First Light with new ASI585MC Pro.: Page 10

First Light with new ASI585MC Pro.

w7ay

Kevin_A

Whoa!

https://www.reddit.com/r/canon/comments/1agbiyo/was_set_on_picking_up_a_sigma_40mm_f14_art_but/

Adorama no longer sells it, and Amazon does not have the EF mount.

Looks like ART 35mm for me, or get the Nikon version (couple of sellers still at Amazon) and a ZWO Nikon/filter drawer. MIght be worth it, seeing lenstip's comments about it.

Chen

Kevin_A

w7ay personally get the Tamron SP35 if the Sigma 35 does not work out…. I read that the sigma 40 was the best but just buy thru Amazon so you can return it! Maybe the 40 was discontinued due to bad sample variations and returns or just too heavy and lack of sales.

w7ay

Kevin_A I read that the sigma 40 was the best but just buy thru Amazon so you can return it!

I ordered the ZWO->Nikon filter drawer from Cloudbreak. Should get it tomorrow (UPS Ground from Seattle is overnight). So, give you one guess what I did with the lens selection :-).

I will have to frankenstein the Nikon adapter with a 54mm back plate -- like the stock EOS filter drawer, it comes with 42mm back -- sheesh ZWO, 42mm for a camera lens where people might use full frames with very fast lenses? Last time I measured, the ZWO 42mm interfaces only have 38mm clear openings. 28mm is not enough for APS-C even, exect for the optical center. Depending on the f-number, 38mm is not enough for APS-C. Needless to say, I have frankenstein my ZWO EOS filter drawers to become 54mm.

The 40mm size is a niche. Plus, the Sigma boys were trying to outdo their own 35mm, and were apparently given all sorts of liberty... and ended up with a large and heavy lens (after the XP50/1.2, lens weight no longer bother me :-). Like the FOA-60Q, I definitely don't mind owning one just to own something that measures well (me engineer :-).

I will still need to find a way to mount it, of course. No lens barrel dimeasions from the spec sheets.

Anything has to be better than the Samyang XP50/1.2.

BTW, as I was tuning back focuses, an idea occured to me -- it is much easier to collect filters with different glass thicknesses, and use glass filters to tune backfocus instead! As long as you have a filter drawer in the train, tuing backfocus is painess this way.

I know most of the Baaders and the Optolong (at least the L-Pro) are 2mm thick. The Astronomik L1 is 1mm thick or 1.1mm thick, depending on where you read, the Radian Quadband is 3mm thick. (All my chromas are 3mm thick, but are unmounted. But they have a clear 2" mounted filter (parfocal with their other filters) that is 3mm thick.) No filter is 0mm thick.

Since mechanical backfocus is 1/3 of the glass thickness, this gives four backfocus values that are 0.33mm apart by just swapping filters.

There are other odd ball values. The ZWO UV/IR cut (I will in the near future show that they don't really cur UV worth a darn) is 1.8mm think. But who knows, really. The Baader CMOS optimized UV/IR cut is 1.1mm (at least according to Agena).

This, by the way, is another way to determine the length of an EAF step size -- switch between an Astronomik and a Baader, the two EAF step values represents 333 nm :-).

Chen

Kevin_A

w7ay the svbony uvir cut is 1.85mm thick!
Good luck with the lens…

w7ay

Kevin_A the svbony uvir cut is 1.85mm thick!

That is just 0.15mm glass delta, or 0.05mm backfocus change. I was going to order one after seeing your post until I realized this :-).

But I may get one anyway as part of my bunch to measure UV and IR cut effectiveness. Wouldn't suprised me if it is identical to the ZWO. Both companies like to blindly OEM from the same low bidders. I wonder if ZWO even owns a spectrometer.

BTW, your SW 100ED blue bloat is most certainly due to the D60 UV/IR cut window of the ASI2600MC. Be sure to add a real UV/IR cut filter when you use the 2600MC with that OTA (all the Optolongs seem to really cut UV well). Or any ZWO camera that uses their D60 UV/IR cut window. The ZWO window probably came from same low-bidder OEM as their UV/IR cut; even though they don't have the same characteristics. It is amazing what junk they pass on to their customers who have no facility to measure. Their YouTube shills don't care, either.

Chen

w7ay

Kevin_A

The "use filter thickness to adjust backfocus" method still needs you to refocus the OTA. Not much we can do about that. But it sure saves a lot of time swapping spacers and then rotating the camera angle. With the glass filter swapping, the camera angle stays the same!

