Which lenses / eyepieces do we use to stargaze ?

ddlong

Hi there. My son and I had a very disappointing first attempt at stargazing last night so we need some help from you experts out there ! We got a new Newtonian telescope but the instructions are very poor so we don't know how to use it properly. There are various bits and pieces that came with it but we don't know which ones to use. Here are the details:
Telescope:
Model= Newtownian 76700
Diam = 76mm
Focal Length = 700mm
Accessories:
1.5X Erecting eyepiece
3X Barlow lens
H20 lens
H12.5 lens
SR4 lens
During the daytime, when we screwed the H20 lens into the side and looked at a house down the road, everything was upside down but I had been told this is normal. We then put the '1.5xErecting eyepiece' between the H20 lens and the telescope and this flipped the picture seen through the lens through 180 degrees (the picture through the viewfinder is still upside-down though). We then replaced the '1.5xErecting eyepiece with the '3xBarlow lens' and the picture went upside-down again. However, last night we had a lovely clear night so we brought the telescope outside but we couldn't see any stars at all through the viewfinder or through any combination of the lenses/eyepieces. We could see some lights from a town on the horizon, but couldn't see any stars at all even though with the naked eye we could see the stars perfectly. The moon unfortunately didn't come out until after we went inside. Can anyone advise on which lenses/eyepieces we're supposed to use to look at the constellations ? Thanks.

peterako

Very sorry to hear that all was not well last night.

But, persevere! It will be worth it.

First thoughts:

If you could view terresterial objects by day you should have been able to view celestial objects at night....

How did you 'point' the telescope at the star/planet etc?

I'm not sure what your viewfinder is like on your scope, but try and make sure that it is aligned by daylight (find a distant tree/house etc. and make sure that it is centrally located in the eyepiece AND viewfinder by adjusting the viewfinder).

With your scope the finder may be a bit basic so only fnction as a guide.

When I had my Celstron Astromaster i gave up on the finder and used to aim teh scope by lining up teh scope and looking along its length at the object before trying then to find it with the eyepiece with small movements left/righ/up/down.

Second:

Use your lowest magnification first! (Unless you have a great finder).

Don't bother with the erecting eyepiece. It will only rob light.

This gives you the widest field of view and you're more likely to find the object this way.

Once you've centred the object at low magnification you can change eyepieces upping the magnification etc.

Third.....

It may be that there is not enough movement on your scope's focusser to attain focus.
This is unlikely, but may be the case if you're barlow is moving the focal point.

Back to second point above....try with your lowest magnification lens only first.

Lastly.....

with a 76mm reflector you will not be gathering a lot of light.....and will not be able to get very high magnifications.

A general rule of thumb is (from memory):

(Diameter of primary Mirror in inches) x 50 = the theroretical max useful magnification

But in practice it's often lower.

So yours will be 150 x magnification all else being perfect. But I would recon maybe 100 - 120 woudl be pushing it.

If you try and magnify higher than that you really won't be seeing anything useful.

Magnification can be calculated (for your scope) as:

(Focal Length of Scope)

---

(Focal Length of Eyepiece)

An example:

H20 I presume is your 20mm eyepice....

700 / 20 = 35 magnification

After all the above we're into alignment issues......

But....try with the 20mm eyepiec first. Maybe Saturday moring on the Moon in daylight?

Clear skies,
Peter

ddlong

Thanks a million for taking the time to type all of that ! I'll print it off and try and digest it at home tonight !

Drake66

Hi you can align the finderscope in the night as well. If it is clear tonight find the really big bright object high in the southern sky; the planet jupiter. Put the 20mm eyepiece into the telescope and point it towards jupiter. Try to align the telescope on jupiter by eye; by looking down the tube. Check the eyepiece to see if you have it; it may take a while. When you have jupiter, center it in the eyepiece and don't move the scope. Now adjust the finderscope using the little lugs until it is centered on Jupiter and lock it in place.

Forget the barlow lens, errecting eyepiece and the sr4 for the moment.