Testing lens for suitability when cloudy

[TiffsAndAstro]

Is there anyway in daylight or a cloudy night, to test my lens at various stops to see how it effects aberations? I'm thinking bright led behind foil with a pinhole at one end of garden and 30feet away at other?

[Albir phil]

If you have sun you can use a ball bearing this will give you a point of light to focus on ,or a Christmas tree bauble will work and it larger .Set it 30 ft away 

[pipnina]

I bought one of these and it seems to be quite good

Of course it will be most effective once the sun goes down. You can position it some distance away while your camera is on a tripod, focus on it and the picture will show you the sharpness you can expect from stars. Just adjust the camera's orientation to position the fake star at different parts of the frame: https://www.firstlightoptics.com/other-collimation-tools/hubble-optics-5-star-artificial-star.html

For smaller camera lenses (nifty fifties?) I'd guess the larger holes will be suitable and nice and bright once more than 10m away. For bigger lenses ideally you want as much distance as possible. Most lenses will decrease in quality as you focus closer.

[vlaiv]

+1 for artificial star.

Small ball bearing or similar can be used to simulate it, or you can get commercial one.

There is also a way to DIY one from optical fiber and white led with a bit of soldering.

If you have a good small scope (even achromatic refractor if stopped down to F/10 or so while being slightly larger aperture than lens will do)  - then you don't need a large distance. You can use scope as collimation lens for the beam to make it close to infinity.

[RT65CB-SWL]

Another +1 for the https://www.firstlightoptics.com/other-collimation-tools/hubble-optics-5-star-artificial-star.html

Useful tip!… So I don’t loose the small magnetic metal strip, I put a 1.25” vinyl end cap over the end of it when not in use/storage. It will also protect the mask from damage… and do remember to remove the batteries. 

 

Edited March 18 by RT65CB-SWL

[TiffsAndAstro]

1 hour ago, Albir phil said:

If you have sun you can use a ball bearing this will give you a point of light to focus on ,or a Christmas tree bauble will work and it larger .Set it 30 ft away 

I remember seeing the sun on television a few months ago.

[vlaiv]

1 minute ago, TiffsAndAstro said:

I remember seeing the sun on television a few months ago.

You can use something like torch - one that focuses to tight beam to illuminate the ball bearing from the side.

Use very small ball bearing to get small reflection.

[TiffsAndAstro]

Ty all for all the solutions. 

I'm sure at least one will do the job. The flashlight thing from flo seems a great idea at a decent price.

[Louis D]

To see the aberrations, you really need darkness.  That means either testing at night or in a dark or near dark room of sufficient length.

I would recommend a black nickel ball bearing.  The chrome ones seem to reflect too much stray light from the light source, making it difficult to see aberrations.

I've yet to perfect an indoor technique after trying clear Christmas tree light filaments, chrome and black ball bearings illuminated by a tactical LED flashlight (torch), or a backlit foil pinhole using various light sources.

Good luck and let us know what works for you.

[TiffsAndAstro]

Well I found shiny thing and found i could focus on its reflection.

No use for fixing bird shapes but, I could watch that focus change as the purple fringing was replaced with green fringing

I think that will help me focus when it's dark though.

I'd say the fringing doesnt really change much even stopping down to 6.3

[Louis D]

4 hours ago, TiffsAndAstro said:

I'd say the fringing doesnt really change much even stopping down to 6.3

Check the far corners to see if the artificial star shape become more rounded at smaller f-stops.

[TiffsAndAstro]

2 hours ago, Louis D said:

Check the far corners to see if the artificial star shape become more rounded at smaller f-stops.

didn't do this and will give it a go tomorrow, though its possible clouds will clear a bit in an hour or so

[TiffsAndAstro]

On 18/03/2024 at 00:16, TiffsAndAstro said:

didn't do this and will give it a go tomorrow, though its possible clouds will clear a bit in an hour or so

Finally have some test results. Will convert to jpg and post soon ish for feedback.

200mm f3.5 but stopped down one to 5.6. 30 180 and 300 sec subs all looked trail free and I never took caps off polar scope.

Stars have more red than Donald trump's bank account though. 

Also 60x120sec on m51 and 30x120 on iris nebula. I have chance of fixing Chromatic aberration in siril than bird shape stars though hopefully.

I'm still a tad suspicious that my Blue Peter style focus  (here's one I made earlier) is unfocused enough to hide any trails though.

Orion or m31 tonight hopefully for science.

