First Light with new ASI585MC Pro.: Page 23

First Light with new ASI585MC Pro.

w7ay

Kevin_A

Another by the way, the Sigma 40mm has very little light falloff over an APS-C frame at the f/2.4 that I am using.

Looking at a flat capture (cooled to +15ºC), I see Max ADU of 37600 and a min ADU of 31700 with mean ADU at 35400.

I have turned gain to -2.5 dB to get more dynamic range. Heck, I can get almost one more bit of dynamic range by using a longer exposure than 0.2 sec.

Flat dark has Min ADU of 400-ish, and max ADU of around 1000.

Chen

w7ay

Kevin_A

Hmmm, B&W UV-IR cut may cut too much IR for astrophotography... H-alpha at 680 nm is attenuated by quite a bit:

https://schneiderkreuznach.com/en/photo-optics/b-w-filters/filtertypes/protection/uv-ir-blocking

Chen

Kevin_A

w7ay I did try f4 images with the new lens and it did not improve. I might just get the old Rokinon adjusted to the point it is ok and then play with my Esprit 100 and a Starizona Apex ED L refucer that turns it from a f5.5 to a f3.6… maybe!

w7ay

Kevin_A

Hmm, I think Siril's HFD/FWHM is confused by background gradient. It is showing tilt when visual tilt is quite small. I need to look more into this.

This is what I now have with the aberration inspector with the Sigma 40 at f/2.4 (rotated 180 degrees). Image is 17 subframes of 180 second exposures at 10 dB gain; scaled down for JPEG to fit forum, no sharpening in post processing, so what you see are the real stars. Slight contrast change using macos Preview:

Sadr is at center. Almost the entire Northen Cross part of Cygnus is in frame. Albeiro is just barely outside the bottom edge. Camera angle not perfectly north-south since I can't include the camera angle adjuster.

I might just get the old Rokinon adjusted to the point it is ok

Yeah, I am going to leave the camera lenses for perhaps a week too, to get the large tilt plate made before I do more on the Sigma 40 and Samyang 135. Otherwise, I Iwill end up doing it over again.

Chen

Kevin_A

w7ay that image my friend is actually very good and flat especially considering the aperture. It is well balanced and stars are tight. I wish my copy had been half as good. I know with mine that I can only fix certain optical issues but not multiple issues with decentering, coma and Other CA. I just want a copy I can work with. Haha

Kevin_A

w7ay i can even see a faint Crescent nebula too.

Kevin_A

w7ay did you use any filters?

w7ay

Kevin_A that image my friend is actually very good and flat especially considering the aperture.

Thanks; I am pretty happy too, considering all I did was to convert away from bayonet and adding a tilt adjuster to the lens.

I am almost tempted to open it to f/2, but I not tempted enough :-). I like sharp pinpoint stars :-).

I should be able to do a bit better once I have the larger tilt plate (easier to get to all 4 push-pull screw sets -- right now one set is under the camera are almost impossible to get to accurately).

This is probably the best Milky Way that I have been able to get from this location. Bortle is probably around 6 where Cygnus is, and not being able to wait for ideal darkness through the small opening in the trees. I may even have picked up a sporadic meteor:

i can even see a faint Crescent nebula too.

Yes, quite a lot of stuff with the wide field. Cygnus Loop is a little disappointing though.

did you use any filters?

Yes, the 82mm Hoya UV/IR filter stacked on a 82mm Maven Night FIlter (pretty generic didynium glass). A lot of hydrogen alpha came through. The didynium is probably still slightly effective against little city lights, but it won't be long before they are useless when the sodium lights disappear. I will do a on-off test one of these days.

Back in 2015, I had pointed the telescope downhill to one of the Portland suburbs (Hillsboro) and got images with no-filter, IDAS HEUIB-II and IDAS LPS-D1. This is a composite:

Once you see this, you will never go filterless again :-) :-).

To get decent colors on a ZWO camera, B&W UV/IR may actually work OK at 656nm to be usable. I may give that a shot one day. But for now, both Hoya and Haida UV/IR cut filters appear to work; I haven't seen anything obviously bad yet. But I trust Japanese glass infinitely more than Chinese glass, and German glass even more (just not Astronomik); so Hoya it is for now.

