Modified cheap telescope. image quality? edges less blurry

[realflow100]

I got a cheap 64$ 70mm F5.1 telescope from amazon and I actually got the image quality to be somewhat useable by removing the spacer between the 2 lens elements and using 3 tiny pieces of scotch tape instead of the thicker plastic spacer it came with.
And also fixed the light baffling to not cut into the light path nearly as much. and shortened the focuser tube to just under 1/2 as long. it was far too excessively long even for prime focus!
the edges were ridiculously blurry (looked like coma in a fast uncorrected reflector telescope but extremely dramatic and unusable image quality)
I got the image looking like this so far
only added a small amount of sharpness contrast saturation and exposure to the image to make it more interesting to look at. the edges were beyond unusably blurry before the modifications i made to the telescope

[OzEclipse]

Great work. In my early days, I found a 2nd hand dept store 60mm refractor. The lens was a glass achromat and it turned out it was quite good. The problem was a horrible plastic barlow to give it the 560X magnifying power. Once removed and the tube modified for the new focal plane it served me well for many years until its untimely demise during a desert 4wd trip to see a solar eclipse in Dec 2002. The achromat came loose and bounced around chipping all the edges. I could still use it but would have to mask it to 40mm.

Joe

[realflow100]

Ouch! dang.
Mine doesnt have any kind of barlow in it luckily its just an open tube.
there is some definite chromatic abberation and purple fringing but its not nearly as bad as the blurry edges were before i modified it! the edges were just horrible beyond unusable

I'm pretty sure the glass isnt that great of quality or anything special. but the image quality is ok. its a pretty cheap basic refractor telescope.
only problem is the image gets dark fast beyond like about 25x magnification. its decently bright with a 32mm and 20mm eyepiece but smaller than that it gets really dark fast. probably because of the tiny exit pupil so cant really use any eyepiece smaller than 10mm
The included eyepieces are beyond junk plasticy dark and small field of view. and dark image. and magnification and focal length seems "wrong" for their rated focal length. seems too zoomed in.

my plossl eyepieces are fine though with it fairly sharp image with plenty field of view.

my 100mm F4 reflector telescope stays plenty bright even up to 200x magnification but it has the most extreme coma ive ever seen. beyond unusable for astrophotography. and no coma correctors for 1.25" scopes. even with an adapter I dont think one would work or be worth it. as coma correctors generally cost more than the telescope itself is worth might as well just buy a better telescope at that point.

[Bigzmey]

Nice mode!

[realflow100]

Mode? What do you mean?

[OzEclipse]

I think he meant 'mod' as in modification

[realflow100]

Ohh right haha ok

its still not perfect. but you cant really expect perfect from a cheap barely-achromat telescope

[Bigzmey]

Yep 'mod'.

Well this is the hobby when you need to tweak most of the products before use, no matter cheap or more expensive.

[Sky Tinker]

I got a cheap 64$ 70mm F5.1 telescope from amazon and I actually got the image quality to be somewhat useable by removing the spacer between the 2 lens elements and using 3 tiny pieces of scotch tape instead of the thicker plastic spacer it came with.
And also fixed the light baffling to not cut into the light path nearly as much. and shortened the focuser tube to just under 1/2 as long. it was far too excessively long even for prime focus!
the edges were ridiculously blurry (looked like coma in a fast uncorrected reflector telescope but extremely dramatic and unusable image quality)
I got the image looking like this so far
only added a small amount of sharpness contrast saturation and exposure to the image to make it more interesting to look at. the edges were beyond unusably blurry before the modifications i made to the telescope

Congratulations! These smaller, shorter and cheaper achromats have a lot of potential, and for the DIY'er.

I got a Barska 70mm f/4.3 achromat recently myself, with a 300mm focal-length, that I'm now in the middle of working on. I, too, have removed the baffles from the OTA and the draw-tube of the focusser. Also, I cut the draw-tube down to less than half its original length. As it arrived, it was operating as a 30mm f/10, but now as a 60mm f/5 at least. The next steps are to blacken and flock it throughout.

Best of luck with your own, and clear skies!

[realflow100]

nice!
I haven't done any kind of blackening stuff just fixed the baffling so it wasnt so aggressive (shortening the tube helped the most and moving the telescopes inner metal baffle ring further away from the lens helped a lot too. nearly no glare and decent contrast.
image is kinda soft unless you get it focused very perfectly.
I actually added a removable baffle by using a small black plastic ring with a hole in it attached to a sorta small plastic tube that fits snugly in the end of the focuser tube so I can adjust the position for various situations.
if i'm at a really good location with no streetlights around i don't need the focuser baffle at all. can just leave it out for maximum possible light gathering and no vignetting.
its good enough to see andromeda galaxy on a full moon! (just barely because we have pretty bad light pollution here) but still visible.

As far as I can tell there's no blurry at the edges visually at all. its quite flat. only warps stuff a bit (straight lines a bit bendy) but stuff is still sharp which is most important.
visually it looks crisp across the whole field of view.
it is SLIGHTLY harder to focus with a camera because the sharp focusing point isn't isolated to the middle of the frame (I think thats normal?)
but before there was extreme blur at the edges. and quite strong vignetting. as well as being a dark image.
I think the image is only slightly darker than its rated F5.1
its 360mm I couldn't find any 300mm telescopes at 70mm but that might be a bit too extreme anyways.

in daytime the telescope works pretty fine for birdwatching or stuff like that. though chromatic aberration is slightly more noticeable (though it doesnt bother me too much so im fine with a slight bit of CA.)
better glass material and quality and there would be far less CA as well and it'd be pretty decent for a small telescope.
Maybe I could modify it to fit a 2" focuser and have even better quality images. less problems with the focuser tube and baffling it right.

