More homemade parts for your telescope and modifications
					Menu
					Modify or build a finder Homemade collimating eyepiece Oil-spaced achromat Solar filter cap Homemade tube extension/anti-reflection cap	M E N U

Modify/build a Finder

I had a finder to "molest" laying around. Actually it was a cheap finder of my son's cheap department-store telescope. Because I had no use for the finder on my homemade telescope and needed an optical finder with a bit of magnification. My Skywatcher-Pro 130mm Newtonian telescope has a red-dot finder, which is excellent, but to be able to use my Skywatcher telescope for dusk-/dawn-/day time observation and to better pin point the stars (my red-dot finder has no magnification) a secondary finder was needed. So I decided to make my own finder with an achromatic lens. The cheap finder I had wasn't suitable because of its plastic lenses with a huge aperture stop. It had a 20mm aperture, but only 1 cm effective aperture because of the field-stop behind it...
no need to explain any further!

• Not only had the cheap department-store telescope finder plastic lenses and a field-stop, it had a shiny black interior. Yep, that shiny black plastic! So we need some work on darkening the inside tube of the finder to improve the contrast and reduce reflections and shimmer. It will greatly improve observer comfort by minimising glare and providing sharper images.
  Remove the finder from its holder and unscrew the tiny dewcap (needs our attention too*) that also holds the plastic lens and remove the finder's eyepiece.
  Sand the inside of the little tube and paint if flat black.
  
• I wanted to use an achromatic lens for my finder. And yes, those cheap 10$ tiny binoculars have such lenses. They had the ideal size for my finder: 25mm aperture. A bit bigger and no need for a field stop (achromatic lens). So it also fitted on the 25mm dewcap. Plus, the prisms and eyepieces are perfectly suitable for making homemade eyepieces (certainly better then those cheap 0.96 lenses that come with those cheap (toy) refractors. Please read this about making your own eyepiece out of binocular lenses. The prisms are ideal for making a rectifying eyepiece (put everything right side up, like a diagonal).
  this is how the eyepiece of the binocular looks like
  
• and here you can clearly see the prism removed from the eyepiece
  But I needed the 25mm achromatic lens. So we just remove the rubber cap and unscrew the top.
  The top has a metal threaded ring where the objective lens fits. This ring is exactly the size to fit on the finder. We need this ring later on.
  
  You need to remove the field-stop. You can drill it out or cut it. The field-stop of the original finder had an aperture of only 1cm do to the plastic objective uncoated lens.
• all the parts, left to right: finder tube, eyepiece + crosshair, achromatic lens in the lens holder ring of the binocular, binocular eyepiece, dewcap
  
• I wanted to add a crosshair to the finder's eyepiece, so on the bottom edge of the finder's eyepiece I glued (superglue) two thin wires (like from a transformer, or sowing thread) after measureing the exact center.
  
• Here you can see the achromatic lens mounted on top of the original dewcap. The achromatic lens was fixed to the ring that came of the binocular. Because of the longer focus length of the achromatic lens I needed to lengten the tube. This way the finder dewcap became a part of the tube A(needed for compensate the longer focal length of the achromatic lens) AND enlarged the aperture needed for the 25 mm achromatic lens.
  
• I needed to cut 1 cm from the dewcap to compensate the achromatic lens focal length. You'll probably need to get the focal length and measure how much is needed to lengthen or shorten the tube.
  
• The homemade/modified telescope finder mounted on my Skywatcher telescope. I drilled a hole through the bottom of the original finder holder to allow the finder to be mounted on the telescope's camera piggyback.
  You also notice the "new" enlarged dewcap made out of 32mm PVC pipe of 45mm length and painted matt black.
  After all these changes I got a pretty usuable finder. Together with my red-dot finder they are a perfect pair.
  
• Tip: If you wanted an upright image in the finder, then simply use the prism mount it in front of the original finder eyepiece.
  Why not take the whole binocular eyepiece lens and prism? Well, you could if the magnification of your binocular isn't to high. Mine was 12X and to much magnification for the aperture. Keeping the exisiting lens drew the magnification way back to 6X. And, prism design (Schmidt-Pechan) employs silvered surfaces that reduce light transmission by 12% to 15%.
  
  It uses a Schmidt-Pechan prism that is a type of optical prism used to rotate an image by 180°. They are commonly used in binoculars as an image erecting system.
  