Why the heck did it take me this long to come up with the "backfocus with glass filters" method? It seems so obvious now. (Have you ever seen anyone else mention doing it this way?) It could have saved me a lot of time in the past. Heck, I am going to go back and check backfocus on my Baby Q again, since it is so easy. I might make up a 2" filter wheel (I think I have a spare ZWO 5x2" EFW around) with different glass thickness and that should make life even easier. Filter wheel trick does not work with camera lenses, though. That wheel has the same 20mm mechanical thickness as a ZWO filter drawer, so easy with real OTAs. Can't wait until someone sells a set of clear glass filters of different thickness (I want royalties! :-).

Having to refocus often is where the Kase 82mm Bahtinov mask comes in. The 82mm "filter" is actually in two pieces. One looks like a regular 82mm filter, and the other is a 82mm-82mm adapter that has a magnet. The Bahtinov part can simply be yanked out quickly with fingertips.

Now, if you know the glass thicknesses to a good degree of accuracy (we can measure them once, and put µm values in notebook), then the number of steps to refocus an EAF can be used without actually refocusing.

Here is the Kase Bahtinov mounted on the stacked lens hood on my Rokinon 135:

The front of the lens hood is thread for a 72mm filter, So I simply used a 82mm to 72mm step-up ring to the Kase.

All the extra bolts and pillow bearings is for me to do some careful measurements to draw up spme "declutching" plates :-).

Chen

w7ay

Kevin_A t

OK, the "use EFW to swap filter thickness" will work with these (asuming ZWO can machine the filter drawer version and the EFW version precisely to 20mm delta):

https://cloudbreakoptics.com/products/new-eos-lens-adapter-for-efw
https://cloudbreakoptics.com/products/efw-nikon-adapter

I don't (currently) have either one to try out. Will fix that "problem" ASAP. Cloudbreak only has one of the Nikon ones left, and two of the EOS one left.

Chen

w7ay

Kevin_A

Here we go. Star is Alioth. 3 second exposure ASI6200MC with gain 100. Rokinon 135/T2.2 with ZWO EOS adapter with 3rd GEN filter drawer (the latest, that finally does not leak significant light). Rokinon is stopped down to f/3.1 with an aperture mask (ergo, no confusing spikes for the Bahtinov focusing)

First screenshot: Kase Bahtinov mask, no filter (empty filter holder), EAF at 21248:

Second screenshot: nothing changed (i.e., EAF still at 21248), but swapped in a filter holder with a Baader Semi-APO that is supposed to be 2mm thick. The backfocus presumably changed by 0.66mm. :

Third screenshot: still with Baader filter, refocused to center Bahtinov spike. EAF now reads 21503:

So, I see a 255 EAF step change for the 2mm filter, presumably 667 µm change, or 2.61 µm per EAF step (my FSQ-85 has 4.2µm per EAF step).

I will go enjoy looking at backfocus now, after I test the Astronomik L1's EAF delta and the Radian narrowband filter EAF delta (that is where you really need the Kase Bahtinov mask). Those two are suppose to be about 1mm and 3mm respectively, but I want to measure it than guessing it.

I can then refocus instantly (perhaps just refocus for no-filter case when temperature changes) when I change filters. Don't even need the Bahtinov mask anymore once I get the Astronomik and the Radian EAF deltas.

Painless! Even with just a filter simple drawer -- with an EFW, I won't even need to go outdoors.

Kevin, re etched glass Bahtinov masks, see why I like them for their cleanliness? The Bahtinov core does not bloat in brightness, and there are no extraneous splikes. Especially when compared to the 3D printed crap. The R, G and B chroma channels overlap perfectly. You get what you pay for, and it pays off with tight stars (if the OTA is up to it -- the darn Rokinon is pretty darn good, from this Bahtinov image).

Chen

P.S. someone is lying about their glass thickness! The Astronomik L1 (claim 1mm glass) and the Baader Semi-APO (claim 2mm glass) are essentially parfocal! I can't focus any better with the Bahtinov (and we saw the huge difference between no-filter and the Baader). They have the same glass thicknesses. This hobby is full of charlatans.

P.P.S. I tried the Baader Neodymium (claimed 2mm thick) and it has a different focus than the Astronomik and the Semi-APO. So, the conclusion is Semi-APO has the same thickness as the Astronomik -- it is most certainly not 2mm thick.

P.P.P.S. The Baader Neodymium gave an EAF step of 21700. I then swapped in an Optolong UV/IR cut filter (claimed also 2mm thick glass)-- parfocal with the Neodymium. So much of a coincidence, so both Neodymium and Optolong UV/IR Cut are 2mm with very high probability. (No, I am not going to put calipers on a glass filter!)

