TheSkySearchers

I have a question
I dont know if its the optics or just the sensor resolution of the camera im using causing a problem with my images

When I take a photo without the barlow. the smallest details look a little soft. and trying to sharpen them makes it look weird and sorta "blobby" or noisy.

but with a 2x barlow theres almost a 2x increase in the smallest details that can be seen. and the smallest details with the 2x barlow dont look any worse than the smallest details without the barlow.
so overall the 2x barlow gives a higher detail image. ESPECIALLY if you did a mosaic. but the focal ratio would be F12


is it due to the focal ratio becoming F12 (similar to stopping down?) causing the image to sharpen up and reveal more details?

if I stopped it down to F12 without the 2x barlow. would the details become significantly sharper?
Or what?
I'm really confused why its doing this
Heres some example images for showing what I mean.
Small image is without the barlow.
large image is with the 2x barlow.
the moon is approximately in the same phase as well in both images
theres nearly 2x as small of details visible in the image with the 2x barlow
Why does the image without the barlow look so much lower detail. even though it should be the sharper one due to undersampling?
The smallest details are no larger than the smallest details in the image with the 2x barlow

This is only a guess. I "think" I see what you are driving at, but.....

My first guess is that it doesn't have to do with the f ratio at all. It could be that without the barlow the pixels on the sensor are under sampling the image? Not sure. If you like I could later dig up a formula that would allow you to see whether you need to barlow for your images or not.

Hi,

Go and input your telescope and sensor info into this website and see what results it gives you whether you're in the ideal range or the under or over sampled range. Bookmark this website.
https://astronomy.tools/calculators/ccd_suitability

Cheers

I did just a few minutes before checking this forum actually lol
and this is the result



but if I change the "seeing" to be good or higher it tells me this
"This combination leads to significant under-sampling. This reduces the influence of guiding errors and improves signal to noise at the expense of finest detail. May be OK for widefield imaging but might result in softer images."
why would the image get softer in better seeing?
Would undersampling cause softness for some reason? Why would that be?

I also did a test with my canon SL1/100D vs my canon 500D
The smallest visible details for some reason looks SOFTER with the 12MP 500D.
and the 15MP SL1/100D looks sharper in the smallest visible details
Also there seems to be more chromatic aberrations with the 12MP 500D somehow.

Two things, the SL1 (18MP) has a smaller pixel size than the 500d (15.1 MP) 4.3 vs 4.7 microns (respectively). So the SL1 should give you the sharper image. Also you can't completely disregard the seeing either. There's no way to tell if the seeing was identical between your two images.

the seeing should be finel. I took the sharpest looking image with the lowest atmosphere turbulence out of like 20 photos. for each photo

but why does it seem like the image is so soft and not super crisp or sharp. even when i pick the sharpest image out of 20 with the best atmosphere/seeing

realflow100 wrote: ↑Sat Oct 23, 2021 7:09 pm the seeing should be finel. I took the sharpest looking image with the lowest atmosphere turbulence out of like 20 photos. for each photo

but why does it seem like the image is so soft and not super crisp or sharp. even when i pick the sharpest image out of 20 with the best atmosphere/seeing

Because the pixel size of your camera sensor is under sampling the unbarlowed image. The barlow helps with that a bit.

So would i actually get a sharper image if I got a barlow with the right value like 1.2x or 1.5x to get a perfect balance of under/over sampling so its right in the middle or something. so the scope resolution matches my sensor resolution as closely as possible?
or should I try to make the scopes resolution match the sensor as closely as possible for optimal sharpness? (optimal sampling?) so that for example 2 arcseconds of detail in the scope would be 2 arcseconds of detail on a pixel of the camera sensor?

Or would it always look the same softness for the smallest visible details (Not talking resolution of the image. just the sharpest/finest detail size visible at 100% crop)

is 2x maybe just at tiny bit too much. but no barlow is too little?

Does a barlow magnify the chromatic aberration? (Making halos bigger?) or do halos stay the same relative size at 100% crop when using a barlow?
does using a barlow since the focal ratio is extended cause the CA to be proportionally similar amount for some reason? (since the focal ratio is longer. its similar effect to stopping down?)

if I stopped it down to F12 would the image sharpen up or not? would the CA be smaller scale or something?
Does the focal ratio being extended cause a more significant flattening/enhancement of the field toward the edges than expected? your not only using a smaller portion of the optics. but also the longer focal ratio is also causing a stopped-down effect at the same time?
Or does it cancel out?
I have so many questions about these specific things

Here are two good articles that should answer a lot of your questions.

https://diffractionlimited.com/matching-camera-optics/
https://www.peterzelinka.com/blog/2019/ ... ope-/-lens

with my 100D and 420mm focal length I have arcseconds per pixel of about 2.11

the smallest stars I could find in my recent photo of polaris are very faint ones about 4 or more pixels wide/across diameter. looks perfectly round and smooth to me.
stars just the slightest bit brighter than the faintest ones visible are already 6 to 8 pixels wide.
but I dont know if its due to focus not being absolutely 100% totally perfect. or due to the optical quality not being enough to make stars super sharp.
or if its just the atmosphere/seeing.
and I'm not sure if my processing of the image has bloated the stars or not.
the unprocessed image has vaguely sharper looking stars. but they are much dimmer and harder to see too..

Go with focus and seeing first. Also, do you use a Bahtinov mask to ensure you have achieved the best focus possible?

I use 10x digital crop and do it by eye in live view with my camera on a star.
I used polaris and had to play with the iso to find the best star brightness where its not blown out. and can make the white core of the star as small and sharp as possible. it takes about 5 minutes to really nail the focus this way.
and the seeing/atmosphere sometimes makes it hard to truely find the absolute best focus. so I have to wait a while for the atmosphere to get calm enough sometimes.

also re-check my post I found better smaller stars and updated the image

With a Bahtinov, focus takes about 15-30 seconds. They are very inexpensive and when you own one you realize, how did I ever live without this. Here's one at Agena.

https://agenaastro.com/farpoint-bahtino ... fp400.html

Also I'm not quite 100% sure how stiff my focuser knob should be.
Its just barely stiff enough to keep it from going out of focus when I leave the telescope alone with my camera attached to it.
but I feel like if its any tighter it makes it more difficult to find perfect focus.
its such a tiny tiny tiny spot where perfect focus is. like 0.05mm or smaller.

I did a quick surgery on my 70mm frac. Because it is a cheapo frac it has a plastic focuser. I drilled a 3/16" hole in the top of the plastic housing of the focuser, then tapped the hole for a nylon screw that now screws down to the focuser tube (but not into!) and locks the focuser in place. So no more focus drift when the camera is installed.

I made a home-made star focusing mask out of a paper plate. a sharpie marker and a razor blade

Its BARELY good enough to work. on the brightest stars. and I still have to use magic lantern to set a lower 10FPS framerate. 10x digital zoom. and max iso on the camera. to get the spikes to be visible.

heres some sample images. taken at 1/4 shutter. iso 12800
First image is bright centered star. Second image is a faint star off to the right a little ways. and yes it actually really is a star. however faint it may be. its actually a star. surpemely dim and faint but visible as a tiny white dot. like 1-3 pixels across

heres another pic of the smallest stars I can see
this is after using the home-made star focusing mask. focused on Capella
first one is a pic of polaris. and I circled a very faint star next to polaris (confirmed in stellarium its really a star)