@"Kevin_A"

Sigma 40/1.4 at f/2.4, APS-C frame,Alioth at center:

Center and right looks good. Let side shows severe tilt (an almost identical tilt exists for both the M54M-M48F-2 and the PreciseParts M54 adapters, so the tilt is farther on the lens side). It looks like a moderately "simple" tilt -- the left side of the frame is too far from the image plane:

Tuck tape might be able to fix it.

However, look at the top right and bottom right corners. Not bad.

Now get this: very low level diffraction spikes with the lens closed down from the fully open f/1.4 to f/2.4 here.

Here is a comparison of the Samyang 135 and the Sigma 40 irises:

Samyang 135/2 at about f/4 (smaller than my fixed iris):

Sigma 40/1.4 closed down to f/2.4:

You get what you pay for, again.

But this is good news. I won't have to open up this lens at all (scary at f/1.4 when putting it back together :-).

Tomorrow, I will first try going back to the original Nikon bayonet. If the tilt goes away, I can just use it as is, with the QHY Nikon adapter (10mm thick). Unlike the ZWO Nikon adapter, the QHY adapter has 18.7mm of available backfocus (the 10mm QHY EOS adapter only has 16.2mm of backfocus because the EOS flange distance is only 44mm).

Perhaps I can incorporate a push-pull tilt adjuster in that set of plates from the web machine shop. I will look into that -- should not be very difficult. I think that is the best thing to do.

Chen

Kevin_A have you tested the WO P68 yet and fine tuned the backfocus?

Yeah, no problem. Don't know what the Cloudy Nights people were complaining about. The backfocus just has to be tuned to within 0.1mm. Even 0.2mm off is a bit questionable. It is a very fast scope.

Only complain I have is that after fine tuning backfocus, the focal length came out to be 270mm. Plate solves don't lie. So this is not a f/3.8 scope as advertised, but more like an f/4. Yeah. Don't sound as good now, eh? As usual, never trust people trying to part you from your money by offering something that sounds cheaper than it should be.

They may have simply taken a number that is 4% less than the designed focal length to make the advertised specs look better, and that way, they can't get sued, since a lawyer can weasel their way around 4%. I want to see what the designers think the focal length is :-).

The fact that 270mm focal length gets you almost precisely f/4 makes me think there is some marketing hanky panky. They just don't think hobbyists will measure that carefully. Their shills don't.

Except for the high end equipment makers, this hobby is full of people bending rules. That is why things like Takahashis and 10Microns are worth every penny they ask for.

Ironically, I once got a fortune cookie that said: "Good things are not cheap. Cheap things are of no value." :-)

Chen

Kevin_A

Quick test with shimming the Sigma 40 (@f/2.4). I added 0.2mm right in the direction of the pole, and this is what the aberration inspector shows now:

The tilt is better, but still there, and it is more obvoious now that it is not just up-down error, but the bottom right is worst.

But if the aberration inspector output is what the tilt produces, I probably don't have to do anything about it :-).

I will play a bit more with the tilt, but this lens is as good as the photo world lens reviewers make it out to be. Especially when it is straight off-the-shelf with a decent iris. It really shows that the Japanese do pay more attention to detail when it comes to optics (thank you, Ed Demmings). Even thier irises are cleaner.

The center of the frame is 2º directly north of Alioth, that is why there is no bright star in the center panel of the aberration inspector. I was trying to place the entire dipper part of Ursa Major inside the frame.

Notice the difference in color of Dubhe vs the color of Alioth, even though I am using a didymium filter. This is 180 seconds at 10 dB gain with your favortite APS-C camera.

The Hungarian wide-angle 100mmx100mm Bahtinov is working well too. Very bright (very high resolution mask deposited on photographic film and sandwiched between plastic plates), and no missing diffrraction spikes all the way to the corners of the frame (mosaic of small copies of the Bahtinov pattern).

Chen

Kevin_A

Moved the 0.2mm tilt adjuster (that 3D printed stuff from Canada :-) to about the corner of the sensor, and I get this:

Still better. I will try 0.3 mm worth of tilt tomorrow, and a tad less rotation to the corner of the sensor. (That's going to be quite a few layers of the 0.075 mm worth of Tuck tape!)

I'll bet most people have more tilt than this :-).

Chen

Kevin_A Is that the Kase Bahtinov mask?

