telescope sharpness problem? edges are really blurry looking

[Lady Fraktor]

Welcome to astronomy
Last night was my first night out in close to three weeks.
Patience

[realflow100]

i found ONE star to try this with and the first thing i notice is the star disappears if i focus inwards. and turns into a sorta colored circle/ring if i focus outwards

[Lady Fraktor]

First you need to get the lens reasonably spaced, then do a star test to see what is happening.
Nor can you just aim at any star that appears, you need a bright star and proper magnification and good seeing to perform the test.

[realflow100]

i dont have any of those things. i'm just using my DSLR with magic lantern and exposure override and zooming with digital zoom to 10x in to a vaguely bright-ish star

if I focus one way it just gets really fuzzy and disappears. and if i focus the other way it turns into a circle/disk/ring

i tried with the original spacing/spacer and its the opposite situation
it turns into a disk if i focus inwards and gets fuzzy if i focus outwards

[Lady Fraktor]

The camera is not going to work to do it, there are specific requirements needed to test properly.
Nothing unfortunately will change that fact.

Two pieces of advice for the future, if the telescope arrives damaged in any way return it immediately for a refund or replacement.
The other is do not take it apart until you know exactly what you are doing or have someone talking you through how to modify things
Much aggravation is prevented that way and more fun using it.

[JayTee]

My teaching experience has taught me that one way of describing a procedure will make perfect sense to some and be utter nonsense to others. So you try a different way to describe the procedure until the person finally gets it.

Here is another attempt at explaining the star test and hopefully you'll be able to use this to your advantage. https://starizona.com/tutorial/star-tes ... pe-optics/

Cheers,
JT

[realflow100]

My telescope isnt even capable of 200x magnification. with a 4mm eyepiece the image is already significantly dimmer! theres no way I could get a useful 200x magnification with a 70m refractor. especially not with the terrible atmosphere conditions and light pollution here.

[Lady Fraktor]

A 5mm with a 2x barlow of good quality will work well, you do not absolutely need 200x just around 50x per inch of aperture.

[realflow100]

i only have a 4mm and I dont have a good quality barlow

I tried arcturus at 4mm and it just looks like a weird diamond shape when in-focus that morphs around into a circle when it goes out of focus

it looks a bit orange-gold in color compared to other stars. but i'm sure i'd need a star as bright as sirius to be able to do this with a 4mm eyepiece

For some reason stars look like diamond-ish shapes or like a slightly verticaly squashed square to my eyes. i mean the brighter stars. like arcturus or some of the ones around big dipper. or orions belt.
even at a lower magnification like with 32mm eyepiece or 20mm eyepiece. they look more like dots but still a bit like diamond shapes.

I did a LOT of testing with my camera and different amounts of duct tape layers and it appears 2 layers is as close as I can get. any more or less and its too far off. I can focus a star to a tiny point only a few pixels across if i nail my focusing at 10x digital magnification with my camera. with arcturus. and then if i increase the exposure. it turns into a bright star with a bit of blue and purple CA visible around it.

its easier to focus on a bright star if i actually reduce the exposure so that the star is less over exposed. so i know when its actually focused

[Lady Fraktor]

I will send you some ideas a bit later today, busy at the moment.

[John Baars]

Theres not enough room inside to add baffling or flocking. it'd reduce the effective aperture of the telescope dramatically with only a 5mm ring in the middle of the focuser tube. also causing a lot of vignetting.

https://www.fpi-protostar.com/flock.htm
This is the flocking material you should stick in the focusser. It is typical 0,27mm thick once installed. No dramatical vignetting worth mentioning.It will work great.

A porchlight is not suitable for startesting as Lady Fraktor already stated. It really must be a point-source of light.

Still, the pictures in #16 show nicely the CA red for outside focus and CA blue for inside focus and both in focus, as should be in a f/5 refractor. So far so good. The pictures show another severe flaw in the lens however of which we haven't spoken yet. And that is a quite severe form of astigmatism. In your pics you see the porchlight as a standing egg in the red aberrated pic and as a lying egg in the blue aberrated one. So it seems there are different ways this lens forms its focal point; different for vertical and horizontal. This is called astigmatism and contributes fundamentally to the unsharp images you experience.