  Enjoy your newly modified or homemade telescope achromatic finder!
  Specs:
  Achromatic objective lens
  F/5.2
  25mm aperture
  125mm focal length
  magnification 6X
  

Improve a air-spaced achromat with and oil-spaced doublet achromatic lens

• 
  In the oil-spaced design, the gap between the concave and convex lens is filled not with air, but will some suitable oil, which ideally should have a refractive index between that of crown and flint. In practice, any clean clear oil can be used. Mineral oil, salad oil, even olive oil or cedar oil have all been used. The advantages of oil-spacing is that:
  
  • the ghosting problem between the lenses is eliminated since no internal reflection occurs.
  • the lens's light transmission goes up by 8% or so because the large loss of light from the additional air/glass interfaces.
  • the optical effect of errors in the figure the lenses becomes nil.

  In other words, small surface irregularities don't degrade the image.
  

• First remove the plastic O-ring that is used to air-space the two lenses (flint- and crown glas lenses).
  To oil-space a lens, wash and clean the lens well in warm mildly soapy water. Rinse well and dry with a cotton towel. Make sure no water spots remain. Then carefully level the the concave-lens (flint) on an elevated ring, and pour a half tablespoon clear oil into the center of it without making bubbles. Any clean salad oil will do, or clear mineral oil. Then dust off any large particles from the dry parts of the the concave-lens and from the the convex-lens and gently lower the convex-lens into the concave-lens, trying to touch the convex-lens to the oil at the convex-lens's midsection. The object here is to try to avoid entrapping bubbles in the oil.
  
  Once the convex-lens has contacted the oil, gently lower it and drive the oil away from the center of the concave-lens into contact with the outer dry parts of both surfaces. If you do entrap small bubbles, then as the convex-lens settles further into the concave-lens, these may slowly migrate to the edges of the lens. If they don't move you can try to move them out by moving the convex-lens around on the concave-lens. It's not important to remove all the bubbles. A few left over will make a negligible difference to the lens's performance.
  
  Oil will now be seeping out of the lens. Dry this off and keep the convex-lens centered on the concave-lens. Eventually, it will stop sliding around. When the convex-lens has come to a rest, let the lens sit for 30 min. in order for more oil to seep out. When the seepage has apparently stopped, you'll need to clean the edges of the lens to remove the oil. Denatured alcohol or isopropyl is ideal.
  
  Once the lens is reasonably free of oil, then you'll want to tape the edges of the lens with cellophane ("Scotch") tape. Two turns around, without overlapping the end of the second turn on the two layers already beneath, should be sufficient. Now you'll need to carefully wash the exposed faces of the lenses to cleanse them of oil, and you're ready to install the lens in its cell. As you can see, this process is likely to be messy.
  If you need to separate the lenses later, they should just slide off of one another with minimal risk of scratching.

Collimating cap

If you do not know what collimation is then please read this first. In short: Collimating a telescope is lining up its optical components (lenses, mirrors, prisms, eyepieces) in their proper positions.
If you have a cheshire collimation piece then you can skip this...but this is a cheap alternative...not as good, but still...
"collimating cap":
- if your scope has 1,25" eyepieces: out of a plastic 35mm film canister (black with gray lid). Drill or punch a small pinhole (+/- 4mm) in the exact center of the lid and cut off the bottom of the canister. Replace the lid.
- if your scope has 0,96" eyepieces: use an protection lid that came with your scope (like one of your diagonal or the one used to protect the eyepiece holder). This is of transparent material. Drill or punch a small pinhole (+/- 3mm) in the exact center of the lid. Draw a diagonal with a thin black permanent marker.

This device will keep your eye centered of the focuser tube.

Homemade Solar filter cap

Please observe the following safety precautions with EVERY solar observation:
1. Prior to each and every solar observation session, check the filters fit and - if necessary - tape it to prevent slipping. Never use the filter at the eyepiece, only attach it onto the front of the objective, in front of the Schmidt-plate (SC-owners) or in front of the tube of a Newtonian-telescope. Otherwise it can become dangerously hot inside your instrument and inside your eyes.
2.If you use a binocular, protect both objectives with a filter. Also make sure, that the viewfinder of your telescope is properly covered, either with foil or with the original dust cover. Unprotected views through your finderscope would have the same catastrophic consequences for your eyes as a look through the main telescope itself!
3. A filter made of foil is relatively resistant to breackage in comparison to a glas filter. However, care should be taken with sharp pointed objects.
4. Emphasize the importance of caution to those observing with you, especially children. Intentionally removing or damaging the filter can endanger their eyesight. This is no place for jokes. Never leave the telescope outside unattended during the daytime!