Chen

w7ay

Kevin_A a

Kevin, remember that I was going to move the ZWO filter drawer (EFW has same thickness, so if drawer works, so will 2" EFW) so that is is on the OTA side of the Takahashi 1.01x flattener?

Well, here is the result without adjusting a thing (I have the backfocus adjuster in there just in case, but had set the backfocus to match the 18mm of the backfocus adjuster). I am using standard spacers and a PreciseParts that is already on hand to get the 56.2mm backfocus of the flattener

The above is 0 dB gain and 180 seconds, to bring out the dimmer stars with Alioth at center. Plate solved focal length is 458mm.

Alioth registers an ADU of 44K with 0.01 sec exposure, and this is 180 seconds, so equivalent to Alioth with an ADU of about 8 million :-).

With 0.1 second, and Alioth moved to right next to a short edge of the image frame, it looks like this. No aperture vignetting (the two notches) in sight.

An IDAS HEUIB is in the filter drawer.

What this means is that filter drawer location (with some M72 adapters) works fine. And when I change filters, I can now simply refocus when I change filter without having to adjust backfocus, since there is no glass change between the flattener and the sensor.

Because the field is flat enough, I had no problem with ASIAIR's "autofocus."

By the way, this is the same ASI2600 that I used on the Sigma 40 earlier. So, the tilt earlier was not caused by the camera but by the Sigma 40 train.

This is f/4,5, by the way (only a little slower than my Pleiades 68, which measured an f/4.0, instead of the f/3.8 lie.)

Chen

Psst: with all the government agencies (even local government) requiring smart phones to do business with, I finally broke down and switched to the cheapest and smallest iPhone (SE).

Kevin_A

w7ay looks good! I have never been one to start using tilt plates on camera as you are guessing that it is a sensor tilt issue rather than optical issue. That is why I would rather return a lens than try to correct one that has multiple optical issues which makes me believe it will never be anything but subpar. A tilt device between the two parts would be a better solution leaving both untouched and fixing or identifying the issue independently. So you finally broke down and are part of the smart phone/dumb generation users now… haha. It is unavoidable as technology starts cutting you off from being low tech!

Kevin_A

w7ay i just started using Siril for aberration inspection and PSF. Is it safe to say that for PSF, if all 4 outer numbers are close to the same size and the centre number is a bit different… then the backfocus is slightly off and no tilt?

w7ay

Kevin_A if all 4 outer numbers are close to the same size and the centre number is a bit different… then the backfocus is slightly off and no tilt?

It could be that, but it is probably as likely that the glass itself has different spot sizes between the center and nearer the corners. Take the FRA300 for example:

The IMA 13.914mm square is pretty much a corner star on an APS-C from (28mm diagonal, thus half diagonal of 14mm), Notice that there is a size difference from the star at the optical axis. However, notice also the displacement of the red spot that I had mentioned before, the light rays don't converged on the same spot, and that can also cause HFD numbers to bloat.

I think all you can do is to first adjust the tilt away, then tune the backfocus for the most "pleasing" result, since the HFD bloat could be caused by this non-APO characteristics, and it might be more "pleasing" to accept a bit more actual bloat and less red/blue displacements.

I have not tried using a red filter for example, when adjusting backfocus and tilt, so as not to conflate the two HFD problems, and perhaps even use a monochrome camera. I do have some Wratten gel filters that I can use with the 100mm x 100mm filter folders. It might be worth experimenting with them, and there are "red" filters in 82mm glass form. For 2" filter, just use the red filter from a 2" LRGB set -- I have a mono ASI2600 with a 2" EFW mounted. However, the images I took last night on the Baby-Q does not seem to have too much of the red-blue displacement, though. But the Sigma-40 has plenty.

In any case, I have found that it is much better to first get rid of tilt, and only then play with different backfocus distances.

Chen

Kevin_A

w7ay i agree with that whole statement. I noticed that my Tamron 35 was tightest with a blue tinge around the stars while my Rokinon was a red tinge. I was going to try my Rokinon again but focus to where the stars look most pleasing and see how the image looks. My FRA300 tends to be also best when on red too but looks fine when I adjust it slightly off towards neutral.

Kevin_A

w7ay i checked my tilt with my original Rokinon vs new on the same camera and tilt was low for old and moderate for new… but abberation inspector showed round stars on old and lots of coma and uneven stars in every corner… so… nope! Haha

w7ay

Kevin_A

Remember the noise table that I estimated for the ASI585?