No. I have the Kase; it produces bright spikes, but like all single Bahtinov grill-pattern masks, do not work with very wide angle lenses.

A skewed ray bundle in a wide angle lens would for example miss one or more of the Bahtinov sections, and you end up with missing spikes. For a star near to the corner of the 40mm lens, I would sometimes see one pair of spikes only, sometimes two pairs only with the regular masks,

The Hungianian one is quite smart and consist of a mosaic of smaller Bahtinov masks, so every ray bundle will see all three of the Bahtinov grill patterns.

The resolution of the thing (you need a magnifier to see the grill patterns) is so high, the spikes are very bright, too. Plus the size of the spikes is humongous. It is not laser etched glass or plastic (like the Kase), but is done using photographic negatives. The negative is then sandwiched between 100mm x 100mm plexiglass to protect it.

https://focusonstars.com

This mask does not work with longer lenses, so I also bought his "Tele" version. Same technique, using very fine lines, so very strong spikes even though it is a single Bahtinov grill.

They are a little clumsy since they need the 100mm square filter adapters. But at least the wide one is indispensable. When done with backfocus adjustments though, you can probably do without the wide mask, since all you need is to focus on a central star. But I find it extremely useful when looking for backfocus. When I am done with backfocus on the Sigma 40mm, I will probably go back the the regular 82mm Kase mask since it is much less cumbersome.

Even the "standard" etched glass/plexiglass lens-threaded Bahtinov masks like the Kase have higher resolution than the cutout metal and 3D printed ones, or the stupid cutout plastic ones in the William Optics lens hood (those cheap junk are next to useless). You will get a culture shock when you start using them.

The Kase one also produces crisp spikes. But lower resolution masks than the Gabor one, so the spikes are not as long and as bright as the Gabor. Also, the Gabor wide mask is still the only one I found that works at the corners of wide angle lenses.

I have since also found a place that makes very large "glass" Bahtinov masks, so I got a 95mm one (mounting on a Thousand Oaks aluminium cell that I cut out the mylar film :-) for my FSQ-85. That makes brighter spikes on the Baby-Q than the standard 82mm and 77mm glass/plexiglass masks.

https://mavenfilters.com/product/starry-focus-filter/

What I have done with the 82mm masks and filters is to mount them on the Kase magnetic rings. This lets me simply plop the masks and filters in and out without trying to thread them.

https://www.amazon.com/dp/B0CH9FNNZB?psc=1&ref=ppx_yo2ov_dt_b_product_details

One ring from the pair is screwed onto the camera lens, and the 82mm Bahtinov masks are screw onto the other of the ring pair. I have also mounted these Kase rings on the 100mm x 100mm filter holders. Once of the magnetic sides is mounted on the lens, you can also buy the filter end alone, without having to buy a pair of them:

https://www.amazon.com/gp/product/B0CG2PH8XX

They are really life savers in the dark. The shock value when seeing bright spikes all over the FOV is almost worth the value alone :-).

Oh, the spikes are as blurry as the stars when it is out of focus. But when these high resolution masks are in focus, the central star in the Bahtinov diffraction is a very small pinpoint, making it easy to judge if the central spike is centered. I have been centering them by skewing my iPad screen so I can see if the central Bahtinov line crosses the central star. Notice the lack of "hairy" stuff coming from the central star too on the Gabor mask. I will make s magnified version of a bright star to show how clean it is. I should be able to find one from my saved images.

Chen

Kevin_A

Sigma 40 @f/2.4. 12 frames (dithered) of 180 sec 10dB gain ASI2600MC centered at Sadr. Nothing special - default AstroPixelProcessor settings. No flats, no darks, no nothing. 82mm Didymium filter + 82mm Hoya UV-IR cut (essential with the ZWO window). Tilt was the value I reported in earlier post. Moon was 2 nights from full moon.

No crop, scaled down to 25% or original.

North America is quite obvious, next to Deneb. Even the Pelican showed up. Cygnus Loop, too.

Light falloff at f/2.4 is surpringly good for a 40mm lens (since I didn't even take a flat). At f/2.4, I woudn't dare put any filter between the lens and the sensor.

This is Deneb at full resolution of the camera:

Some random spikes. No iron cross nor notches -- so no aperture vignetting. Albeit, it is not at the corner of the frame.

Chen

Kevin_A it looks like a keeper! Now imagine 3 hours of subs…. Nice!