[Sky Tinker]

I'm configuring my own for maximum aperture, therefore no baffles whatsoever, albeit with maximum false-colour. The flocking should substitute for the baffles. I also plan to use it as a finder-scope for a Maksutov on a manual mount. That one has a 1900mm focal-length, and blind as a bat under the night sky.

[realflow100]

Whats flocking?

[Dragonsfire]

Its a material that absorbs stray light so you dont get reflections and the tube is dark.
http://www.deepskywatch.com/Articles/fl ... onian.html

[realflow100]

I think I would only need to apply flocking to the focuser tube. and the inside of the telescope tube near the objective. so I wouldn't even need very much. but I would need some REALLY good kind of flocking material. as its quite severe with my telescope for some reason

I'm also thinking of doing a simple way of blackening the edges of the objective lenses which might help improve contrast further just using a simple sharpie marker would probably be good enough.

Whats the best kind I can get from amazon? I dont need a whole lot either.

[Thefatkitty]

Hi, quite like your thread I did the same sort of thing with an objective from a 60mm Tasco 55VTE into a cut-down Celestron Astromaster 60mm tube.
For the $35 and time it cost me, the pics are great; from garbage pandas to conjunctions!

As for blackening the lens edges, I found one of these at a dollar store that worked great:

IMG_4177 (2).jpg

IMG_6733.JPG

IMG_1319.JPG

Also, I found your experience with shortening the distance between the two lenses intriguing. I have an old Jason 80mm f/15 that just seems a bit "soft" on focus. Having had to clean the objective (thankfully it was marked for alignment afterwards) I noticed that the spacer between the two objectivesseemed a bit thick from previous experience with these. Could that be the reason for the soft focus issue ...????

Thanks for the idea, even if it doesn't work, it's always worth trying!

All the best,

[Sky Tinker]

Flocking is like very low pile carpeting for your telescope. It absorbs, eats, stray-light. This is where I had blackened and flocked a 6" f/5 Newtonian...

I use the self-adhesive flocking, and cut it into strips...

The sheet has a wax-paper like backing, and it's super-slick. You have to be careful and not let the paper fall on the floor after removing it from the flocking, else you may fall on the floor after stepping on it.

I get my own from Protostar... https://fpi-protostar.com/hitack.htm

In the past, it has been acknowledged as the best. But nowadays it's the same as what Scope Stuff sells... http://www.scopestuff.com/ss_flok.htm

The stuff I'm seeing on Amazon may or may not be suitable. Flocking specifically for telescopes is dead-black...

flocking2.jpg (15.03 KiB) Viewed 3798 times

My camera had a difficult time taking that image, as a result.

Given that the length of the tube of your achromat is, what, less than 6"? I think you could splurge and flock the entire tube. Flock the inside of the dew-shield whilst you're at it, if you choose to flock indeed.

I blacken where I cannot flock...

I either spray it, or paint by hand with small brushes, depending.

Those matte-black paints are the deepest you can get readily, and from Wal-Mart or Home Depot. Always shake the living devil out of the can before spraying, and so to mix it well.

When blackening the edges of a doublet's lenses, I paint those by hand...

Note how the doublet almost disappears afterwards...

That's what you want it to do. You actually don't want anything between your eye and the object you're observing. But, alas, we must make exceptions, and include the doublet.

In the end, you want the inside of the telescope to be utterly dead to reflections and stray-light sources.

[realflow100]

interesting. and neat.

heres another test photo (bit fuzzy from taking a photo through a closed window with bedroom lights on. but still looks okay)

[Sky Tinker]

It is noteworthy that when removing an achromatic-doublet for treatment, that the two lens-elements may become out of order. This image I created illustrates the correct order...

Since the convex surfaces of a crown-element appear very similar, flipping the crown may be required. You simply test each side, and pick the one that gives the sharpest view.

[realflow100]

Yes I have them correctly oriented.
The slightly more convex side of the crown goes into the concave side of the flint. and the less convex side of the crown faces the stars away from the observer.

thats how it came from factory at least.

I dont think I could flip any of the elements a different way and have a better image. the image quality seems fine visually. only a bit of CA on very bright light sources and stuff is a little bent/warped near the edges but i think thats just normal.

I could get a better diagonal (seems to be the cause of the blackout im having with my larger eyepieces) and improved or eliminated baffling and potentially also adding that flocking stuff throughout the whole thing. focuser. telescope tube. diagonal. everywhere I could use the stuff.
but I havent been able to find any available to buy from amazon. im only able to buy the stuff from there

I could instead buy some black 3.0 paint (super black matt paint) and just paint the whole thing up if that would work good enough. which seems to be a little cheaper and potentially easier to apply

[realflow100]

I found out the telescope focuser itself even though its shortened to half still causes some serious vignetting with larger eyepieces
and even the diagonal causes some vignetting too
Im at a loss as to how to fix that besides getting a better focuser assembly and better diagonal