After initial test of placing the filter drawer ahead of the FSQ flattener with the ASI2600, I tried a few exposures of M27 with the cooled ASI585MC. Based on my table, I chose HGC (25.2 dB) gain and 30 second exposures; cooled to my usual -10ºC. This is 35 subframes (about 15 minutes) before the remaining frames got clouded over:

No bias, no darks, no flats. 457mm focal length.

My table shows that 20 seconds may be even lower noise than 30 seconds, but this does not look too dirty already. And I am not used to short exposures :-).

That small central star in M27 makes using small pixels on the FSQ worthwhile :-).

My ancient IDAS HEUIB-II filter. Next time, I will check out the IDAS NBZ, which should have better SNR (but may have to wait a few days for clouds to clear). I also want to see what the repositioned filter location does to the light halo from the NBZ.

So, HGC + short exposures is the trick. HCG brings down the read noise, and short exposures bring the dark current noise back down. 25.2 dB is a whopping linear factor of 18x!! They should have switched to HGC much earlier to keep the dark current noise (the real weakness of this sensor) low. I'll need to buy a QHY version when they come out with a cooled version of the IMX585.

No spikes, no iron crosses, no notches. So far, I am very happy with this arrangement. With the ZWO filter drawer in between the flattener and the camera, I was getting notches from aperture vignetting. No more. Stars look clean, and the HEUIB-II shows pretty good star colors.

Chen

Kevin_A

w7ay that is my feeling regarding the 585! You either use 10dB and lots of 60-120s subs or 30-45s at 25dB and a huge amount of cleaner subs but lots of HD space. That way you keep the dark current low and do not oversaturate the FWC. HCG should have been at 15dB.

w7ay

Kevin_A HCG should have been at 15dB.

Even that would only allow 60-second type exposures before the dark current noise is equal to the read noise. This is just a poor sensor for DSO. Too bad, since the pixel size is closer to what I need.

Unfortunately, people who are not thinking tend to buy based on price alone, and end up spending more by having to replace it with a real DSO camera later. One can only hope that that next camera is not from ZWO.

I have reasonable sized Samsung T9s connected to my astro processors; and the IMX585 sensor is so small anyway. Plus, I chose the Ultra version (20 cores) of the Mac Studio when I bought this desktop; so no problem with how long it takes, either. Short exposures are fine with me; I am just a creature of habit, and used to the CCD type read noise when it comes to choosing exposure times :-). The proper thing to do is to forget the past, just balance the read noise vs dark current noise to minimize the total RMS camera noise. For this camera, short exposures.

When it comes to read noise vs dark current noise, this camera is almost the complete opposite of CCD cameras.

Problem is going to come with narrowband filters, where it might take a long exposure to bring the signal up to 10 dB or more over the camera noise. I'll find out when the sky clears.

Chen

Kevin_A

w7ay I am not planning on using this camera for narrowband. I purchased it solely for tiny faint galaxies using my bigger scopes. I hated 90mb files and cropping 80% away! The smaller pixel size was the main feature but damn that tiny FWC and dark current is making it hard to don anything other than broadband. I have an i9 processor 16 core so I am fine with processing but maybe I might need to add a few more terabyte drives! Hahhaha

w7ay

@"Kevin_A"

Hah! I just figured out how to very easily install a fixed iris on the Samyang/Rikonon 135!

There is no need to remove anything but the front glass group, and that involves a simple lens spanner without teeny tiny screw, nor deal with fragile cables. And it is completely independent of whether the Samyang lens has a EOS, Nikon or Sony mount.

Then apply some glue to the iris mount just as if the iris is outside the barrel, and drop the fixed iris in place.

Basically, glue the top piece to the bottom piece in this photo, with the bottom piece not removed from the lens barrel:

If you look down the lens from the front glass of your current lens, you should be able to see the bottom piece of that photo. The top piece comes from the machine shop.

I am going to fabricate a new fixed iris, but this time, not adding the ZWO fisheye retaining rings, but just get the machine shop to create that small aperture directly. The mitered hole that they mill appears to be smooth enough.

The machined plate is 1.5mm thick, but even that can cause some aperture vignetting for large angles from the optical axis. So I have them miter the hole down to an almost knife edge (0.2mm thick).