Yeah, I think it is a keeper too. Not shabby at f/2.4. I wonder if it is worth opening it up to f/2 even and use 40% less exposure times :-).

BTW, because of the trees, I may need multiple days to get 3 hours worth of subs. Speaking of subs, for the 40mm FOV, I should move the center of the image a bit south and east of Sadr, since north of Deneb is not that interesting, and it can include more of the Cygnus Loop. I can't move that much south along the Milky Way though -- city lights start getting really bright as I move south, and the gradient is going to screw things up. Cygnus is pretty much as far south as I can get for the Milky Way.

OK,given the results, I've decided to make a tilt plate for this, since I have backfocus to spare. Currently it looks like this.

There is a small 2mm thick circular plate (almost hidden from view by the Sigma's rubber weather proof gasket) that is placed right on the Nikon bayonet flange. Over that is 3mm circular plate that has the same diameter as the PreciseParts M54 adapter (black plate).

I can make the 3mm diameter plate larger (since it will clear the Sigma), and add a second 3mm thick tilt plate (4 corners push-pull screws) that clears the smaller black PreciseParts plate. The push-pull only needs about 0.3mm of movement. An M2.5 screw has a pitch of 0.45mm, so it need less than one turn worth of push pull screw movement; and I can always pick and choose screws of different lengths.

I can finish the drawings and send this to Seattle tonight to let them to start playing with it. In the meantime, I can start testing the Samyang 135 that has the fixed iris. BTW, if the reassembly really miscollimated everything, I do not need to replace the Samyang, since I have the same lens in Rokinon Cine form that I have not touched. Just need to remove the back end and replace with M54 thread.

Chen

Kevin_A definitely a fabulous mask…

OK, here is the "long" version of Gabor's Bahtinov Mask, for 50mm focal length and longer. 100mm x 100mm filter format.

The first image is a full FOV of the ASI2600MC with the Samyang 135/2 lens (with my fixed iris at f/3.3). This gives you an idea of the enormous size of the spikes.

Notice Mizar at the bottom right corner, with it's dimmer double.

Here is what Alioth (central, brightest star in above image) looks like when magnified a little. Notice how compact the center point of the diffraction looks like. Makes it really easy to determine correct focus.

Notice too the central spike... nice clean transition between R G and B components. Lens is very "APO." Many non-glass masks produce a line that is too dirty and fat to see how clean this lens is.

The only downside is how bulky a 100mm x 100mm filter holder is. But it is worth it, seeing the results. And I have a Kase 82mm magnetic ring screwed on to it, so removing the filter is just a matter of pulling it away from the lens.

Chen

Kevin_A

Samyang 135/2 with the fixed f/3.3 iris.

Some tilt, so I may want to also make a tilt plate for it too before wasting too much more time adjusting it for now.

Good news is that there is no 18-spikes anymore.

However... see that red hair like thing eminating from about the bottom of the bright central star (Alioth)? It shows up also on Mizar. I need to look carefully if I left some dust inside the lens.

I'll send off the tilt plate drawings for the Sigma 40. Too tired to try to rush off a tilt plate drawing for the Samyang for now. The Sigma result is quite exciting. The Sigma stars have a snap to them that I had only seen on the Baby Q when it is focused very carefully. Things like Askars never have a snap, however you focus them.

Tomorrow night's project: check to see if the filter drawer for the Baby Q that is placed before the 1.01x fattener vignettes. That part of the scope uses 72mm parts, but the flattener glass appears to only be 40mm or so in diameter, so a 2" filter there may be OK. This arrangement is so I won't have to readjust backfocus when I change filters and not have to step outdoors.

Chen

Kevin_A i would check for a tiny hair or smudge.

I did find a small grease smudge at the outside of the rear lens group. Must have transferred over from the lens barrel when I was going through the pain of reattaching the rear lens group with the three half hidden screws with the nylon shrouds. A little alchohol got rid of it, so I might try the lens out tonight.

I still have no idea how Samyang themselves align that rear lens group to those three hidden holes when they installed the component. There must be some trick I haven't discovered. If not for that, removing the rear lens group to get to the iris is child's play.

Got email from FrontPanelExpress that they have started to work on the tilt adapters which I submitted last night. I don't think they can get it ready for UPS pickup this afternoon, though.

