What size telescope to see Hercules D crater on the moon?

[John Baars]

Can fixing the overcorrection improve the image

You can't do it yourself. Such corrections can be done by a high skilled optician in his optical shop. Applying coatings must also be outsourced. But the costs transcend the price of a new and better telescope.

I just need some additional spacer rings I could stack to get the right spacing. but where to buy them from cheap?

You can use three alufoil spacers and take the foil from the kitchen. See my post from August 23.

my star tests look almost exactly like that! based on that result would there be a recommended spacing to aim for? to at least get it closer?

That would be a case of trial and error.

Surprised I can even see 2 bands on jupiter with correction that bad

With a well corrected refractor you should be able to see more than that, if seeing is on your side. Saving for a high-end telescope seems more sensible to me than spending money on low-end telescopes every time. It seems to me your hobby is to get the most out of the simplest instruments every time. But by now all this must be old news to you, right?

[realflow100]

the foil would be too thin. I need something more between the size of 0.2 to 0.5mm to see what is closest
but its really hard to position them and have the lens aligned. thats why I wanted ring spacers instead of little squares/pieces of something.

[realflow100]

ok I tried re-spacing and adding spacing but the image didnt seem to change in any meaningful way it looks about the same with about twice as thick spacer.. Maybe a tiny fraction better? but its too hard to tell.

I tried even THICKER spacers. and it still looks the same. what is going on??

I stuck back the original spacer and it looks the same again.

[MistrBadgr]

Looking at the website that John was using, the craters you have circled match with Edege B and C. Wikipedia says B is 8 km across and C is 5.
A long 70mm refractor has a Dawes limit of about 1-3/4 arc seconds, but I am not sure right now how that computes to km on the moon.

[John Baars]

Looking at the website that John was using, the craters you have circled match with Edege B and C. Wikipedia says B is 8 km across and C is 5.
A long 70mm refractor has a Dawes limit of about 1-3/4 arc seconds, but I am not sure right now how that computes to km on the moon.

There are several ways of finding out. One of them is looking in Stellarium.
There you'll find that the current Moon is 3474.8 km or 29 arcminutes. The size in arcminutes / arcseconds is decreasing by the minute at the moment.
29 arcminutes = 3474.8 km.
1740 arcseconds is 3474.8 km
1 arcsecond = 1.997 km
So 5 km = 2.5 arc seconds.

ok I tried re-spacing and adding spacing but the image didnt seem to change in any meaningful way it looks about the same with about twice as thick spacer.. Maybe a tiny fraction better? but its too hard to tell.

I tried even THICKER spacers. and it still looks the same. what is going on??

You won't be able to see the difference on objects like the Moon or daylight-objects. Only critical startesting will show a slight and subtle difference.

[realflow100]

I used star testing. not the moon or daytime objects.

[GCoyote]

Don't forget to check a weather site like Meteoblue for tonight's "seeing" conditions. It includes an estimate of best angular resolution you can expect each hour.

It sounds like you are getting good views from an economical scope just by being diligent in fine tuning the optics. I'm encouraged by what you have accomplished under bad light pollution and restricted set up locations.

Keep us posted.

[John Baars]

ok I tried re-spacing and adding spacing but the image didnt seem to change in any meaningful way it looks about the same with about twice as thick spacer.. Maybe a tiny fraction better? but its too hard to tell.

It is rather difficult to make the difference between 0.5 and 0.3 Wave SA, especially when one doesn't have the the comparison material side by side.

[realflow100]

ok thats barely any difference at all.
How much of a difference in spacing do I need to make it noticably better?

[John Baars]

As I told before that is a question of trial and error for us amateurs. I suppose an optician could do some calculations.
Since you already did all trials and errors, from a very narrow gap to twice the distance set by the manufacturer, I think you are at the end of the possibilities the lens gives you and within your reach.

When you haven't done yet, you could think of flocking or internal baffling all shiny surfaces you see when you aim at the clouds with your eyepiece removed. That makes a good difference in contrast- mid tones, together with an aperture stop. (I remember stopping down a 60mm departmentstore toy to 45 mm, with good results.) When you have already done so, there is nothing much left to be done, except for saving for a better telescope in future.

And meanwhile: keep observing and enjoying! Your current telescope is ten times better than the one I started with (made of spectacle lens, cardboard tube and single magnifier) Still I kept on observing!

[realflow100]

Oh Ive flocked the focuser tube so theres no shiny surfaces left anymore
gave a big boost in contrast.
I also blackened the edges of the lens elements with a sharpie too

I think maybe the spacing is too large by a fraction of a millimeter. think i been going about it backwards?

[John Baars]

(...) think i been going about it backwards?

