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.
Step 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:
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..
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.
Step 2: The list of improvements
Improve the basic functions
1. Stabilize the finderscope
2. Make a finderscope yourself
3. Increase your eyepiece collection
4. Build an eyepiece of your own
5. Control the stiffness of the focusing system
6.Control the stability of the guiding system
7. Stabilize the tripod
8. Mount it on a photographic tripod
9. Use it as a spotting scope
10.Mount a camera on it and use it as a telephoto
Make it faster to setup and easier to transport
11. Use marks for the equatorial mount
12. Make it portable
Step 3: The rules of the game
1. Keep your hands away from the main objective lens !
Whatever you do, don't mess with the main objective lens and the buffers in the tube. You may cause a misalignment that you may not manage to repair. Keep the objective always covered when you are working on the tube.
2. Make all changes reversible
This is important. After making a change you may find that it worked better before, specially if you do not have much experience. All changes proposed in this instructable are reversible (with the exception of the box modification). When replacing small parts e.g. screws and nuts always keep the old ones.
3. Spend your money wisely
Buy things that will be also useful for other scopes such as eyepieces and optical add ons . Think always in terms of added value.
4. Build the more you can !
Making things is a part of the hobby of amateur astronomy and one of the major motivations to get into it.
Step 4: (1) Stabilize the finderscope
The finderscope can be a cause of frustration. This specific one is a 6x25 (6 times magnification , 25mm objective diameter) which will do. It is supported with three screws, one of them containing a spring. The view through it is rather disappointing because the objective is a plastic lens and diffuses light too much, however it is usable as a finderscope when you focus on a bright star.
The major problem is that if you loosen two of the screws to align , it may slide in its holder and this can be a problem in the middle of the night. I used two zip ties to restrict the movement along the tube and also mounted a piece of plastic foam on the body of the finderscope with 2-sided adhesive tape. Problem solved.
Step 5: (2) Make a finderscope yourself
I found a broken pair of 10x25 binoculars with a much better view through them compared to the finderscope. The diameter just fits into the original support of the finderscope so I decided to make a finderscope of my own. I removed the plastic cover, disassembled everything and sanded the aluminum tube. Then I remounted the optical components and made covers from layers of black canson paper for the eyepiece and the objective, since I could not find any plastic at the appropriate size.
I did not mind to make a cross hair for centering the target because I prefer to have a clear view through the finderscope. The field of view is about 4.5 degrees. If you get your target within 1 degree it will be enough for the 24mm eyepiece.
Making a crosshair is not a big deal. I have repeatedly made/repaired crosshairs on other occasions. This can be done with very thin wire and an instant glue. They are placed after the eyepiece of the viewfinder. and close to it.
Step 6: (3) Increase your eyepiece collection (if necessary)
Now we arrived at the most critical point for a scope : eyepieces. Are they enough? How do they perform?
In the case of a 70mm/700mm scope one needs magnifications in the range x20-140. A higher magnification is not so usefull because the resolution is limited from the size of the objective lens anyway. You may magnify but you are not getting any more information from the image. The rule for finding the maximum useful magnification is to multiply the diameter of the objective in mm x2 or in inches x50.
If the scope had no eyepieces at all, I would buy at least three of them, the best quality I could afford :
1. A wide field 24mm or larger
2. A 10mm (70x) and
3. A 5mm (140x) or a Barlow Lens x2 to use with the 10mm.
The scope included three eyepieces a 20mm, a 12mm both adequate but not optimal and a 4mm (too much distortion and aberrations). I intended to buy a Barlow but the nice people at the shop advised me differently and I am glad I listened to them. I got an 8-24 zoom Celestron wide field which has become my main tool, since it covers the 20mm and 12mm eyepieces with a much better performance. Furthermore it has a 42mm threading so that you can mount a DSLR camera and photo using the "projection" method (see step 13 for discussion on this).
Still, I intend to get a 5mm in the future.
Step 7: (4) Build your own eyepiece
I found two planoconvex lenses with focal distance 42mm , probably from binoculars and I decided to make a low magnification eyepiece with them. The simplest and probably the most standard design involving only two lenses is the Plossl. This is obtained by placing the convex parts facing each other. The focal distance depends also on the actual distance of the two lenses (see relation). In the case of two identical lenses of 42mm the focal distance is between the limits 21mm < F < 42mm. I selected 34mm which gives a magnification 700/34=20.6x and a large field of view of 146 arcmin=2.43 degrees.
This is useful for two reasons a) actually it substitutes the finderscope and b) large open star clusters may fit fully in the frame. For example the Pleiades open cluster in Taurus extends 110 arcmin.
The realization of the eyepiece was easy. I used a piece of a 40mm diameter PVC tube, black canson paper for the spacer between the lenses and a film canister which fits nicely in the 1.25" tube of the scope. Using the canister for this purpose is an old trick.