I had considered using the ZWO 90mm tilt plate for the ASI2600 as the tilt adjuster (my design is already 85mm in diameter anyway), but passed on that because it is 5mm thick. The advantage of the ZWO part is that it has the foam ring to prevent light leaks, which I have to use electrical tape for my tilt plate. If there is a 0.3mm gap from the tilt, I would definitely need tape to keep away stray lights.

The 40mm lens is a challenge. We are talking about 0.05mm type backfocus, and just 0.3mm tilt produces a humongous change in HFD across an APS-C frame.

When it all settles down, I will try the Sigma and Samyang on a full frame sensor. I have never had a single OTA or lens (even the the Baby Q) that works to my satisfaction to a full-frame corner, so I don't hold up much hope. But they might be good for an extra 10% FOV , which sometimes really help.

In general, I think it is easier to get a good image from a shorter lens (like using 35mm instead of 40mm) than taking an existing lens to a larger sensor. But 40mm is already too wide for me -- there are not many places in my skies that don't include tree branches. This (SkySafari screenshot) is all the clear sky I have from my Mortar tri-pier:

The rectangle is 40mm at APS-C. I was not joking when I said I have no more than a couple of hours of integration time. The clearing is larger further north -- that is why I use Alioth :-). Fortunately,my interest is in instrumentation and not TikTok photos, so it is not a problem for me.

Chen

Kevin_A if I were you I would cash in some of those AAPL stocks and get a different rooftop!

Nah. Portland is very good for health services, especially opthamologists (my spouse has macular problems on one eye, and both of us already have cataract surgery done; she also had Lasik done a couple of decades ago).

When you are old, that kind of thing is infinitely more important than a silly hobby. Plus she has a nice garden on part of our 2 acre land.

We used to live in Phoenix two decades ago with the clear skies, but summers are a pain. Between life and hobby; I choose life.

If I didn't care about being near health services, I would move to Bend, Oregon, which is actually not far from that SkyLark Bortle 2 dark site. Bend itself is probably no worse than Bortle 4. A bit warmer in summer and a bit colder and more snow in winter than Portland, but livable, unlike Phoenix. My old boss (he was the creator of ColorSync when he was at Apple) retired from his job as VP at Xerox to Bend; he chose it because of golf -- in his youth, he went to Göttinghen for his doctorate because he drove race cars. He definitely chose to live for his hobbies :-).

For playing with instrumentation, my skies are more than adequate. Just need clear skies now and then to do testing of ideas.

Chen

Kevin_A One thing I did read on the internet regarding the red halo is that some fast lenses are designed to have a slight red halo when best focussed.

Both Askars and Taks tend to have smaller red spot size at the optical axis vs green and blue spots. The Pleiades 68 does have a large red spot.

(See how Askar always cheats by showing 200µ x 200µ squares. Tak and WO use 100µ x 100µ.)

Maybe I will manually focus instead of letting the EAF do it and see what happens to the star size if I balance out the RGB visually.

I have no problem with the EAF to do the focusing. The problem is ASIAIR, which is independent from the EAF. The thing to get around the EAF backlash is to mount it on the fine focuser shaft of the OTA, but you then run into the backlash of the OTA focuser.

BTW, the EAF commands is easy to reverse engineer. But so far no one has done it because those who know how also believe in not violating intellectual property :-). I actually have an idea of how to make ASIAIR control any well documented electronic focuser (or ones that have SDKs), without reverse engineering the commands.

What you do is to connect the ASIAIR to an EAF. But do not connect the EAF to the focuser. Instead, connect a good focuser like the ESATTO (check out what essato mean in Italian :-) to the OTA. You connect the EAF to a shaft encoder, instead. Read the shaft encoder with an Arduino and translate the movement to the ESATTO. You don't even need high resolution shaft encoder, since you can make 1/10 of a revolution of the EAF translate to a 1 micron movement of the drawtube -- i.e., let the Arduino be the one that reduces the movement. That way, even a 5 or 10 degree of EAF backlash is not a problem, since it gets reduced by the Arduino. And, no violation of intellectual property.

Did you see my measurement of the EAF after it was first released? Same idea, but the Arduino idea can use a much cheaper encoder. I was seeing 3 degrees of backlash, and the curve did not recover full linearity for another 3 degrees. Cheap gears.

https://bbs.zwoastro.com/d/12562-5v-eaf-linearity-and-backlash

Chen