L1/Semi APO delta from no-filter was 255. 2mm glass from no-filter is 452 EAF steps. Half of that is 226, not 255. So, the L1/Semi-APO is thicker than 1mm. (I have read somewhere that the Astronomic is 1.1mm, not 1.0mm, but no confirmation from Astronomics. )

If 2mm (since Optolong and Baader Neodymium (both claim 2mm), the 226 step per mm of glass (333 µm of backfocus) is probably somewhat reliable. This makes the EAF scale 1.47 µm per EAF step.

And if the Astronomik L1 is 1.1mm thich, it would be 1.1*226 EAF steps, or 249 EAF steps, which is pretty darn close to the focused Astronomik L1 of 255 steps.

So, my guess is Astronomik L1 is 1.1mm or so thick, and the Baader Semi-APO is the same (since they are parfocal). And Baader Neodymium and Optolong UV/IR cut are both parfocal at 2mm.

Whew. No more Baader filters for me. Their specs cannot be relied on.

It is amazing what you learn about this hobby, once you dig deeper.

Going forward, I will be using no-filter to represent a glass delta of 0mm, Astronomik L1 to represent a 1.1mm glass (actual EAF 255), and Optolong UV/IR cut to represent a 2mm glass (actual 452 EAF steps).

Now to see if I can actually measure the Radian, with its very narrowband passbands. If not, I have a Chroma 2" Luminance filter coming my way, and that too claims 3mm glass. But, at least for now, I have 0mm, 0.3663mm and 0.667mm for backfocus deltas.

P⁴.S. Wow, more inaccuracies. This is what a focused Radian Quadband looks like. Notice the EAF setting? 21436. Remember that no-filter EAF step was 21248? This makes the Radian just 188 EAF steps from the no filter case! It is not 3mm, it is not 2mm, it is not even 1mm. 1mm glass is about 226 EAF steps, so the Radian glass is barely 0.83mm thick! I rechecked the no-filter case, and the EAF remained at 21248, so no gear slippage (I use a 0.8MOD follow-focus gears, with the follow focus ring gear that is buit into the Cine version of the Samyang/Rokinon 135). Zero remained zero.

(Notice that Alioth's neighbors are Meridian flipped :-)

I await the Chroma to get 3mm glass....

Kevin, notice that the exposure time for the narrowband Radian remains 3 sec? Another advantage of these masks is how bright they are.

Chen

Kevin_A

w7ay I think Svbony is identical to Optolong uvir cut not zwo… graphs are not identical to zwo.

Kevin_A

w7ay I havent tried my SW 100ed yet, you must mean my Meade 115mm triplet that has lateral CA… orange one side and blue the other. But I am going to add a uvir cut in front from now on to test as you said regarding the 2600.

w7ay

Cloudbreak only has one of the Nikon ones left, and two of the EOS one left.

OK, Cloudbreak is now out of stock of the Nikon one, and down to one EOS one. Guess why? :-)

Whoops... got the wrong adapters (wrong filter wheel size. SHould be these:

https://cloudbreakoptics.com/products/eos-lens-adapter-for-2-filter-wheel
https://cloudbreakoptics.com/products/nikon-lens-adapter-for-2-filter-wheel

Looks like both of these needs the ASI2600 tilit plate to be removed to get the correct baseline backfocus.

Chen

Kevin_A

w7ay I too am a little OCD when it comes to anything touching a filter glass… heck, I cannot allow even dust to touch them for a minute. How I measured mine was to use a clear poly bag… the same bag my microfiber cloth came in. I measure the total poly thickness, put the filter in it and lightly close the caliper at the unused edge of the filter until it stops and then back it off. Voila…

w7ay

Kevin_A I think Svbony is identical to Optolong uvir cut not zwo… graphs are not identical to zwo.

I just ordered one from Amazon, to add to my UV/IR cut testing (washed my hands afterwards - yeech, never thought I would buy a single SVBONY product -- even lower end than ZWO). The collection of filters that I have gathered together now includes 2" filters from Chroma and Radian Quad, all the way down to ZWO and now SVBONY.

The claimed thickness of the SVBONY is similar to the ZWO's (1.85mm), and very different from Optolong's standard 2mm. If the SVBONY measures like the ZWO, it is also landfill material.

