A cheap telescope can discourage anyone from pursuing the astronomy hobby. Usually the effect of lowering the price of a scope is associated with compromising its performance. In many cases the vendor includes additional features that intend to attract the buyer and have no real value.
The point is that if the main features of the scope such as the quality of the objective and the focusing mechanism are acceptable we can deal with the rest and make improvements.
All the tips and modifications described here are common knowledge to experienced amateurs. A fresh user may find this information helpful in evaluating and using her/his own instrument.
I also demonstrate how to make a finderscope out of a small set of binoculars and how to use discarded lenses to make your own eyepiece. Some of these modifications are shown in the photo.
The point is that if the main features of the scope such as the quality of the objective and the focusing mechanism are acceptable we can deal with the rest and make improvements.
All the tips and modifications described here are common knowledge to experienced amateurs. A fresh user may find this information helpful in evaluating and using her/his own instrument.
I also demonstrate how to make a finderscope out of a small set of binoculars and how to use discarded lenses to make your own eyepiece. Some of these modifications are shown in the photo.
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Signing UpStep 1: My Bresser Junior 70/700 EQ-Sky
I bought this small scope as a department store offer of LIDL in Athens a few years ago for 70 Euros (the current internet value is about 150Euros). I found a lot of reviews and modifications for it, coming from Germany, Austria and England although for a different equatorial mount with a counterweight. This scope has been included in a comparison test for 70mm refractives in the BBC edition "Sky at Night " November 2010.
I summarize how I feel about it:
ADVANTAGES
1. Light weight, it will go anywhere, the biggest advantage.
2. The objective is good. I tested the field of view with a Celestron 8-24mm zoom eyepiece and the result was about 90%.
3. The 20mm and 12mm eyepieces are usable.
4. The focusing mechanism is acceptable.
5. The tripod works both as an Alt-Az and Equatorial. This is useful as well as instructive..
DISADVANTAGES
1. The viewfinder is not too secure on its holder but this can be dealt with. The viewfinder's lenses are plastic and diffuse light.
2. The 4mm eyepiece is inevitably of very low quality with plenty of distortion and chromatic aberrations. This was included in order to advertise 175x magnification. A 2x Barlow lens for use with the 12mm eyepiece would be preferable but I suspect that at this low price the Barlow would not be good either.
3. The tripod tries to do it all but there is a cost. It is too light and unstable. Even a weak breeze will make the scope tremble. The guiding system that supports the scope is well designed and well made. The main source of instability are the legs of the tripod. Hanging a weight makes the whole system more stable.
4. The set included also an orthoscopic tube with a plastic lens in order to turn it into a spotting scope. To be discarded. I use a 45 degree prism instead.
WHAT YOU CAN DO WITH IT
You may use a scope of this size and type to observe the planets (zones on Jupiter and satellites, the phases of Venus, Saturn, Mars), the moon, nebulas, clusters and double stars.
The theoretical minimum feature size that a 70mm objective lens can resolve is ~2 arcsec. To compare with, the apparent size of Saturn is 15-20 arcsec. Saturn with this scope appears as an oval shape , no chance of seeing more features.
I also have resolved double stars distant 8-10 arcsec apart, provided their magnitude is high enough. For example Mizar A (magnitude 2.25) and Mizar B (magnitude 4.0) in Ursus Major , distant 14.5 arcsec is an easy target. Everybody's favourite Albireo in Cygnus (35 arcsec) is even easier to resolve in its two colourful components.
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Sorry I can't be more specific as I can't get to either right now as the weather just went bad here and I keep them both in my trailer in my back yard (it is a long way away). I have a picture of the one (I buy a lot of junk at garage sales) I want to put the bigger focusing mechanism on. But no picture of the one I want to pull the focusing mechanism off.
They look a lot alike to me except for the diameters. You know the flanges that connect to the tubes, rack and pinion etc. But is there anything that would stop me from doing something like this that I should know about?
Mainly I want to change them because I have the lenses for the larger diameter focuser, but not for the smaller one. Thanks.
If you mean can you swap the smaller focuser for the larger one, it is possible, but you would need to make sure that the secondary mirror (the small oval diagonal mirror that reflects the lightpath into the focuser/eyepiece) is large enough to take advantage of the larger eyepiece diameter.
I do not know the condition of you 4" criterion. However if the two mirrors are not oxidized or scratched you could restore it. The tube can be cleaned. Forget about the tripod. If the mirrors have even the minimum damage then you can forget about it.
The tripod will be another issue I need to address. But I figure I'd better get a working scope first before I worry about it.
I just remembered I have another telescope and it may even have the right diameter lenses with it. It is complete in the box so I'm not as keen about breaking it apart. Though when I used it I can't say I was too thrilled about it.
It is a Meade Jupiter model #60AZ-M I think it has the skinny lenses. I pick a lot of junk up at garage sales. Sometimes I forget what I have. At least I could borrow a lens from that and see what the Tasco is like with it. Can I do that?
From the photo I deduce that the this scope is about F5 to F6 (ratio of mirror diameter to length). A possible problem when dealing with short F numbers is that when you change the focuser you may need to approach the secondary mirror more in order to focus, that is to move the focuser more into the tube (a few mm). I have met this problem. The solution is to shift the rack lower , or to change/modify the internal tube of the focuser. It can be solved.
