View Full Version here: : Baby Dob query
02-03-2012, 07:35 AM
Hi all, I'm John a newbie to this caper and I am trying to set up a 5 inch Heritage 130p mini-Dob SkyWatcher for my son. we have been given a 3x Saxon achro Barlow recently and we attempted to marry the Barlow with the mini-dob with very limited results. While we had good views of the moon with both the 10mm and 25mm eyepieces as soon as we put the Barlow between the eyepiece and the telescope the lunar image became blurry and no amount of focusing helped. The same problem was noted with stars. Am I doing something wrong? Should I be using a 3x Barlow with this device? I was told it would be fine to use.
The advice I have been given is that it is possible to get 50x mag for each inch of aperture, so we should have an upper limit of 250x before losing resolution through the eyepiece. Using the 10mm eyepiece with a scope with a focal length of 650mm I have a mag of 65x, add to this the 3x Barlow and that brings us up to about 195x mag, below the recommended threshold. Am I missing something or doing something wrong? Please note that it was a fairly light sky last night and also we were about 15 metres from a small light, so there was a fair bit of pollution. Your guidance on this would be very much appreciated. :thanx:
02-03-2012, 11:02 AM
John, I think the Barlow may just be physically too long to allow the focuser to reach focus. In other words, if you could get a few more turns of the focus knob it would probably come into focus.
You may just need a different brand or physical size Barlow.
Did you purchase the scope from a local dealer? If so they can probably help.
Hopefully others will chime in and offer other possibilities.
02-03-2012, 11:41 AM
The 50 per inch is a "theoretical" maximum assuming very dark skies and perfect seeing. In reality is is closer to 10 or 20 per inch and in a small scope like yours, 50x would be usually adequate for reasonable views.
A 130mm scope at 50x will show quite a few DSOs nicely, EG M42, Omega Cent, NGC 2070 and should be able to pick up Cent A Galaxy with little problem.
02-03-2012, 11:44 AM
G'day John, :welcome: to IIS!
Like Michael said, some barlow lenses may not work, needing too more in-travel than your focuser may allow.
However, by the sounds of it, you did achieve focus, which leaves one of two possibilities.
The first was that the prevailing atmospheric conditions didn't allow for 195X to be used. The typical maximum possible is only 100X to 120X, REGARDLESS of the size of the scope you've got! 250X is pot-luck to achieve with minimal "boil" of the image. Anything more and you should be buying lottery tickets. My 5" scope gave fantastic images at 250X, but only when the atmosphere was stable, but it too was a very, very good scope.
The other thing it could be is that the mirror of your scope is spherical in shape, and not parabolic. Spherical mirrors are cheaper to produce, and if your scope has a long focal ratio, these spherical mirror can perform well. However, with the fast focal ratio your Heritage scope has got, if the mirror is spherical, you've got no chance of pushing the scope's magnification. The light from the primary mirror just won't come to the one focal point.
The GOOD news is that it sounds more like it was the prevailing atmospheric conditions that were the problem. A quick test would be to use your 25mm eyepiece with your barlow. If the image was still good, I'd suggest it is the atmosphere.
If things don't improve, I'd then suggest you talk to the folks who sold you the scope. If it is spherical, all the scope will be good for is low powers. You will not be able to get a good high power image from it.
I hope this makes sense.
02-03-2012, 01:35 PM
This would make sense. It was a gift so no money lost, sounds like we may just need a bigger Dob:) Might have to shelve this Barlow until we get some experience under our belts.
Thanks Malcolm, that's encouraging.
Thanks for your response, it makes sense. I have overestimated the magnification capabilities of the scope (probably a common problem so I'll console myself with that). I didn't actually achieve focus with the Barlow even with the super wide 25mm eyepiece so it may be atmospheric conditions or the Barlow simply isn't compatible. The mirror on the scope is a parabolic mirror (at least that's what is says) so that doesn't seem to be the problem. I'll reassess tonight as it looks to be a reasonable night for observation. Thanks all.
02-03-2012, 03:15 PM
I believe that the mirror in these is parabolic. I had a look at one of these and for what they are they are not too bad mechanically.
Have a play with it at low power and see what you can get, it will whet your appetite for more aperture and before you know it you will be shopping for a big dob!
