Got a beginners' question? No matter how elementary, our friendly forum community and magazine writers will answer it.
Wed Jan 27, 2016 7:58 pm
The telescope came Friday so was up most of the night, saw the moon in some lovely detail, also not too bad images of Jupiter although a little blurred I could however see lines on Jupiter unsure if this could be down to the eye piece that came with the telescope, I did try it with different magnifications the 10mm and 25mm and tried both with the 2x barlow lens.
I attempted to see Saturn, it eventually came above a cloud bank around 7am but I think there was too much atmosphere as all I could see was a white blob.
Looking forward to another clear night now to see Jupiter in more detail and possibly Saturn.
Could the blurriness be down to the eyepiece? and if so can anyone recommend a premium one?
Wed Jan 27, 2016 9:01 pm
Let's see, you have a focal length of 1500mm. So your 10mm eyepiece will give 1500/10 = 150x, and your 25mm will give you 1500/25 = 60x. Double those magnifications with the barlow. So you have a range of 60x, 120x, 150x and 300x. It is debatable, but I was always brought up with 50x per inch of aperture for maximum, so that mean a maximum useable magnification of your telescope (5 inch) of 250x. Therefore you should not use the 10mm with a barlow. To get the best magnification out of your telescope you need an eyepiece which will give you about 200 - 250 times. That's a 6mm or 7mm eyepiece. (Alternatively use a 12 or 14mm eyepiece with the barlow.)
Other things to consider - Your Barlow lenses might be a cheap one and not up to standard. You might have to buy a better one. Also consider that Saturn is pretty low down at the moment and not much more than a blob in my telescope either!
Features on Jupiter are quite subtle, and I've never found them particularly sharp. Seeing conditions might not have been particularly good that night. You might find persistance with it might reward you there.
Wed Jan 27, 2016 10:28 pm
Aratus is right and I have a lot of respect for him. However, I disagree with him (and most other astronomers!) in that a Skymax 127 is capable of supporting maximum magnifications above 250x. It is a Maksutov and not a Newtonian. I have been known to push it to nearly 500x to see features on the Moon.
On the other hand, I used a 32mm focal length eyepiece and 0.5x focal reducer to get some of the Messier objects.
Wed Jan 27, 2016 11:15 pm
My respect for Philip is reciprocated, but I've previously never come across such claims for any
5" telescope. However, I'm all for challenging the status quo!
However, can I suggest than any discussion on the subject be started up elsewhere on the forum.
I think we can agree that given Stu's current range of magnifications, whatever else he buys he does need an eyepiece that will give him a magnification in the 200x - 250x area.
A focal reducer and/or larger eyepiece will certainly be useful to give a wider field of view - a good investment. However there is a limit to the width of field before vignetting occurs (you see the circular edge of the telescope). Its always a good idea to check before you buy (bitter experience!
Thu Jan 28, 2016 10:35 am
Once again some fabulous help and information, I will look at changing the barlow and picking up some other eye pieces just to try out and see what the difference will be.
Thu Jan 28, 2016 4:32 pm
There are plenty of 6mm eyepieces out there. One issue you might want to consider is that with small eyepieces you need to get the eye nearer the eyepiece. Also looking directly into the centre of the eyepiece is more essential. They are a lot less tolerant when you move from side to side causing the image to disappear.
Getting say, a 14mm eyepiece will give you 1500/14 = 107x magnification which would fill a gap in your range. With a barlow that would be 214x.
It may not be a issue with you, but with my aging eyes I tend to use a barlow.
re the 10mm eyepiece with the barlow giving 300x magnification, there is no harm in trying, but if it is too wobbly and/or dim, you will know why, and you can go down to a lower magnification. Basically that is the procedure. Increase your magnification until it becomes too dim or unstable to view, and then go back to the previous magnification. That will be the best for that night's conditions.
Consider Philip's advice too about getting a focal reducer to get good, bright, wide angle views of larger objects.
Fri Jan 29, 2016 12:32 am
As I keep saying on this forum, there is no point over cooking the magnification because you'll end up seeing less! The reason for that is due to the eye's central region, called the macular, having a finite size. If you under illuminate it, that is with a cone of light that is smaller than the macular, you will see less. It's as simple as that.
The general accepted limit is an eye piece exit pupil of 0.8mm. You can work it out by dividing the eye piece focal length by the telescopes f-ratio. For example an F10 with a 10mm EP has an exit pupil of 1mm. A 20mm EP would have a 2mm exit pupil, and a 5mm EP would have a 0.5mm exit pupil. You get the picture.
By the way, the upper limit is defined by how wide you can dilate your pupil. Having a wider exit pupil simply means you can move around in the light cone, and you will be illuminating all of your retina in the process.
I strongly recommend a zoom eye piece for ease of use, and fun. Swap out to a wide field EP for specific observations when that suites.
Sat Jan 30, 2016 10:55 am
Stu - Amateur astronomy always brings up several different solutions to any issue. It's quite rare to meet someone who isn't happy with the solution they chose. Whatever you choose to do now, I'm sure you will be pleased with it. In the end it is all about getting the best advantage out of what you buy - whatever that is.
Sat Jan 30, 2016 11:27 am
Well that's true! You can find as many answers as you can find people to ask. I hope that you find my answers reasonably well reasoned though.
Sat Jan 30, 2016 9:31 pm
Hi, I am happy with the setup I have decided on.. just want to get the best out of it
I really appreciate all the advice given its helping me understand this science much quicker than just trying to do it on my own.
Learning from experience is the best experience
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