TheSkySearchers

What size telescope to see Hercules D crater on the moon?
Like just seeing it as a tiny crater (smallest visible crater)?
what magnification?

its just next to the main hercules crater (larger crater with almost 2 craters inside it)
What size is that crater? arcseconds? miles?

Hercules D measures 15.5 X 7.6 km. Which is around and about 8,4" X 4,1 arcseconds.
You will need a 3 cm telescope to "see" 4,1" In reality I think a 6 cm telescope should be more comfortable.
In order not to have an empty magnification I think 120X and crystal clear steady skies are necessary.



Source:
https://quickmap.lroc.asu.edu/?extent=- ... F8BdC0yioA

I can just BARELY see it in my 70mm F6 refractor achromat telescope. at 140x its barely even a speck. hardly visible. I have to go even further. 200x magnification to really see it as more than just a speck.

is my scope very badly over/under corrected causing this?

I can see airy disk in bright stars at 140x magnification. but they have too many rings. and its hard to focus them perfectly. i have to rack focus back and forth to find the real middle point a few times before i get it right
on one side of focus (racking the focuser OUT) i get sharp concenctric rings in a crisp out of focus "disk"
but racking the other side of focus inward (focuser racked inwards)
around the same distance from "perfect focus" the airy disk is still visible until i get 2-3 times more distance out of focus and it basically blurs and fades away. no crisp out of focus disk or concentric rings. (the rings are technically still visible. but so difficult to make out i doubt that even matters)
would that make my scope under or over corrected?
The plastic spacer ring is very thin. not even 0.5mm thick

Too many rings in focus means a spherical aberration of around 0.5 Wave or more. ( Rather usual for economic refractors)
With the focusser racked in, a darker center means undercorrection.
A certain amount of undercorrection can be corrected by decreasing the space between the lenses. How much, I don't know, trial and error, but I doubt if you can cut the undercorrection in half.
Instead of a 0,5mm plastic ring, use alufoil from the kitchen. Alufoil is approximately 0.01mm thick. Double it and you have got 0.02mm. Double again gives 0.04 mm. Double again and it is 0.08mm. Again is 0.16mm. Fold it for the 5th time and you have 0.32mm. You could start with that, making three spacers of 0.32 mm.
When you decrease the distance between the lenses, focus is slightly shifted to red. I doubt if it is visible.

Nice literature for you:
H. Suiter, Startesting Astronomical telescopes.
Nice freeware program:
Aberrator by Cor Berrevoets

the spacer it has is more like 0.1mm I dont think I could feasibly get thinner spacing. I think its over corrected actually.
I see a brighter spot in the middle when racking the focus inward.
Then the whole thing goes fuzzy and blurry and gets very hard to see
with the focuser racked outward I see a very crisp sharp bokeh "ball" with concentric rings which is very slightly darker in the center

The Hercules D crater is approximately 8km (5 mile) across and your telescope size in excellent conditions will let you see down to 6.4km (4 miles)
At best you could see it but detail will be minimal at best.

Occasionally, I have found the flint lens turned around backwards on entry level scopes, which reverses its correction. If you can turn the flint lens around and try it, you might find the scope works better. The flint is normally the rear lens with concave surfaces. The front lens with convex surfaces is the crown.

Also, if the ground glass circumference of the lenses has not been blackened, you can help yourself if your blacken it. I use a medium size black Sharpie with a cone tip. I blacken with the side of the cone and not the tip to keep from accidentally running the tip across the optical surface. Wear a cloth glove or a sock over the hand holding the lens by the edge. Be patient and do maybe a quarter of the circumference of a lens at a time, put it down for at least 15 minutes, thirty is better, to let the ink completely dry before blackening some more. One coat is enough to get rid of most of the light reflecting back across the lens surface, but I normally put two coats. Three coats can be too thick for some lenses to fit easily back into their cell.

Blackening that edge can change the whole "feel" of the lens when you look through it, with better contrast. That will help you see small craters better.