To aim the telescope, do not use your finder! Instead, point the telescope until it casts the smallest shadow possible behind it.

What do we need?

• Well, a sturdy round plastic container-like to fit the outside of your telescope depends on the diameter of the tube of course. I used a cheap but sturdy round plastic cookie container with a lid (more like a bucket). This fits perfectly around my telescope.

• Baader Astrosolar safety film. An A4 (7.9 by 11.4 inch) sized sheet cost around 29$.

• some white and flat black paint, glue, Scissors, razor and/or x-acto knife, fine sandpaper, pencil

It is all pretty strait forward as you can see from the images. Just be sure it does not come of your telescope easily! It takes only a moment to damage your eyes.
The film must be mounted flat and free of any tension - Only this will provide first class Solar images. The quality Baader Astrosolar safety film is very high.
Try to prevent as much as possible any wrinkles or strain on the film as this could lead to deterioriation of optical quality.

don't let the little wrinkles fool you, it seems worse then it is and it stretches perfectly flat once placed over my telescope.

Cut a perfect circle of the size of your telescope aperture out of the lid.
Sand the entire cilinder (bucket or whatever) inside and out with fine sandpaper. Paint the outside with white paint (prevents from the cilinder heating up from the Sun).
Measure the size of Baader solar filter you need according to the aperture, but make it a bit larger. Add glue all around the inside of the opening and press the film on it. Slide the film evenly over it so you don't have any large wrinkles. When satisfied let it settle. Just to be 100% sure it does not come off I have placed some thin transparent sticky tape in a hexa layout. Sand the top of the tape.

Finally, paint the inside flat black as you can see above.

I bought a second round cookie container (bucket) to use the lid as protection cover.

Please observe the above safety precautions with EVERY solar observation!

Homemade tube extension Dew/Anti-reflection cap

All finished. Looks pretty cool doesn't it...

If you live in a light polluted area like I am a tube extension to the front of the scope does really help alot. It keeps out unwanted stray light hence improving contrast.
What do we need?

• Again, some kind of cilinder that fits the diameter of your telescope. I used a cheap plastic round cookie container.

• Paint that fits the color of your telescope (aesthetic reasons).

• For best results use some flockingpaper, black velvet or black felt.
  If you do not have or want to use flocking paper, then flat black paint will do fine as long as you roughen up the inside of the cilinder with sandpaper. Sand it with medium coarse type sandpaper. Structure paint can also be used as this has a coarse finish. Just be sure it is painted flat black.

• Velcro if the dew cap is mounted over the scope
  If you're planning to use the dew cap to fit the inside of the scope, then - if needed - you'll need some double side sticky tape and some dense cardboard (I used painters cardboard, it looks just like canvas).

This is the cookie bucket with the bottom cut off. As I wanted to fit this particular type of container, I needed to fit it on the inside of my Skywatcher scope. Actually I've found that this works very well in any case. It's a matter of slidng it in the scope (works only if you have Newtonian telescope of course).
Anyway, to make it fit my Newtonian I placed double sided sticky tape as you can see above...

Here is where my painters cardboard comes into action... I've cutted a strip of this pretty sturdy material of 6 cm and ...well the diameter length you need ;-)
Sanding the outside and the inside of the container will not help only to hold the paint, but also the glue for the black felt (flocking paper) and holds the double sided tape much better.

I used some black felt and glue spray to fix the felt inside. The black felt, or even better; flockingpaper, really does a better job preventing stray light or reflections then flat back paint does!

Finally finished... As you can see I've painted the outside blue and the lid. The lid is ideal to cover up the dew cap, hence your telescope when using it or storing this homemade extension cap.

The homemade extension cap on my Skywatcher Explorer.
It's a pitty I didn't had the exact type of paint of my telescope for the cap at the time...

Astrophotography, info about telescopes, tips & tricks, homemade scopes and lenses etc.
Home