Question about my telescope...

[Fr0do]

ok, last time I posted here was about me seeing saturn, some of you thought I was seeing a star but I know I'm not, cause I know what a star looks like through a scope and that was not a star... and I no longer think I saw rings, I'm almost 99.9% sure I did..... but I was using a 20mm eyepeice that came with the scope, now in my manual it says the 20mm eypeice will see thigns smaller than with a lower mm eypeice, now I only got 2 with my scope, the 20mm and the 4mm, now the other night I tried looking at saturn with the 4mm and I saw a really big blob, now I wanted to focus it but I couldn't, cause I couldn't keep it in my sight, my scope isn't very steady sooo when I get it in my view I let go of the scope and it moves away, and when I don't let go it shakes sooo much it just goes in and out of sight..... soooo anybody got any tips on keeping this object in my sight? when I use the 20mm it's not a big problem because it's smaller so easier to keept in sight... somebody please help!! are they all unsteady or is mine just an el-cheapo model that will always do this?

[SarahMc]

ok, last time I posted here was about me seeing saturn, some of you thought I was seeing a star but I know I'm not, cause I know what a star looks like through a scope and that was not a star... and I no longer think I saw rings, I'm almost 99.9% sure I did..... but I was using a 20mm eyepeice that came with the scope, now in my manual it says the 20mm eypeice will see thigns smaller than with a lower mm eypeice, now I only got 2 with my scope, the 20mm and the 4mm, now the other night I tried looking at saturn with the 4mm and I saw a really big blob, now I wanted to focus it but I couldn't, cause I couldn't keep it in my sight, my scope isn't very steady sooo when I get it in my view I let go of the scope and it moves away, and when I don't let go it shakes sooo much it just goes in and out of sight..... soooo anybody got any tips on keeping this object in my sight? when I use the 20mm it's not a big problem because it's smaller so easier to keept in sight... somebody please help!! are they all unsteady or is mine just an el-cheapo model that will always do this?

The shorter the focal length of the eyepiece, the more magnification or "power" you'll have. You'll also have less contrast. The only time I use that kind of power is when I'm collimating the scope, and the conditions allow for high powers (a very rare occurrence).

One thing you'll learn after a while is how to observe at lower powers, and still be able to see objects and features that you didn't see previously when you started. My experience with EP's that come with most scopes are that they're not of the best quality. Most are older, less expensive optical designs and usually aren't a/r coated very well, if at all. You might consider investing in some decent plossl EP's, or some of the lanthanum EP's on the market.

High powers are really only good for splitting double stars. The vast majority of the time, I use much lower powers and wider FOV's - from 40x to 200x,most commonly from 75x to 150x. I'm not sure what the focal length or f/ratio of your scope is, but calculate the power that the 4mm EP gives you - chances are it's far too much except for extremely good seeing and transparency. You might be more happy with EP's ranging from 10mm to 40mm.

Bad mounts, IMO, are the primary reason for people becoming disinterested in amateur astronomy. A poor mount is frustrating at best, and more often than not, they end up in a closet somewhere gathering dust. The scope may be just fine and be of a decent quality, but if the mount can't support it properly, the scope is worthless.

An alt-az mount is probably the worst, as it's near impossible to track objects properly. Cheap equatorial mounts are just as bad because even though you can track using the RA axis, the mount wobbles and focusing is next to impossible.

I'm not sure what to tell you, other than do whatever you can to make the mount more stable. Anti-vibration pads will help. Adding weight to the centerline of the tripod near the base helps (lower the center of gravity). Making sure the gears have as little backlash as possible, and making sure the axis bearings are tight helps. Make sure the scope is balanced properly.

Lots of people are happy with their mount, but not the tripod. Many have replaced the typical aluminum legs on less expensive tripods with wooden ones, providing a much better support for the mount. Always make sure the spreader has the legs spread as far as possible when you set up.

If you have an aluminum tripod that has legs with a center section that are adjustable, try using the tripod with them fully collapsed, rather than extended. The higher the tripod is off the ground, the less stable it usually becomes. There's some after market stabilizers for those legs, they're like a clamp that goes around the leg and squeezes the center extendable section. Those help quite a bit.

