32 replies to this topic

[Mbenj2405]

Hello, new to optical testing, this is my first time doing a dpac. The scope is my WO flt 156, I would like to think it is a upper mid tier refractor, priced just below a Skywatcher 150 Esprit. Mine has the great FT3545 focuser, much better than the stock 3.7". My camera is a Samsung S23 Ultra, I learned a lot using the cameras expert mode, which is mandatory. The grating is 2.5 cy/mm chrome on glass from Applied Image (Great product!) and the flat is 200mm in diameter. My power source is 3v, mated to a cheap rheostat controller, the adjustable voltage makes life a lot easier as the different color led need different brightness to achieve decent focus. 

 

[Mbenj2405]

Blue OutsideBlue Inside

[Mbenj2405]

White Inside White Outside

[Mbenj2405]

Green Inside Green Ouside

[Mbenj2405]

Null

[Mbenj2405]

Red Outside Red Inside

[Mbenj2405]

To me, I see a little under correction in red, the polish looks good. Maybe someone with experience can chime in but to my eyes this telescope is excellent. It seems obvious the William Optics do care about their products, especially their top end products. 

[Mbenj2405]

I just redid outside blue, less grainy. This looks the weakest to me, fat fingered near the edge.

 

[T1R2]

Looks like a very nice lens you have there

[Scott in NC]

Spherical correction looks very nice to me. You’ve got a good scope.

[RichA]

Null

You can definitely tell the high-performance grating in-use from the cheap printed plastic sheet ones.  A major improvement.  One thing people need to be on the alert for, crappy printed Ronchi gratings can give the impression that the optics might be rough, when in-fact it is the printing of the grating under high magnification that is bad.  Easy enough to test for and once established can let you know the optics aren't the issue. 

[Mbenj2405]

Yea, I first tried to print off Ronchi lines using a decent laser printer on film, the results were poor and I didn't even bother to try to use it as I knew the results would be poor. Garbage in = garbage out.

[scoale]

Very nice scope!  Congratulations on first DPAC.

 

Do you have a test certificate for this scope?  Not needed but would be interesting to see correlation with DPAC results?  I had a 132 FLT that was very nice - but heavy.

 

[Mbenj2405]

Very nice scope!  Congratulations on first DPAC.

 

Do you have a test certificate for this scope?  Not needed but would be interesting to see correlation with DPAC results?  I had a 132 FLT that was very nice - but heavy.

Thank you. I didn't get the test certificate for this one. About four years ago I purchased a flt132 with test report (0.977) and when I posted on here I was basically met with a incredulous reaction. The current owner of that scope doesn't live too far away, hopefully we can get it on my test bench. Um, the 132 feels like a baby compared to the F7.8 156, lol.

[CHASLX200]

Looks good to me.

[starman876]

Very nice lens.  I have seen similar DPAC's on lenses I have and it is such fun to do it in the rest of the color spectrum.   When I first started the LED I had was blue.  So I did my first DPAC in blue.  Then I got the other color LED's.  

[Mbenj2405]

Very nice lens.  I have seen similar DPAC's on lenses I have and it is such fun to do it in the rest of the color spectrum.   When I first started the LED I had was blue.  So I did my first DPAC in blue.  Then I got the other color LED's.  

Thanks, led are cheap to pick up in a multitude of colors nowadays. 

[scoale]

Thank you. I didn't get the test certificate for this one. About four years ago I purchased a flt132 with test report (0.977) and when I posted on here I was basically met with a incredulous reaction. The current owner of that scope doesn't live too far away, hopefully we can get it on my test bench. Um, the 132 feels like a baby compared to the F7.8 156, lol.

I would be willing to bet that FLT 132 will perform well on the bench.  That's nearly identical to the FLT I had (.978 w/ a PV of .135 & RMS of .021) and the optics on mine were sharp.

 

Ya, I looked up the weight on the FLT 152...that's a bruiser for sure.  If I knew for certain that I would get one as good as yours I might be tempted to lift it

Edited by scoale, 15 September 2024 - 12:34 PM.

[Jeff B]

Mbenj2405, I am glad to see you are stepping into the double pass world.

 

And a good start too!  The scope certainly seems quite well corrected spherically and color wise.

 

If I may, some suggestions. 