Sorry, your words and my foreign understanding of the language don't seem to match. I should have shown this before.

image:


0.5 Wave undercorrected. In focus resp out focus.
If it looks like this, it is undercorrected. If it looks the other way around it is overcorrected.

my lens elements had so much slop/tolerance between them and the holder they are in that there would be awful ugly blue red aberration shift left-right across the whole image. and a generally horrendously soft image too.
I had to put 3 small strips of packaging tape around the lens elements just to take up that slop and keep the lenses more aligned too. which fixed the blue and red aberrations. so the image now looks uniform. airy disks are centered within rings.
i dont think any amount of coats of sharpie would add any measurable thickness lol
I think my 6mm eyepiece has a glare problem though. Need to fix it somehow ore replace with a better eyepiece.. my 9mm eyepiece seems fine. but I get less magnification of course.
and the elements seem to be in correct orientation at least.

i just attempted using a sharpie and i think its a success. but the edges of the lenses seem a tiny bit beveled. by like a fraction of a millimeter. so it was a bit hard to get sharpie at the edges of the edge.
seems to of worked ok though.
tested it but I'm not sure if theres a big difference.
Maybe a percent or 2 more contrast? hard to tell. maybe would make more difference at night.

Focusing inward looks like the right pic.
Focusing outward looks like the left pic
so reversed from what you showed.

realflow100 wrote: ↑Thu Aug 26, 2021 9:44 pm my lens elements had so much slop/tolerance between them and the holder they are in that there would be awful ugly blue red aberration shift left-right across the whole image. and a generally horrendously soft image too.
I had to put 3 small strips of packaging tape around the lens elements just to take up that slop and keep the lenses more aligned too. which fixed the blue and red aberrations. so the image now looks uniform. airy disks are centered within rings.
i dont think any amount of coats of sharpie would add any measurable thickness lol
I think my 6mm eyepiece has a glare problem though. Need to fix it somehow ore replace with a better eyepiece.. my 9mm eyepiece seems fine. but I get less magnification of course.
and the elements seem to be in correct orientation at least.

i just attempted using a sharpie and i think its a success. but the edges of the lenses seem a tiny bit beveled. by like a fraction of a millimeter. so it was a bit hard to get sharpie at the edges of the edge.
seems to of worked ok though.
tested it but I'm not sure if theres a big difference.
Maybe a percent or 2 more contrast? hard to tell. maybe would make more difference at night.

Focusing inward looks like the right pic.
Focusing outward looks like the left pic
so reversed from what you showed.

The way you describe your objective cell tolerances, you are right about the thickness of the sharpie deposit not affecting things, but I had one that was tight enough that three coats would not allow the lenses to slide in. I polished what I could off of the circumference with a cloth. Whatever deposition that was there was gone and the lenses went in ok. The problem can be at the wrong temperatures, the differential expansion can, in a case like mine, cause the glass to distort a tiny bit and throw the image off. Will not happen with yours.

As far as the effect of blackening the edge goes, the first time I tried it on a 60mm by 700mm refractor, I could not see any defraction rings on a relatively bright star before the modification from the very light shot back yard I had at the time. After the modification I could see the first ring and a small indication of the second and the image background did look a bit darker. That was the only thing I had changed on the scope.

I think I need to flock my focuser tube. Thats whrere the majority of the glare seems to be when i look through my telescope focuser without an eyepiece installed.
Any idea what the best kind of flocking stuff would be? Something thats suuper cheap but very effective? Well known? Also would need to super thin so it doesnt act like an aperture stopper
preferably purchasable on amazon?
Ive always seen airy disk and multiple diffraction rings. on even medium brightness stars.

The material I use, even in focuser tubes, is sold by Scope Stuff. It has a sticky backing. It is not terribly thin, but it works in the draw tube on 60mm refractors. I cut it in one inch wide strips, then use a Sharpie to going along the edge of the strip. The backing is white, so that blackens whatever might show. I also spray paint the inside of the focuser tube with tiny little spurts of flat black paint, to dull the surface, before installing the flocking, trying hard not to get any runs, which are more reflective. That way, if I mess up and leave a tiny bit of the focuser tube surface showing, it is dulled as much as I can get it without the flocking. That material will absorb 99% of the light that hits it, or more, depending on the angle of incidence. So far, all this works.

Sometimes I have to patiently work with a strip some to get it to line up beside a previous strip the way I want it to. On the last piece, it has to be custom cut to width. Somehow, the width can vary a tiny bit from one end to the other sometimes. I am not sure if it is me doing something wrong or if the inside of the tube varies in diameter a bit.

On the main tube of a 60mm, I cut 2 inch strips, go through all the same motions as the focuser tube, but I pull the backing off and lay the strip sticky side up on a yard stick, then run the yard stick down the length of the tube. I have to have some sort of light near the far end of the tube so that I can see the strip to get it gradually lined up with the edge of a preceding strip. I have a quarter inch dowel rod, maybe 18 inches long, that I glued a three quarter inch wooden ball that had a hole drilled in it. I run the ball up and down the length of the tube to press the flocking down firmly onto the tube, once I know I have it the way I want it.