Edited March 30 by TiffsAndAstro
example subs

[TiffsAndAstro]

blue screen of death trying to remove stars for iris nebula. siril probably just can't handle so many and such high quality stars....

also phot0metric colour calibration failed but i can actually see the iris nebula. if i squint. seems my set up is working pretty good, except my lens is a bit crap. 

[Elp]

I'd say that's a result.

For starless try installing the GUI version of Starnet separately, mine doesn't work via Siril (haven't bothered finding out why as the standalone works fine).

Regarding NGC7023 Iris, it needs many hours if you wish to separate the dusty regions from the background, never properly managed it myself.

[Elp]

Regarding the red halos, it's likely the lens is optimised to focus red first (similar to my Takumars), I had to change the infinity stop so that I could minimise the red to a point.

[TiffsAndAstro]

and m51.... 200mm crop sensor is not idea for this. or my $10 lens. but at least (i think) it shows i have everything working, ready for m42 or andromeda tonight. need to choose soon ish

[TiffsAndAstro]

24 minutes ago, Elp said:

I'd say that's a result.

For starless try installing the GUI version of Starnet separately, mine doesn't work via Siril (haven't bothered finding out why as the standalone works fine).

Regarding NGC7023 Iris, it needs many hours if you wish to separate the dusty regions from the background, never properly managed it myself.

i cropped it a little different and it worked fine. i think it didn't like having 703 sub parts to process

i quite like it without stars. if im really stupid i might try it again with my 500mm mirror lens, er i mean my schmidt-cassegrain telescope tonight once m42 or andromea have fled the scene. 

[TiffsAndAstro]

4 hours ago, Elp said:

Regarding the red halos, it's likely the lens is optimised to focus red first (similar to my Takumars), I had to change the infinity stop so that I could minimise the red to a point.

i pre focused it on the moon last week and taped it down. its probably the best focus i've managed so far. infinity seems before the marker on the barrel, but its probably a 50 year old lens. m42 might look better if i can grab 90 minutes on it tonight. 

*******

faffed about for too long with 3ppa. 30 sec test shot on orion looked great, 180 sec even better, so i've set it going after using nina framing assistant to get most of m42, all of hh and some of flame nebula. might get an hour before it drops too low. high hopes for this one

then its time to break out the sct   and get m51 for like 5 hours maybe? 

Edited March 30 by TiffsAndAstro

[TiffsAndAstro]

4 hours ago, Elp said:

Regarding the red halos, it's likely the lens is optimised to focus red first (similar to my Takumars), I had to change the infinity stop so that I could minimise the red to a point.

how did you change the infinity stop? mine seems to stop past the infinity marker.  still don't like dropping it to 5.6 but there is no stop between wide open 3.5 and 5.6. might try 3.5 again now its focus is locked. lol. i bet there are people choking back vomit but i find getting 'perfect' focus on lenses diffcult and this seems ok for me for now

[TiffsAndAstro]

also, chrome remote desktop seems to work pretty well. also just put the app on my phone lol. works great there too

Edited March 30 by TiffsAndAstro

[Elp]

2 hours ago, TiffsAndAstro said:

how did you change the infinity stop?

You'd have to research your particular lens but with the Takumars you usually get to the stop, unscrew the three retaining screws around the focus barrel so it can rotate freely, turn it a bit, then retighten. Yours may or may not be similar.

[TiffsAndAstro]

2 hours ago, Elp said:

You'd have to research your particular lens but with the Takumars you usually get to the stop, unscrew the three retaining screws around the focus barrel so it can rotate freely, turn it a bit, then retighten. Yours may or may not be similar.

i can focus back and forth through infinity as the stars get smaller and bigger if i rotate focus back and forth? so i wouldn't need to moving the stop?

[Elp]

Maybe not, but at infinity my Takumars create red halos. It's only until the infinity stop is done you can reduce the size of the red halos further. But, if at infinity your star sizes start increasing again then the mod likely won't help.

 

[Ratlet]

It might just be you're in focus is why you get the red halos.  I found best results by getting it in focus and then backing off slightly.v if you use a bahtinov mask with some vintage prime focus you'll get awful red stars.

With the takumars you get red halos on one side of focus and very light blue on the other side.  The red is very noticeable, but the blue much less.  You basically want to err on the blue side as it ends up looking like very slightly bloated stars.

I've noticed that going by Nina if I focus for the 'best' focus my stars look awful, but if I back it off slightly they look better so long as I head towards blue halos.