Kevin_A if it were tilt I would expect different elongations but what I am seeing is very bad long fuzzy comet coma in two kitty corners and not just on one side or one corner.

I was also getting terrible fuzzy (coma filled) stars on some corners before I started to focus [sic] on getting tilt adjusted to 0.02-ish mm.

For a quick look, take the lens out of focus so you get large Airy dics (HFD perhaps over 15). Are the corner disks all circular? If not, it could be a tilt problem. If none of the corners is circuar, it is a backfocus problem. So, at least get the backfocus adjusted first so that at least one corner has a circular disk.

If the non-circular disks turn circular when you change backfocus, it is a tilt problem. I.e., as you change backfocus, the Airy disks of different corners become circular. This is just a 1 minute exercise if you have the Askar backfocus adjuster.

BTW, the 40mm Sigma is not perfect. When tilt, backfocus and focus are adjusted properly (so that the corner stars are tack sharp), you can see a small red spot deviate (x-y) from the "white" spot. Tiny amount, perhaps less than a pixel or two, but it is visible. You can see this in some of the Takahashi spot diagrams, by the way.

When slightly defocused, this looks like coma, but it is just a short red tail. Not 100% APO from red to blue.

I have decided to just use the tilt screws to create a +/- 0.05mm offset for backfocus, by the way (use the Blue Fireball metal shims to get down to 0.1mm). I has considered using a 54mm stop plate, but am afraid the stop plate will itself introduce 0.02mm type errors.

However, I will create a larger tilt plate, so that the screws will clear the 90mm body of the camera. I.e. place the tilt screws 90/2mm + 1.5mm from the center of the tilt plate. A 95mm or 96mm plate should work. Then stick in 4 (non-ball head) long 2mm L hex wrenches into the pull screws, and strop the four to the body of the camera. The ball head hex wrenches do not provide very precise angles, clearing the camera body (by half the diameter of the wrench -- that 1.5mm) will allow the use of non-ball headed wrenches.

I may have to buy an additional tilt plate as I want to keep the 2600 as is as it is fine on all my other scopes.

If you do, get the new one, which is a pair of plates. This lets you adjust from the back of the camera (like what I am planning for my next set of tilt plates). You still end up with 5mm backfocus consumption, same as the original.

https://www.zwoastro.com/product/m54-tilter-2/

Like I said, I think they send the crap to Canada as my last Rokinon buy took 3 copies to get a decent one so I am not surprised by another sub par copy.

You may just have had 2 copies with a bad tilt. Not bad glass. If so, you should be able to correct by just correcting tilt. I am not kidding when I say that the backfocus is sensitive to even 0.025mm correction.

Now that I understand the problem better, I m going to one day play with the Sigma 135/1.4 ART again. I think now that it was just a tilt problem.

I don't think it is a Canadian problem. Heck, my Samyang and Rokinon came from some Amazon third party that sells candy, even though the web page says ships and sold by Amazon. The Sigma came from a NY electronics store that sells compuer games.

Chen

Kevin_A

Oh, BTW, I can confirm that the red glitch that I saw when I used a Hoya UI/IR cut and a didymium filter is not caused by the Hoya, but by this particular didymium filter:

https://www.amazon.com/gp/product/B07NP774RH/

The didymium filter from Maven (cheaper than above) produced no red globs and glitches with the Hoya:

https://mavenfilters.com/product/night-sky-filter/

I then tried a slightly cheaper Haida UV/IR cut filter than the Hoya with the Maven, and also no red blob.

So, I am back to using 82mm UV/IR cut and didymium filters now.

I did find a B&W 82mm filter that cuts both UV and IR. I may get it if I play more with wide angle lenses. B&W are my go-to filters for terrestial photography. Using a Chinese Haida filter on a Canon lens is like putting an SVBONY UV/IR cut filter on an Takahashi. In both cases, there is no point screwing up thousands bucks worth of optics by saving a few bucks on the filter.

Chen

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

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 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

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 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

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