Not considering Optolong's crummy primary functions and the halos, etc from their narrowband filters, the ones I have looked at initailly Iincluding their UV/IR cut, L-Pro, L-Enhanced, L-eXtreme, and L-Ultra, all block UV and IR really well. Better than other brands when it comes to blocking UV and IR. The Astronomi L1 was dissapointing -- I had considered it earlier as my narrow-band "pre-filter," but will probably go with a Chroma luminence filter instead, now.

Chen

w7ay

Kevin_A I too am a little OCD when it comes to anything touching a filter glass… heck, I cannot allow even dust to touch them for a minute. How I measured mine was to use a clear poly bag…

These are boxes I use. The 2.76" x 2.76" x 0.67" fits the ZWO filter holders quite well:

https://www.amazon.com/gp/product/B08DY1ZTBF

The filter holders are labeled this way, to make them easy to find. Easy to see thru the plastic box:

The better filters are actually well coated. No problem washing them using lens paper and alcohol. No need to baby them. But using a pair of ordinary calipers on them is not my cup of tea. Perhaps I can look for the calipers that are meant to measure lenses.

Chen

Kevin_A

I was just rechecking my backfocus on my rigs and noticed that my zwo filter drawer for my big scopewas not painted flat black inside on the exposed threads compared to the Nikon filter drawer adapters I have that are nicely painted matte black… off to paint it now! Haha Ahhhh QC at its best!

w7ay

Kevin_A Ahhhh QC at its best!

Why is this news when it comes to ZWO? :-) :-)

BTW, after buying some EOS and Nikon couplers that can use filter drawers/filter wheels, I just figured out that there is no need for filter wheels and filter drawers, if I can measure the difference in EAF step from a Bahtinov focus between a star on the optical axis and a star on the corner. You do need a halfway decent electronic focuser. The high end QHY Q-focuser is probably an overkill, even. The ZWO EAF probably suffices.

You only need one filter with a known glass thickness! Together with a filter holder that is empty (no glass).

Why?

OK, take a look at the numbers from last night. Assuming that the EAF step sizes for no-filter and 2mm filter are accurate (fuggetabout ASIAIR; it simply is useless for anything to do with accurate focusing).

I had measured EAF steps of 21248 for no filter (glass thickness == 0), and 21700 for 2mm filter. That is a difference of 452 EAF steps (for my 0.8MOD geared EAF) for 2mm glass (or 0.667mm of mechanical backfocus). This comes to 1.476 µm per EAF step (0.001476 mm, but easier to think in terms of µm at these kinds of scale).

What we do is focus a bright star at the optical axis really well with no filter; note down its EAF step size. Now find a bright star at the corner (say 12mm away on an APS-C frame; an APS-C frame is about 28mm from one corner to the diagonally opposed corner, so corner is about 14mm from optical center).

Refocus on this bright corner star (this is the part I am not sure I can do with the ZWO EAF's sloppiness, will have to test during clear nights). This is where you want to use the best Bahtinov mask that money can buy.

If this is do-able, note down the EAF step difference between the center star and the corner star. Make sure to record the sign of the delta. This is the EAF delta for no-filter case.

Repeat the above with a 2mm filter. This is the delta for the 2mm filter case.

Now you have two EAF deltas, one for a 2mm filter, and one for no-filter. You can now interpolate (or extrapolate if the backfocus is more) for the equivalent filter thickness that produces an EAF delta of precisely 0.

Remember, the whole exercise of backfocus adjustments comes down to finding where the image plane is flat (i.e., no difference between focus of center star and focus of corner star). Forget about the "stars have radial pattern means backfocus is too short" nonsense -- we go back to the original mathematical definition, and not some hobbyist rule of thumb.

I.e., if the corner-to-center EAF is +100 steps for the no filter case, and the corner-to-center EAF is -200 steps for the 2mm filter case, using linear interpolation, the filter that produces +0 EAF steps would have a thickness of 0.333*2mm.

Go back to our scale above to now convert this to the number of µm of mechanical backfocus.

Done. Punkt. All with one filter thickness (plus empty filter holder), an accurate Bahtinov mask, and an electronic focuser that has no slippage and as small backlash as possible.

No need to add shims, rotate camera angles, reinspect the corner stars, and all that nonsense. Just use good old mathematical linear interpolation of what we see as field flatness of the corner stars. Just four measurements: EAF for focused center star with empty filter holder, EAF for focused corner star with empty filter holder, EAF for focused center star with 2mm filter, EAF for focused corner star with 2mm filter.

I'll bet the four refocusing and EAF measurements take less than 15 minutes, and just use a slide rule after that.

And it works especially well for camera lenses where you cannot insert any mechanical backfocus tools (most of them will rotate camera angle too, although there is at least one that does not).

