside the tube.
Aimed too low, and cats love to go exploring.
Cheers,
Renato

[noeyedeer]

I have a 10" gso and love it. I have some tmb 6, 5 and 4 mm eye pieces and Saturn is never bigger the say the size of a pea. I can take an uncropped pic to show you what I mean (thou it was last year and I don't remember the specifics) thou it was my phone video held onto the ep and that's the frame I took when looking at the video. no editing.

realisticly you need to factor in that Saturn is a long way away compared to the moon. it will never be bigger than a pea sized object. the goodness of aperture is the light it gathers from an object be it millions of kms away or light years.

how does Jupiter look .. or has it gone beyond the point on no return for your viewing session? planets are only the beginning .. you can see so much more with your scope then just those.

have you seen omega centauri .. it's bigger than Saturn and just as spectacular and look sideways while looking at it and it'll pop and then you'll never forget that ball of stars ..

matt

ps I can post crappy hand held phone pics of Saturn if you want to compare .. but they don't do the justice of what's seen at the eyepiece

edit ok maybe I have big peas on steroids .. but it'll drift through the ep in a few seconds but won't be no bigger than a pea from what I remember

that pic is from a video frame on my HTC one xl held to the ep which I don't remember .. maybe a barlowed 9mm or one of the tmbs .. I dunno .. but it's defective quality is my hands and the video .. Saturn is crispy thou the ep. that's just for comparison thou .. the moon at the same length would show a few craters like this ..

the moon is an actual pic, with 114kb and Saturn is a frame from a vid and is 8kb ... I think I used the same ep. but something to compare about what you're expecting ..

sorry he 3rd pic is what you would see with the same ep as Saturn .. if you looked at the moon with it

edit: if you're really concerned, download stellarium. you can enter your scopes focal length, and eyepiece length and it will show you what the object would look it at magnification. it's a pretty cool piece of kit for free.

[PSALM19.1]

Hi Herschel,

Joy of beginning in Astronomy! I wish I had a 10" Dob to begin with...mine was a 76mm Firstscope..anyway, I still had fun! It seems to me that collimation is not really your problem...I have an 8" Dob and move it in and out of the garage often and the collimation stays good for months on end. If you've Cheshired it and Lasered it and done the star test then I think you are a-OK!

Certainly I can attest to cooling the scope down and also to "seeing". I have looked at wonderful views of Jupiter and Saturn but can remember a few times when it wasn't too impressive - in fact, one morning Jupiter was a shiny, blurry blob! Don't want to repeat the great advice you've been given, but let the scope cool and make sure Saturn is at some altitude and I reckon you'll be winning! :)

Oh and, you can see the Cassini division in a lower powered EP than a 9mm and that may actually help in clarity!

[209herschel]

Hi Matt, thanks for the pictures, they're brilliant for comparison. I'm away for a week but I had a go the night before I left and I was pretty happy with Saturn. It was small but sharp in the 30mm superview I just got then I pushed it all the way to the 9mm which was really great. I also 2x barlowed the 15mm and I thought that worked well. I'll for a few more times on the moon and planets before embarking on DSOs. Cheers

[209herschel]

Thanks very much for the advice. My last try was the best yet. A clearer Saturn, and I could make out the Cassini Division even if the gap was only tiny. Another thing occurred to me last time was that I live under a flight path so I'll need to be very late so that the added turbulence may have settled. Cheers

[noeyedeer]

hi Herschel,
the pics aren't great but for comparison size wise, it should give you an idea. glad ya had fun with your new ep, it'll come in handy when looking at clusters and and for star hopping for faint fuzzies .. good luck!

matt

[209herschel]

Thanks very much Matt. The pics have been a great help. I've got a book on the Messier Objects, so I'm really excited about trying to find a few of the large ones because I think the light pollution will be a bigger problem when the DSOs? One last question - I'm trying to answer is how best to understand the coordinates of objects in the constellation? So the book gives me the coordinates of the Orion Nebula but I'm not sure what's the best way of finding it. Is there a book that's useful for that type of thing? Thanks again for the assistance.

[noeyedeer]

no probs. is the book Deep Sky Companions - The Messier Objects? that's an excellent book! the easiest way to find objects or to understand where to begin is to get either software such as Stellarium, or smart phone apps and by learning constellations, and especially the brighter stars in them. and then by star hopping to the object in mind. some are easy to find and some are tough, but all the messiers are visable with binoculars from dark skies. lots of open clusters to see from light polluted areas thou..

some objects are not visable from certain locations and times of the year, for the Orion nebula you will have to wait till summer for it to reappear again.

knowing what season the constellations are visable is the easiest way to focus on which objects to look for. (most books are published for the northern hemisphere so when they say "summer constellations" for us, they're winter ones etc.

the sky at the moment is filled with many messier objects. in your book have a look at how many messier objects are in Scorpius and Sagittarius.

have fun there's always lots to explore up there no matter what time of the year it is.

matt

on a side note, one of the biggest light polluters there is, the moon, is entering the best stage for observers.. a new moon :)

[astro744]

Stellar coordinates on star charts use RA and Dec which is similar to longitude and latitude. What you also need to know is Local Sidereal Time. LST is approximately four minutes faster per day than standard clock time. You can get software or an app to tell you LST.

The LST equals the RA of any object crossing the north south meridian. In other words an object that has risen and reaches its highest point for the evening will have its RA equal the LST at that time. This gives you another way of determining LST if you know the RA of an object and it is just crossing, then that is the current LST at that time.