I will even do silly things like blacken the chrome-like end of the focuser tube with the Sharpie, just in case a reflection off of it might reflect again off of the back side of the objective and in a tiny way mess up the image. I will also cut of any excess amount of screw sticking into the main tube and round off the end with a file, just not mess with any of the threads that actually hold parts together, then spray it flat black.

Basically, anything that I can imagine that might be causing light interference is fair game, no mater how small or seemingly insignificant. Some things may not amount to anything. But if I think something might help a tiny little bit, I will feel better about it if I do that thing. Hopefully, all the tiny little bits add up. If I do everything I can imagine to the scope, then I am satisfied with the result. It is the best that I know to do.

Hope this helps!

i decided baffling the focuser tube would be the easier route (And it was since I already had the parts to do it)
I used an old no-good 1.25 filter removed the glass filter from it and put a ring from an old barlow in (acts as a baffle thingy perfectly)
then screwed it in snug then fitted it into the focuser tube a ways and kept checking the distance with my largest eyepiece to make sure it wasn't cutting into the exit pupil (making pupil smaller)
and once i got it just right you couldn't see the focuser tube reflectiveness anymore! much less distracting when observing in the daytime and higher contrast at night!
And I still get a perfect exit pupil
all i'm missing is a few spacer rings to stack and try to get the best possible star test result I can.
but idk where I'd buy them.

Normally, you do have to cut strips or half rounds out of foil or different types of black paper. The only place I can think of that might have materials of some specific thickness is Scope Stuff. At least they might have a better idea of where than I do.

If you are using this telescope for imaging I would just paint the drawtube with chalkboard black or krylon ultra black.
Putting a baffle into the drawtube restricts the light and putting an eyepiece in to gauge the size is field stop not exit pupil. Every size eyepiece has a different field stop size so unless you are only using the one it will not work well.

Blackening all inside surfaces of the telescope will help a bit. But none of them solutions can definitely solve the problem.
One can't alter the overcorrection of a lens as an amateur. ( regrinding and polishing of all lens-surfaces). Pity.
The severe overcorrection is responsible for a big dip in the contrast transfer, all midtones are "grayed", call it "misty" of "soft".
I run the Aberration program on your lens with the specs I knew or suspected.



As you can see in the Multiple Transfer Function - diagram the dashed line ( your telescope) dives well under the perfect line. It brings down the Strehl factor to 0.42 (PST or point spread function)

The same picture for comparison, but with a perfect telescope: For dimension- reasons the in-out pictures for a 200 mm are shown ( otherwise too big for the frame)

Can fixing the overcorrection improve the image?
I just need some additional spacer rings I could stack to get the right spacing. but where to buy them from cheap?
a couple 0.1 to 0.25mm spacer rings. and stacking them until I get the right spacing. maybe 5 of each. to get the best combination. until the correction is as close to middle as possible.
My focal ratio is 6 by the way

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?

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

Heres some test images I took
one with the baffle I added to the focuser tube. and one without.
Theres a MASSIVE HUGE increase in contrast. but I'm not sure if the image is darker because of the higher contrast. or darker because the light from the objects is getting blocked from reaching the sensor?
The more grey/washed out pic is without the baffle I added to the focuser tube.
if I try to brighten the pic with the baffle to match the pic without the baffle. the highlights get clipped to kingdom come. even if I constrain the highlights to prevent them from getting blown out. the shadows have lower noise in the pic with the baffle installed.

I thought your telescope was a 70mm f/5.1?

No I have a new one. SVBony SV501 70mm F6 now. got it for 30$ and made some improvements to it also.
I dont use the old one anymore. it had major problems
I got another look at the moon and this is the smallest craters i could see (I circled 2 of them. the smaller one is just a vague dot but I can see it and the other one is a circle/crater shape. but still kinda hard to see)
Any idea what these 2 craters are? stellarium says egede C and B I think
I was able to go up beyond 200x magnification about 230 to 250x magnification! and focusing was the hardest thing ever. but once I got the focus right it was decent at that crazy magnification.
The atmosphere was fairly stable too most of the time!
EDIT I just saw an additional crater! egede F! on the other side of the egede A crater