 

Purchase a "warmer" white LED.  The white image you show is very "blue heavy" and seems to be saturating the sensor in the blue causing considerable blooming.  You can see that if you separate out the RGB channels in monochrome.  But also decrease the overall exposure and, if you can, get into the camera's menu and set the color balance for warm as you can, set your ISO as low as you can and experiment a bunch.  Also, use image stabilization if you have it.

 

I looked at the Applied Image site and the Ronchi screens seem to be of excellent quality.     If I'm reading the site right, your current screen frequency is ~65 lines per inch (LPI), which, if correct, is low.  You will, as you have done here, get a good idea or what's going on, but the resolution is low.  The 5.0 screen is close to the 133 LPI "standard" and will show you considerably more, especially at focus.

 

And "fat fingered" at the edge in the outside of focus images are "normal", especially below F8.  You may have noticed that the "outside" images are more difficult to take and their contrast is higher than those taken "inside".

 

What does a high power star test tell you, especially with a green filter?

 

Well done and keep testing!

 

Jeff

[Mbenj2405]

Thanks Jeff, I find this testing quite interesting and fun to do.

 

I looked at the Applied Image site and the Ronchi screens seem to be of excellent quality.  waytogo.gif   If I'm reading the site right, your current screen frequency is ~65 lines per inch (LPI), which, if correct, is low.  You will, as you have done here, get a good idea or what's going on, but the resolution is low.  The 5.0 screen is close to the 133 LPI "standard" and will show you considerably more, especially at focus.

 

"Hello Mike,

1 cycle (cy) represents 1 line pair, or 1 black + 1 white/clear line.
5 cycles per mm would be 10 individual lines in a 1 mm width.
5 cycles x 25.4mm = 127 cycles per inch, or 254 lines per inch.

133 (lines per inch) / 25.4 (mm's per inch) / 2 (lines per cycle)  = ~2.6 cy/mm

The closest equivalent we offer as a standard product would be 2.5cy/mm

Nicholas Mancuso | Technical Sales"

 

And "fat fingered" at the edge in the outside of focus images are "normal", especially below F8.  You may have noticed that the "outside" images are more difficult to take and their contrast is higher than those taken "inside".

 

Yup, definitely more difficult. I was experimenting with the cameras pro mode- ISO, Speed and WB. That made a big difference, but I will take you're advise next weekend- I keep my gear at the cottage.

[Mbenj2405]

I just noticed the led kit I bought off Amazon (Bojack) for testing also includes "warm white", I was using the ones labeled "White".

[Mbenj2405]

Anyone on the east coast of Canada looking to get their optics tested, let me know.

[peleuba]

Hello, new to optical testing, this is my first time doing a dpac. 

 

 

The density is a bit low on the Ronchi screen but there is enough information for it to be useful especially as you learn about autocollimation testing.  And Jeff's post above - #19 - in the thread HERE - is a good cookbook for you to tweak some things.  Also, you can exceed the 133LPI, 2.5cy/mm threshold.  I routinely test with 200LPI screen.  

 

Great start to what will be, I am sure, an aspect of the hobby which you will enjoy.  I would encourage you to pursue it with vigor.  You may be surprised to where it leads you.

[Mbenj2405]

The density is a bit low on the Ronchi screen but there is enough information for it to be useful especially as you learn about autocollimation testing.  And Jeff's post above - #19 - in the thread HERE - is a good cookbook for you to tweak some things.  Also, you can exceed the 133LPI, 2.5cy/mm threshold.  I routinely test with 200LPI screen.  

 

Great start to what will be, I am sure, an aspect of the hobby which you will enjoy.  I would encourage you to pursue it with vigor.  You may be surprised to where it leads you.

Paul, after doing this testing, I can see that a higher density grating would require the inside and outside of focus be closer to the null position, how does this affect resolution as you hinting at?

[Mbenj2405]

I bought a new grating, double the resolution 5 cy/mm which equates to 127Lpi if you only count the dark lines, not it's transparent neighbor. Some confusion with this. Hear is the redo and it does show some flaws, but the lenses are still really good! It looks to be best in red, by just a little. Green and blue are about the same. 

 

   Red looks really good.

Edited by Mbenj2405, 19 October 2024 - 03:02 PM.