You may have to make a couple of sets of measurements to average out the errors from ZWO EAF's sloppiness.

Chen

Kevin_A

w7ay the poly bag method works well. I use a better 5x7 poly bag meant for artwork that is 0.05mm thick. I used my old school metal callers and no marks ever. Easy peasy. The svbony and Zwo uvir cut filters do not look similar at all when you look at them… not the same that I have for sure.
Regarding EAF… I cant rely on zwo due to changing backlash. Mine are all between 20-30 steps of backlash but change randomly +/- 10 steps when changing each different direction… so I set my backlash compensation bigger at 30 steps to eliminate it. I wish it were less as I do not find zwo EAFs very accurate, especially on petzval scopes since they have a smaller focus range compared to my regular typical scopes.

Kevin_A

w7ay mine are similarly stored but I do not label them as I have just a few filters and I shoot mostly just uvir cut and narrowband with l-enhance filter only. I hate loss of true star colour.

w7ay

Kevin_A I used my old school metal callers and no marks ever.

I ordered a +/- .001mm "digital thickness gauge" from Amazon. My plan is to use lens paper on each side of the glass. I can zero out the thickness of the lens paper (I think; it would be silly if they don't have a way to zero calibrate). Considering a single Chroma luminance filter cost 10 times the cost of the whole digital caliper, I don't even mind hacking the caliper to make it work better (like buying a lens caliper and removing the tips from it to glue on to the digital gauge :-).

The same brand's 0.01mm tolerance gauge is only a little more than half the cost (they probably just sorted the same product off the production line, and most of them fell into the 0.01mm bin), but heck, you only live once.

Skies are really deep blue. Think I will be able to test the "one filter glass" method tonight. If it works, this could reduce measuring backfocus to something a grade school pupil can do -- I will definitely write it up in a web page if it works. Just need a single removable 2mm filter, and an electronic focuser to measure the amount of off-flatness for no-filter and 2mm filter cases (or x-mm, but most cheap filters like Optolongs and Antlias that you can buy are 2mm). I will be using the Optolong UV-IR filter tonight. The magnetic carrier for the Kase Bahtinov mask is already mounted on the Rokinon (that is probably the best part of the Kase!). Interesting thing is that all four measurements of this backfocus measurement method involve the use of a Bahtinov mask. I don't think any auto focus program out there allows you to select a star (center and corner) to use for focusing, and something like ASIAIR is so sloppy at finding focus that you can't use its auto focus tool even normally (if you want well focused stars).

Dubhe is behind those foilage :-)

Heck, if I don't need to change shims to adjust backfocus, I will only need my Rokinon EAF clutch only when I want to switch camera angles between long-edge-declinaton or long-edge-RA (I don't belive in other angles; those are for artists, not scientists).

By the way, as long as you are willing to measure multiple sets of numbers and average them, I think the EAF might work -- especially if you always first set the EAF to some large (1000 steps is sufficient, if your backlash is less than 100 steps) positive offset, and only move down from that offset. If you always do this, you do not have to worry about backlash (it is cleared when you move sufficiently down from that large offset), and all you need to worry about is stepper motor non-linearity or slippage. Just like adjusting alt-az bolts during polar alignment, never go back and forth to zero out the Bahtinov spike. Just move in one direction, slowly.

Chen

Kevin_A

w7ay I agree about the EAF… no backlash in one direction… that is why the Asiair automatic AF is not great as it goes back n forth and a lot of times I double-check with a mask.

w7ay

Kevin_A Asiair automatic AF is not great as it goes back n forth and a lot of times I double-check with a mask.

It has bigger problems than that. It is for example using a general conic section (creating a U curve, not V curve). To get a V curve, they need to contrain the equations to use only the hypobola part of a conic section.

The V curve is the mathematical model for geometric optics (the optics that you learn in high school , without diffraction limited terms). The U curve models.... nothing.

Every time I see a complain about ASIAIR autofocusing, I always tell them to use a Bahtinov mask. Buy a good one that can be adapted for different OTA, and you are set for life.

After playing with the cheap 3D printed crap, I came to the conclusion that they are not designed by someone who understands Fourier Optics. The masks used to be a bit more expensive because they were laser cut; some of the large ones still are. But now lots of people are using the 3D printed ones just because they are cheaper.

BTW, I still have to test if that smaller lens hood I have on the Rokinon makes calibration flats less of a problem, without having to use a gigantic light source (such as using the whole sky, as in sky flats :-). The 40mm lens would be much, much worse, so I better find the fix now, with the 135mm lens.

Chen

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