The important thing is to decide which telescope is actually better. This is what I would do. I would remove the focuser from this one and substitute it with a temporary plywood base with a hole and a tube PVC , plastic cardboard or whatever to hold the eyepieces. Then I would setup both scopes a clear night and perform these three tests:
1. Compare the image with the same eyepiece (10-12mm, ~100x). Check for diffusion , chromatic distortions in the center of the field for both. Focus and defocus to see which distorts less (usually the larger F).
2. Find the field of view. Select a middle eyepiece 10-12mm , center a star and try to determine when distortions start to appear as the star moves to the edge. The ratio of the two diameters is the FOV number. 80% or larger is good.
3. Try to resolve double stars. A good test is the double-double epsilon in Lyrae (high during summer). It is a pair that can be resolved with binoculars but each of them is a pair distant 2.3 and 2.6 arcsec apart. It is a standard test for telescope evaluation. Your 4" with a 1.2" arcsec limit should be able to resolve this easily.
The size of the mirror is not the only criterion. There are also other things to consider, the quality of the mirror coating and the quality of the mounting (tripod or single pilar). Learn to use the equatorial mount.
Thry to find the manuals or more information on the specific scopes. Check the "Sky and telescope" for more information for Newtonians. There you ill find how to collimate the mirror , another thing to check.
Spend a year or so with the eyepieces you have and there star thinking about better ones.
I just got back from hiking down to my trailer where I have both scopes stored now. The white one in the previous photograph is a Tasco, my much older, and in poorer condition scope is a Criterion Dynascope. The lenses and focuser I have go with it. the Criterion is a lot longer and skinnier than the Tasco is so maybe this focuser change won't work?
I don't know anything about those F numbers you mentioned. I guess I need to find out.
Don't worry about the eyepieces, they are universal they do not depend on the F number, in one extreme case you may have to do a small modification on the focuser to increase the travelling distance.
My main point is that before doing any changes you have to evaluate them first as I discussed in the previous letter. The tripod or other mount should contribute 50% to your decision to select a scope.
Instead of tranferring the focuser from the other scope why not make a simple construction like this for testing the tube:
http://www.instructables.com/id/Simple-wooden-Crayford-style-focuser/
This is called the crayford focuser and is based on friction instead of the rack and pinion mechanism.
I had a problem with my spotter scope moving off center and as such spent a good couple of minutes realigning it before being able to start searching. I found a bicycle puncture repair patch (the big rectangles 150mm x 100mm not the preformed patches) were perfect, you remove the spotter, place the patch (cut to size) replace the spotter and tighten the screws. Obviously you have to align it but it is far more robust. :)
1) when building the eyepiece, it may be a good idea to blacken the edges of the lenses to reduce scatter.
2) zoom lenses are nice, but the downside is the restricted FOV when zoomed out. It is indeed useful to have the zoom to locate objects and then observe at higher magnification, but you'll still want to have that larger EP in your pocket for observing galaxies, open clusters, etc.
3) for someone as handy as you, you might consider replacing the tripod legs with wood. It will be heavier for transport, but more stable.
1. Blackening the edges is indeed a good idea. Specially because the field in low magnification is very wide.
2. I hesitated before buying the zooming lens. The big advantage is that its perfect for a spotting scope and in some cases when I do not want to carry the full bag of accessories , not even the finderscope, its the only lens you need. Normally i use the full range of eyepieces.
3. This is valid tip because the support is OK, the legs are not good.
I really like adding a wwight to the tripod for stability. what a simple idea.
I also like adding zip ties to the spotting scope to help keep it from being knocked out of alignment. I do have that problem when I transport my scope so gonna have to do that one.
here are some things that i've done to my scope.
replace the standard finder scope with a red dot finder. I was always frustrated using the standard finder scope. looking thru it you would see a white dot, but on a clear night with a sky full of white dots, i could never tell exactly which one i was looking at. with a red dot finder you can use 2 eyes and easily tell exactly what your looking at.
I added a clock drive motor to my equitorial mount. i found one for $20 on bay. it really works well. it's not perfect so i the one i use wouldn't work if your doing long photo exposures but it's not bad. i think it's more a problem with my tripod than the drive.
to help with set up i attached a small bubble level and compass to my tripod. makes alignment much easier.
I have small red led's which stick with velco to the bottom of the tripod legs. keeps you from accidently bumping the legs. mine are homemade which i got at a star party but i'm sure you can rig something up using some keychain lights.
with my tripod the equitorial angle set screw has a tendency to work loose after a while so I cut a wood block that is set for my latitude (42 deg). the wedge takes the strain off the set screw so it stays in place. i need a better way to clamp the wedge in place but it does pretty well.
this really isn't an accessory, just somethign that helps me. on my ipod i have the "Star Walk" app. I can hold it against the telescope as i move it around. it's pretty good at identifying what i'm looking at. it really helps my find dimmer objects.
one of my next projects is to make a custom transport box for my telescope. i often have the problem of my finder getting bumped and knocked out of aligment so I want to make something to keep that from happening.
I did make a custom box to hold all my eye pieces. it's just a small tool box with a piece of rigid insulation that has holes for the eye pieces.
for what it's worth those are the things that have helped me.
I actually created an app for this, called SkEye, but it's for Android. You can attach the phone to the scope and then align it (within the software) with a known star. It then guides you as you move the scope around.
And the scope being mostly plastic helps in this case as it doesn't interfere with the magnetometer :-D
Thanks again!
GOOD WORK!