03-03-2012, 11:10 AM
Malcolm, do you or anyone else think I should just concentrate on using various eyepieces (10mm-4mm) rather than worrying about Barlows at this stage? We are interested in viewing the moon (getting the best perspectives we can) and if possible-conditions permitting-planets until my son and I learn our way around the sky. We have just picked up a reasonable set of filters to assist with this. Do you have any advice for the best set up for this taking into account the limitations of our 130mm scope? Many thanks.
03-03-2012, 12:42 PM
The scope came with a 10mm and a 25mm eyepiece which would give you 65x and 26x respectively. As I said earlier the magnification limit is a theoretical 250x, but on most nights getting 20x per inch of aperture would be pretty good which is roughly 100x in your scope. To get that you would need an eyepiece of 6.5mm focal length.
The thing to remember is that dim objects quote often do not take magnification well, it spreads out the avaialble light and makes them dimmer. In theory more power can also increase the contrast by making the background darker but that rarely outweighs the dimming power of trying to magnify too much.
At the stage that you are at I would concentrate on using the eyepieces you have on the moon and planets. There is a lot of interesting detail to be seen at low power. On the moon there are plenty of caters of various types to check out, rays from newer craters, and if you get a run of clear weather you can track the terminator (the edge of the shadow, not Arnie!:P) as it moves across the surface from night to night. Jupiter is getting really low ATM but you should still be able to pick out the moons and note how the change posotion even on an hourly basis.
Mars if probably not going to be much more than a reddish disk. With luck on a good night you may be able to catch a hint of the North Polar Ice Cap. Saturns rings are really nice. A good project would be start to try for some of the Messier objects also. Most of them would be accessible with your scope, though some of the Galaxies will be hard catches.
I would leave the barlow alone at this stage, keep things simple is always a good idea!
If I was going to recommend a filter, if you have one use a ND (Neutral Density or sometimes called a Moon Filter) to look at the moon. This will reduce the glare and make it much more comfortable.
Just some thoughts! Hope they help.
03-03-2012, 02:42 PM
Thanks Malcolm, I appreciate your advice and I'll follow up on it. It's great to be able to have access to a site like this for two budding hobbyist astronomers trying to find a clear path.
05-03-2012, 12:05 AM
This. I used the Barlow again tonight and got impressive views of Jupiter and two of it's moons and just picked up Mars really well and also Saturn and its rings, just making out the Cassini division. To someone who still struggles with time and space and the distances associated with it I can only say wow to have see these things in real life. I have travelled far around the world, including to many famous historic sites throughout the Mediterranean but seeing Saturn tonight like that up in the night sky far surpasses anything I have ever seen.
05-03-2012, 08:46 AM
John, if you can make out Cassini's division, mate, you are on a winner with that scope! :D
A 5" should be able to resolve it without blinking, if it's worth its metal. And, you'll be able to pick up several of Saturn's moons - at least 4. 6 or more if your eyes are 15 years old, :rolleyes: . Including the giant Titan. The tell tale sign is the moons will all be aligned in the plain of the rings and will be changing position over the course of a few hours like Jupiter's moons. Even some soft banding in the clouds.
The only thing I could add to Malcom's sound words is Patiences. Take your time.
If you could go to an Astro club meet or local star party with your scope, you'll be able to borrow a high power eyepiece or two of real good quality. You'll be able to see the full potential of your scope, without a barlow, as Malcom said :thumbsup:
05-03-2012, 10:00 AM
Well done on Cassinis Division. I struggle to see it in my 12", I think largely becauise the rings are almost too bright!
05-03-2012, 10:10 AM
The conditions were perfect last night, very little boil at the upper end of my mag allowance, it was very still, dark and humid. When I say I saw the division I clearly saw a black line running around the middle of the bright ring. I also saw Titan.
To me, that's seeing the division;)
05-03-2012, 10:38 AM
We really do want to build up some experience at low mag, but we couldn't help ourselves last night:) My son and I are busy spending time on star charts when we have time. I would like to explore the potential of the scope for a while and eventually join a club with a little more experience under my belt. Cheers.
05-03-2012, 01:25 PM
Do filters assist with this?
05-03-2012, 03:31 PM
Coloured filters will not assist with seeing the division. An ND filter may help if the glare is making it hard to see, never tried so cannot comment.
Coloured filters are useful for enhancing contrast of faint features on planetary surfaces, particularly Jupiter and Mars. Saturns surface features are really faint.
Having said all that I am not a planetary observer so others may give more useful info!
05-03-2012, 09:49 PM
Okay thanks for all your advice:)
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