Step 8: (5) Stabilize the focusing system
The focusing tube moves with the help of a rack and pinion mechanism. Shake it a little with your hand up and down. If it moves then it needs stabilization.
I used two pieces of tape underneath parallel to the rack. The pinion is kept in place with a cover and two screws. This should be adjusted so that the tube does not slide.
Since I use a camera on the scope, I controlled these two screws in order to hold the camera without sliding.
Step 9: (6) Service the guiding system
I disassembled and inspected the guiding part very carefully. In summary this is well designed and well made and there is no vibration from this part. The basic 6mm screw which holds everything and terminates in a nylon washer and a nut , should be always tight. This is the only thing one should take constant care of.
I also disassembled the two worm gears and removed the excess grease. Then when putting them in place I took care to press the worm gear against the moving part to avoid any "play" of the axis. The moving part is made out of plastic, metal would be better.
Step 10: (7) Stabilize the tripod
The legs of the tripod are the main cause of instability. Hanging a weight improves the situation a lot. I have prepared a rope (see photo) that can be set up very fast. I avoided doing any permanent change on the tripod and mounted it through the top hole.
If your car is close to the observation site a jug of water is the simplest solution. I have used a plastic bag full of stones or pebbles depending on the location. You may also hang a small backpack or turn the rope around a large stone lying on the ground.
Step 11: (8) Mount it on a photographic tripod
There are two advantages from doing this: a) The photo-tripod is more simple and stable and b) I carry it most of the time in the car anyway for the camera.
The modification was simple. I used a piece of scrap aluminium plate and substituted the two screws of the dovetail base with new ones terminating on the plate.
The handle of my tripod helps me orient the scope with small movements. With a little practice one can regulate this fairly well even in large magnifications.
Step 12: (9) Turn it into a spotting scope
The set contained an orthoscopic tube with a plastic lens in the middle, very low quality. I use an orthoscopic prism unit that goes between the eyepiece and the tube. It is worth having one.
Step 13: (10) Mount a camera on it
There are actually three methods to mount a camera on a scope:
1. PRIME FOCUS
The scope is used as a telephoto for the camera. Remove both the eyepiece and the camera lens and mount the camera body directly on the scope. The best method. I use this with my DSLR. You need an eyepiece adaptor for your camera and a T-ring.
The image of the scope is "projected" from the scope on the camera sensor. Remove the camera lens and mount the camera body on the eyepiece. The T-ring for my camera has a 42 mm threading and so has my Celestron zoom eyepiece. You gain in magnification this way but the whole set up is heavy for such a light scope and I avoid this combination. I have used it successfully with larger reflective scopes.
Aim through the eyepiece with the camera lens on. This method is convenient and the only possibility for compact cameras and mobile phones when you cannot remove the lens (See photo). You can either simply approach the camera to the eyepiece or buy/make a mount. In this case the size of the image depends on the ratio of the focal distances of the eyepiece and the camera lens.
Step 14: Shoot the moon
This photo was taken with 1/100 sec exposure at ISO 800 (lower than that would be fine).
The minimum distinguishable feature in this photo is crater Reinhold b (a black dot close to Reinhold) 26km diameter. It is amazing that one can see this, 400000 km far from here! The viewing angle is (26/400000)*(180 degrees x 60 x 60 / 3.14)= 8arcsec that is, 4 times larger than the theoretical limit of the 70mm objective.
Step 15: (11) Use marks with the equatorial mount
Usually one may hesitate to setup the equatorial mount because of the steps it needs. However it is a very basic and useful thing to learn how to do because you can handle the scope better in large magnifications. Bresser has included in the construction helping marks (on the rod and on the main axis) and the procedure is easy.
I found very helpful to make an orthogonal triangle with the angle of my altitude location (Athens 38 degrees). I also marked this angle with a small piece of paper tape on the rod which keeps the equatorial plane in position. This saves some time in the dark.
Step 16: (12) Make it portable
If you store your scope in the attic you may think twice before assembling and using it. I have remade the box and I keep the scope and the photo-tripod in a corner of the living room. I use it much more often this way. Details of the construction are shown in the photos.
Another reason for preparing a portable setup is that you may have to leave the car and go on feet to the observation point walking for some time. Dividing the load in two bags one for the scope and one for the tripod makes this easier for a company of observers.
Step 17: Epilogue
Reducing all the above to a minimum I would say that
a) increasing the eyepiece collection
b) stabilizing the finderscope and
c) improving or replacing the tripod
should be the first necessary steps towards improving a cheap scope.
I am an occasional observer and I have access to larger, heavier and more sophisticated scopes of friends. However this small instrument has been to places where its 30kgr big brothers cannot go.