The last thing I'd suggest is finding out who makes the mount - most are similar to the EQ-1 to EQ-5 mounts, and there's a lot of information on the web on how to tweak those mounts to be more stable. One of the great advantages of a dobsonian mount over an equatorial is the stability, but you lose the easy tracking and have to "bump" the scope every couple minutes. Equatorial Newtonians really require a stable mount and tripod, because of the size and weight of the scope itself.

Hope that helps a little. Don't give up too quick. If you have an astronomy club in your area, join. There'll be people at the meetings that will be more than willing to look at your equipment and offer some suggestions to improve your observations.

[SirThoreth]

SarahMc,

Thanks for posting. That's a heck of a lot more succinct than a lot of the posters on sci.astro.amateur, who discommend the cheaper equatorial mounts as being "bad", but often don't go into major details as to why they're so bad. I ended up deciding today to make myself a barndoor mount for my camera in order to track for astrophotography (seems the best solution for the astrophotographer on a budget) - I looked up some instructions on the 'net, and it looks fairly straightforward to do, and I can even get the parts I need to build a clock-driven motor for it at my local RadioShack. 8)

Fr0do,

What's the focal length of your telescope, and its aperture? Or, if you can only find one of those, what's its f/stop (like f/10, f/11, etc)?

Basically, here's the deal on magnification on a scope, camera, etc. Your magnification is determined by dividing the focal length on your optical system by its "eyepiece" focal length. Say you've got a telescope with a 700mm focal length, and your two eyepieces (4mm and 20mm). Your 4mm eyepiece would give you around 175x magnification, and your 20mm eyepiece a 35x magnifcation. Theoretically, there's no limit on how much you can magnify, but there is a catch - your aperture determines how much you can magnify the image clearly. The numbers I've seen is 50 times your aperture in inches, or three times your aperture in millimeters. So, say you've got a 60mm scope - that's 2.4 inches, so your maximum clear magnification is either 120x (if you're using the Imperial system), or 180x (if you're using metric). Hmm....kinda weird. SarahMc, do you have more exact numbers?

On my 35mm camera, the "eyepiece" focal length (the distance from where a lens focuses the image and where the film is) is around 42mm. In theory, a 42mm lens would be exactly 1x magnification, equivalent to what you can see with the naked eye. Problem is that's a difficult as heck length to make without distortion, so the 50mm lens is considered the "normal" lens, and every 50mm of focal length is considered a power of magnification - hook that 600mm telescope up directly to a camera like mine, and you'll only get around 12x magnification.

[Fr0do]

thanks, you realy clarified alot of stuff for me, I calculated the power that the 4mm EP gives me and the calculation is(in my manual anyway)
900mm/(bye MM of EP)4mm = 225x
that doesn't sound too hot.. the 20mm is 45x so would I be better off with a 10mm EP witch would be 90x....and btw, I won't let cheap equipment steer me away from astronomy

[SarahMc]

Basically, here's the deal on magnification on a scope, camera, etc. Your magnification is determined by dividing the focal length on your optical system by its "eyepiece" focal length. Say you've got a telescope with a 700mm focal length, and your two eyepieces (4mm and 20mm). Your 4mm eyepiece would give you around 175x magnification, and your 20mm eyepiece a 35x magnifcation. Theoretically, there's no limit on how much you can magnify, but there is a catch - your aperture determines how much you can magnify the image clearly. The numbers I've seen is 50 times your aperture in inches, or three times your aperture in millimeters. So, say you've got a 60mm scope - that's 2.4 inches, so your maximum clear magnification is either 120x (if you're using the Imperial system), or 180x (if you're using metric). Hmm....kinda weird. SarahMc, do you have more exact numbers?

The magnification formula is correct.

About theoretical magnification limits, it has a lot to do with the quality of the optics (including primaries, secondaries, objectives and EP's) involved, the coatings on them, and the conditions, but 50 times the aperture (in inches) is about right, under very good skies. For instance, my 8" SCT has a limit around 200x for the majority of nights I observe. There have been times of excellent seeing when I've doubled that to the theoretical limit, and even gone beyond - but those nights are rare occurances in this part of the country.