So... ta da... I can try this on the Rikonon Cine 135/T2.2, which so far has not has the crap at the back removed. That will prove if I can glue a new fixed iris without removing anything but the front glass. I will still need to remove the bayonet plate and replace with M54 and tilt plates to be able to actually use it; but that is independent of the iris.

Here is an offer you can't refuse... if you want to use a fixed f/3.5 (my -1.5 f-stop rule :-) to get rid of the diffraction spikes, I will get two copies of iris plates machined, and mail one to you (gratis, of course, it is just a couple of dozen bucks). Be forewarned that once you glue it in place, you can only use something slower than f/3.5 from then on. Or use double stick scotch tape, and hope it does not fall off :-)

Or (since there is no volume discount other than shipping from the machine shop anyway), tell me what f-stop you prefer, and I will create a second drawing for them. It might be a week before I get to this, so you can sleep on it for a few days. (I will need your street address; send it by email to me at w7ay_at_comcast.net.)

Don't want spikes? It looks like a 3 minute operation that even the unwashed can do, as long as they have a lens spanner (metal crazy glue does take 30 seconds to set, and 24 hours to cure) -- I can believe that some of the SeeStar types have never seen a lens spanner.

I had used some M42 tubes with some weight on it to apply pressure on the plate when the glue was curing; I think you can still do it with the iris assembly in the Samyang barrel as long as you secure the barrel vertically.

This is a clearer view of the iris assembly. The machined fixed iris is glued to that flat annulus around the Samyang iris.

Chen

Kevin_A

w7ay sorry, my wife was asking me a million questions this morning while I was looking over your posts and waiting for the coffee to finish brewing. I almost missed the drawings until I re-read everything again. Do you plan on having the chamfer face forward when you glue it in place? It looks like that would be the logical choice! I decided to go and do some landscaping and gardening to lessen the noise and clear my head! Haha

w7ay

@"Kevin_A"

Looks like it will work. This is looking down the lens barrel of the Rikonon Cine 135/T2.2 with the front lens group removed:

Completely clean path to the iris assembly.

I think I might create an f/2.8 iris for the Rikonon. The Samyang currently has the f/3.3 iris (with the ZWO fisheye retaining ring determining the opening size). A different f-stop should be more useful than a second f/3.3.

Chen

Kevin_A

w7ay thank you for the offer my friend… let me think on it as I want to test out this lens again first to see if the results are acceptable when I focus to a neutral instead of red. If it is not acceptable then i am not sure a modification would be worth it. I would rather use a lens that had good overall stars to begin with. I will test soon as the skies permit and I may try getting another 135 to try again. I do appreciate the offer truly and everything tells me that I am a f2.8 guy when it comes to this lens because it is a fast lens… so for me it still needs to be used fast. Haha So if you plan to get 2, get an f2.8 and if things work out on my end I might just take you up on your generous offer.
If you want to contact me through another avenue my email is kevin_agar_at_hotmail.com

Just out of curiosity… i wonder if the fixed iris will need to be painted the same non reflecting matte black as the mechanical iris? Maybe the angle of light at that section has no effect. I imagine it is a high quality of paint that would be required to not bounce any stray light upon any front or rear facing close elements due to being a thicker structure. Speaking of which, how thick is your fixed iris? (Nevermind I missed the 1.5mm w/0.2mm flat… no coffee yet) I would expect a very thin non reflective paint has to be used too… just more food for thought. A spray can may be too thick causing an uneven blobby edge on the ledge… an airbrush like the one I use for art may offer a finer atomization and thinner more precise finish. If you do spray it then shake that paint up good and very light coats. Haha Back in the day when I was 20ish I started a career as a lighting designer. I designed one of the first commercial compact fluorescent pot lights using PL bulbs in the market. I studied a bit with light bounce, reflectivity and baffling. It was interesting how light can be manipulated and how it can give unintended issues! Iris blades are so thin they have no ledges to reflect light. Yours will be 0.2mm so not too much of a ledge. It may or may not have any effect… I just have a knack of asking and pondering all variables and differences whether valid on not. I am the proverbial person who asks questions mnobody is thinking about that may or may not be relevant.. but may 10% of the time be worth it that it was thought about! Haha Cheers my friend!

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