Dec is simply latitude. 90 degrees declination is equal to you location latitude up from the southern horizon. E.g. At 30 deg south of the Equator the south celestial pole (90 deg Dec) is located 30 deg up from the southern horizon. The celestial equator arcs from east to west and reaches 60 deg high from the northern horizon for an observer located 30 deg south of the Equator.

Orion is low in the west at sunset so you'll have to wait a couple of months before it reappears in the per dawn hours in the east.

At 19:43 Eastern Australian Standard Time the LST was 13:48. Use Stellarium and turn on equatorial grid and make sure cardinal points are showing and then look toward the N horizon.

[noeyedeer]

some good info but maybe confusing for some. with a dob, you need to know altitude (height) and azimuth (direction). if you have an eq mount then Ra and Dec can be used to find objects with all that info.

software / Appz can convert ra/Dec into alt/az. for objects the star charts at skymaps are handy www.skymaps.com/downloads.html just make sure to download the southern hemisphere.

matt

[astro744]

Alt-Az coordinates are good but they are constantly changing and will never match any atlas. The coordinates for Orion for example will be in any book, equatorial coordinates rather than Alt-Az.

I agree with you though, and this applies to either an EQ or Dob mount, that knowing the altitude and azimuth of an object help greatly in locating it on any given night. Knowing the RA and Dec and also the LST will tell which objects are up and when.

[Renato1]

If you don't have a GOTO scope (akin to a GPS Navigator in a car) the process for finding things in the sky is pretty much the same as using Sydways for finding streets. In a street directory you look up a street's coordinates in the back, and find the main page where they are plotted. You then look at the small scale maps in the front to figure out how to get to the map on which the street is plotted. You hop along from street to street as you see them plotted in the directory.

In astronomy you can start out with a planisphere or the monthly sky maps in magazines, to give you an idea of where all the major signposts are (brightest stars, brightest constellations). I had to hold them overhead and used a red torch to get a one to one correspondence. The planisphere is useful because it tells you what is in the sky at any day and hour of the year.

With a 10" telescope, a sky atlas like SkyAtlas 2000 or an equivalent free downloaded and printed out one would be ideal - as your telescope shows most of the things plotted in it, and an 8X50 finderscope makes it easy to hop along the plotted stars to the plotted object. And you start your hopping from one of the signposts that you learned with the planisphere or basic maps.

So when you get the coordinates to the Orion Nebula, you look them up in say SkyAtlas 2000, and you'll see it plotted there. You remember from your planisphere or basic maps that Orion is that constellation up there with two brights stars. You find them in the sky, you match them up with what you see in your atlas, and you put your finderscope on one of them - with a straight through finder, you turn your atlas upside down, with a correct image one you don't - and then you just follow the little star patterns hopping along in the manner indicated in your atlas, as you see them in your finder, till you get to the Orion nebula.

The Orion nebula or another bright object like Omega Centauri are good ones to practice on - since you can see them by eye and just aim your finderscope straight at them. So you can see how well your star hopping is going when you start from the signposts.

When you hear of a new nova or comet at some co-ordinates in the sky, all you then have to do is pencil a mark onto you atlas at those co-ordinates, and then just do the same hopping process to go and observe the new objects.

Good luck,
Renato

[astro744]

A good thing to remember is the RA and DEC (particularly RA) of the Sun at any time of the year. This will then tell you that any object an hour or so either side of the Sun will not be visible until the Sun moves against the background stars (The Earth is of course what is moving in its orbit around the Sun).

The Sun is at the following positions at four important points in its orbit:

March 21: RA = 0 hr, Dec = 0 deg. This point is know as the Vernal Equinox and is the point where the Sun crosses from south to north and spring begins in the northern hemisphere and autumn in the southern (although we have reset this to March 1)

June 21: RA = 6 hr, Dec = +23 deg. Northern summer solstice. This is the point where the Sun is at its furthermost point north of the celestial equator (due to tilt of earth's rotational axis) and the start of summer in the northern hemisphere, (winter for southern hemisphere and reset to June 1).

September 21: RA = 12 hr, 0 deg. Northern autumnal equinox. Sun again heads south of the celestial equator and autumn begins in the north and spring in the south, (spring reset to Sep. 1 for us in Aus).

December 21: RA = 18 hr, -23 deg. Northern winter solstice. This is the point where the Sun is at its furthermost point south of the celestial equator (due to tilt of earth's rotational axis) and the start of winter in the northern hemisphere, (summer for southern hemisphere and reset to Dec 1).

Note if you know the RA of the Sun you can then determine the RA of the north-south meridian at sunset (+6 hr) and this will show you what is likely to be up. e.g. if the RA of the Sun is 6 hr, (currently a fraction over as we are just past the southern winter solstice) then at sunset the RA of the meridian directly overhead is 12 hr. Add approx 1 hour in the west and subtract 1 hour in the east (for twilight) and you can observe objects with an RA of 7 hr to 5 hr in the course of the evening.

Declination of course also comes into it and the further an object is south in the southern hemisphere the longer it is above the horizon and vice versa for objects north of the celestial equator. This of course also affect sunset and sunrise times.

I hope I haven't confused anyone too much. You can search the web for more info as there are plenty of good resources out there.

[WynneP]

If you scope is well-collimated, the problem could be temperature adjustment. Try putting the scope outdoors at least 1 hour before you start observing. This will allow all the components to equalise with the atmospheric temperature and hence minimise tube air currents. Also, make sure you run the fan.

In my experience, it is generally the case that the later in the night you observe, the better the seeing. Atmospheric turbulence gradually dissipates as the warmth of the day disappears.

I have a 12" GSO dob and lately have been getting great views of the Cassini division in Saturn's rings and so expect your scope should be able to achieve similar.