I am not sure if I know exactly what you mean.
But If you think the steps you took were too big, you can do it again.
I would try it in 0.16 mm steps.With kitchen-alufoil being folded 4X you can do that quite precisely.( 4X folded= 0.16 mm)

[realflow100]

its so impossible to cut and space them at ABSOLUTELY PERFECTLY the right distance and size though
Ive tried in the past and no amount of fiddling would stop star airy disks from being distorted out of existence. or the rings and airy disk distorted and bent to one side. or worse. chromatic aberrations across the whole image in some direction.
i need the ring spacers.

[John Baars]

its so impossible to cut and space them at ABSOLUTELY PERFECTLY the right distance and size though

I don't agree. The following three spacers of 0.16 mm were made within minutes. I folded the alufoil 4 times and then cut to size.

the rings and airy disk distorted and bent to one side.

That is the coma you get when two lenses are slightly shifted or when one spacer is too slim. In the last case alufoil should be added under the spacer nearest to the bright coma-center. Í did so to correct my 150 mm f/5 achromat.
viewtopic.php?f=61&t=13533&p=114276&hil ... ma#p114276

[j.gardavsky]

its so impossible to cut and space them at ABSOLUTELY PERFECTLY the right distance and size though

I don't agree. The following three spacers of 0.16 mm were made within minutes. I folded the alufoil 4 times and then cut to size.

the rings and airy disk distorted and bent to one side.

That is the coma you get when two lenses are slightly shifted or when one spacer is too slim. In the last case alufoil should be added under the spacer nearest to the bright coma-center. Í did so to correct my 150 mm f/5 achromat.
viewtopic.php?f=61&t=13533&p=114276&hil ... ma#p114276

Very good!

Here, we call it shimming with the sheet plates
Please, keep with the correct pronounciation,
JG

[realflow100]

I think I need to add more tape to the edges to keep the elements centered in the telescope tube. Too loose and wobbling around.
I have to keep loosening the threaded part and rattling the lens around then snugging it back down until stars aren't coma blobs in some direction. cause if I dont use tape to hold the lens elements together straight and aligned its unusable beyond even 20x magnification.
also trying to get both lens elements aligned is even tougher. Its very difficult to get the lenses to stay aligned while putting 3 pieces of tape in 3 separate corners of them so they are straight and not offset (causes chromatic aberration and color shift issues if its way off)

[realflow100]

Should I get a green filter of some kind. so the other wavelengths of light arent giving me misleading results?

[MistrBadgr]

Opinions vary about what color of filter to use, or even IF a filter is used. Personally, with a a medium or short focal ratio refractor, I will use a #58 green filter when viewing the Moon if it is more than half full. I might go without one in an early or late crescent phase. I think the color that a person likes the best depends on how his/her eyes work and the brain processes the information.

Using a green filter will knock out a large part of what the red and blue cone cells see and, I think, emphasize the green light and how it is refracted through the objective and other lenses. Some people like to use a red filter. I do not remember reading or hearing about anyone that prefers a blue filter for the Moon, but there could be some. You will have to try the green and see if it works for you. Most likely, it will. For me, a green does seem to give me a sharper image as long as there is plenty of light for it to work with. Normally, a little lower magnification works better for me with a filter than without one.

Hope this helps.

[realflow100]

I meant for doing star test. not for observing with.

edit:
ok I dont know exactly what I did but now I see absolutely flawlessly circular airy disks with a ring around it. perfectly round. (for moderate to dim point sources)
brighter ones probably will have several rings though.
They also seem to look much smaller than they did previously.

I can even push my scope past its theoretical limit and still see smaller details when the atmosphere is calm enough. beyond 140x
most of the time its too turbulant for more than 70x!
in/out focus is still inconclusive. i think adding spacing is the wrong way around. it seemed to make it worse. and when i put back the original thinner spacer ring it was better.
might need to reduce spacing but im not sure what the right spacing is. 0.01mm? or 0.1mm? 0.2mm? it'll take forever and i dont want to smudge up the lenses anymore with the prints spacers make.
the alignment is so perfect this time i can't fathom it being any better. the airy disk is so perfectly round with the ring very neatly around the disk. with no distortions.

[turboscrew]

I meant for doing star test. not for observing with.

edit:
ok I dont know exactly what I did but now I see absolutely flawlessly circular airy disks with a ring around it. perfectly round. (for moderate to dim point sources)
brighter ones probably will have several rings though.
They also seem to look much smaller than they did previously.

I can even push my scope past its theoretical limit and still see smaller details when the atmosphere is calm enough. beyond 140x
most of the time its too turbulant for more than 70x!
in/out focus is still inconclusive. i think adding spacing is the wrong way around. it seemed to make it worse. and when i put back the original thinner spacer ring it was better.
might need to reduce spacing but im not sure what the right spacing is. 0.01mm? or 0.1mm? 0.2mm? it'll take forever and i dont want to smudge up the lenses anymore with the prints spacers make.
the alignment is so perfect this time i can't fathom it being any better. the airy disk is so perfectly round with the ring very neatly around the disk. with no distortions.

Congrats, you pushed through!