My 16" dob (which rarely gets used because of it's size) can do much better under the same conditions. Of course, this assumes that both scopes are properly collimated. My smaller refractors will rarely reach about 35 times the aperture - with the exception of a very old 60mm Unitron refractor that has just amazing optics for an achromat.

Add to all the above the quality and experience of the observers eyes, and you have a "theoretical" limit that fits most scopes and observers under most conditions.

[Kaptain K]

Fr0do,
45X is more than enough to see Saturn and its rings. I can tell that it is not a star with 10x50 binoculars (not a point, like a star, but a sort of elongated dot). You should be able to see the gap between the planet and the rings and (when the seeing is exceptional) the gap between the "A" and"B" rings. Saturn currently is 20 arc seconds across. At 45X that translates to 15 arc minutes of apparent size. The Moon subtends 30 arc minutes, so Saturn, through your scope, should appear half the size that the Moon does to the naked eye.

[SarahMc]

thanks, you realy clarified alot of stuff for me, I calculated the power that the 4mm EP gives me and the calculation is(in my manual anyway)
900mm/(bye MM of EP)4mm = 225x
that doesn't sound too hot.. the 20mm is 45x so would I be better off with a 10mm EP witch would be 90x....and btw, I won't let cheap equipment steer me away from astronomy

Don't forget that you can always use a barlow to increase the magnification. A 2x barlow will take your 20mm to 90x. A lot of people use barlows when they have a limited EP set, or to get magnifications between certain EP ranges. I prefer not to use a barlow, since they add yet a few more optical sufaces to the path, resulting in some light loss. The only time I use my barlow now is with the webcam for imaging.

But, it's yet another option that many begining amateurs use, as well as myself for many years until my EP collection grew. There's also the option of variable focal length EP's, something I've avoided over the years. They may be nice for terrestrial use though.

[Fr0do]

I use a 3x barlow lens.... and when I look at saturn(I'm pretty sure it is from the descriptions in charts and such) it's pretty small.... that is when it's in perfect focus...but I'm sure i see the rings.... but it's sooo small... when it's not in focus, the object will be biger but I will see the plastic round thing that is right at the edge of the hole in the scope so it partially blocks out the image.... I'm soooo mixed up right now.... btw these are all the specs I could find for my scope
900mm focal length
4.5" objective lens(f/7.87)

[Fr0do]

and one more thing, do all eypeices fit in all scopes? is there some sort of universal thing going on there?

[SarahMc]

I use a 3x barlow lens.... and when I look at saturn(I'm pretty sure it is from the descriptions in charts and such) it's pretty small.... that is when it's in perfect focus...but I'm sure i see the rings.... but it's sooo small... when it's not in focus, the object will be biger but I will see the plastic round thing that is right at the edge of the hole in the scope so it partially blocks out the image.... I'm soooo mixed up right now.... btw these are all the specs I could find for my scope
900mm focal length
4.5" objective lens(f/7.87)

Skip the barlow then with the 4mm. Just use it with the 20mm until you can get a 15mm or 10mm EP.

20mm is 45x - with the barlow it's 135x
4mm is 225x - with the barlow it's 675x, way too much power for almost all amateur scopes.

4.5" aperture means about 225x maximum magnification under very good skies. You're more likely to use from 50x to 125x unless you live in the desert. That 4mm EP will rarely be of any use. Stick with the 20mm alone, and with the barlow for now (45x and 135x).

As for EP's, there's three standard sizes in amateur use. The older .965" diameter barrels, the common 1.25" barrel, and the larger 2" barrels. Most scopes sold today use the 1.25" EP's, but you can buy adapters for both of the other sizes, or a "hybrid" diagonal that accepts larger EP's than the scope was designed for. For instance, I use a hybrid diagonal for my old Unitron that was designed for .965" EP's. It allows me to use the newer 1.25" EP's instead. On my SCT, the diagonal takes 2" EP's, but there's an adapter that goes in so that you can use 1.25" EP's as well.