I tried arcturus at 4mm and it just looks like a weird diamond shape

That is what astigmatism is doing for you. Nice description of it.

What you can do about it?
You can't alter the shape of the lens nor get rid of internal tensions in the glass. There is only one solution to minimize it, the one lensmakers in centuries gone by also used. Make use of the properties of a lens that the middle of it gives a better image than the edge: stop down the aperture. In your case to 20mm or so.

[realflow100]

I dont notice that problem with a camera though and a proper point-source like a star. it just turns into a distinct circle/disk both ways of focus. after getting the spacing close enough. and the image is usably sharp too. but CA is pretty crazy for bright lights or long exposures of stars.

Im pretty sure that changing of shape is just due to the porchlight being a vertical stripe (it was only visible through a vertical rectangular hole in some panneling. I get the same effect with my reflector. it changes between vertical and horizontal out-of-focus shape) but with a point-source and camera i dont see any problems. even with a barlow and 10x digital zoom. stars are points not diamonds when in-focus. and small disks/circles when slightly out of focus or a lot out of focus.

I only see the weird diamond shape visually and it follows my eyes if I rotate my head around the eyepiece so I think its just my eyes doing something weird.

[Lady Fraktor]

The CA will always be apparent with a short tube achromat, they cannot focus all of the light to the same point.
If the shape is following your eye then you have astigmatism most likely, an eye appointment would verify quickly.

[realflow100]

if it is that would be really weird. because everywhere else I don't have any issues seeing. I don't wear glasses or anything and can focus far away or close up just the same.
I hope there's not anything because I really cant afford to wear glasses and I wouldn't be able to stand contacts.
my only option would be eye surgery but i can't afford that either

I can read text off my computer screen from like 4 feet away and its only a 24inch 1080p monitor. and I see just as sharp on a sign or something else 100 feet away its weird that something with my eyes would only affect using a telescope.

[Lady Fraktor]

Nothing to worry about, many people have astigmatism to some degree.

[realflow100]

so if ive found the best balance for my telescopes lens spacing as close as I can should i get more comfortable viewing and a balanced amount of CA and softness so its not too soft and not unneccessary amount of CA? clearest image?

[Ruud]

I guess so.

The more stray light you prevent, the more contrast you get. If the objective's lens edges aren't blackened, make them black with a sharpy. If the tube is shiny, apply black flocking paper or self adhesive black velour to its inside.

Make a light shield if the telescope doesn't have a long enough one, and make sure that too is a dull black on the inside.

[realflow100]

Could anyone link me some cheap-ish doublet lenses of 70mm diameter I could buy online. cost 60$ or under. with the same or very close to the same focal length?
(below the price of the telescope i payed for. im not paying more than the price of the telescope just for a lens) or at least under 100$
i cant seem to find any useful results online. even the cheapest achromatic lens costs hundreds of dollars and I can't even find any in the 70mm diameter size

I figured out a more accurate test and it seems the lenses definitely have a little noticable astigmatism. not ground with the same focal length across the whole diameter. which improves dramatically if I stop down the telescope. the image gets darker but definitely usably sharp with a 4mm eyepiece (90x magnification roughly) but quite a bit darker as well with noticable dust spots.

somehow this isnt as much of a problem for my cameras sensor when i crop/zoom the image to be the the same amount of apparent magnification as the 4mm eyepiece and its a tad bit sharper looking on my cameras screen.
which is really strange

Maybe it would be better to just buy a better telescope.

[JayTee]

Here you go, try Surplus Shed their stock is constantly changing so be patient and something will hopefully show up that meets your criteria. https://www.surplusshed.com/category/Achromats

Cheers,
JT

[realflow100]

Nothing available there that matches 70mm diameter 360mm focal length. seems a better choice would be to just buy a better telescope at this point. or at least buy an optical tube assembly without a tripod since I dont need an extra tripod.