Kevin_A Remember I said my flats had a weird circle in the centre with the stepdown

No go on comparing spectra from EL anel and LED panel... the Gerd Neumann is too dim to even bump the graph slighty.

However, this is the spectrum of the LED panel (pretty much like white LED and inreasingly more common, light pollution from LED street lamps):

Notice the dip around 500nm -- good for street lamp pollution near OIII, but not good for flat frames :-).

You can see the same shape in IDAS's plot (the NGS-1/LPS-D3 is supposed to be designed with LED pollution in mind;cutting off that very stong peak at around 460nm). Very light cyan curve superimposed on the filter curve:

FWIW, this is a 150 watt (blindingly bright) incandescent Halogen bulb (in E26 base):

And this is the halogen passed through an Antlia ALP-T's OIII line (horizontal scale stretched):

No Gerd Neumann panel spectrum, unfortunately. The input of the spectrometer is a tiny fiber optic aperture.

Chen

Kevin_A The main reason I went for the Esprit was that it has lots of reviews comparing it to the Askar 107PHQ and being better overall and that was the size of scope I wanted…

For that weight class, I would have gone with the FSQ-106. And no need to worry about upgrading anymore after that. Save money on the long run.

I am still looking at building low, short bases (tripods, tripiers, whatever) that might let me use an FSQ-106. There is no way I can lift a full sized Q up to the mounting plate of my current setup (800mm W.O. Mortar, not even the 1000mm one).

Chen

Kevin_A I hear you about the need to move the lens and remove the belt… a pain.

OK, quick test...

Eppur si movre :-).

http://www.w7ay.net/site/Images/Rokinon%20M0.8%2038T.mov

The Rokinon Cine comes with the 0.8 MOD ring gear (for both focuser and aperture control). Here you can see the EAF driving a standard M0.8 38T pinion gear.

The EAF is mounted to one of the Astrodymium rings (some unused old plates I'd made up for various OTAs); a quick hack. Once I test enough to know that it works resonably well, I will have a rod from the gear all the way to the front Astrodymium ring to a flange bearing -- that will give it a more solid attachment.

With the particular angle the EAF is mounted on the Astrodymium ring, when the ring is completely loosened, the gear lifts from the focus ring, and the lens is now free to rotate (if the second Astrodymium ring is slightly loosened).

I should also be able to "hinge" the EAF plate (nice to have a web machine shop nearby :-), so that it can be lifted without even lossening any of the Astrodymium rings. This will allow fast manual focus and also allow the Astrodymium to be just loose enough to change camera angle.

OK, problem over... no more tearing hair out in the dark trying to reattach belts. Or when adjusting shims for backfocus.

By the way, you can get rubber follow-focus M0.8 ring gears that fits over regular lenses, so you can play this trick with the non-Cine verion of the Samyang, too.

https://tilta.com/shop/seamless-focus-gear-ring/

Chen

Kevin_A

Bingo! as they say....

I decided to be adventurous, and used the 55mm Tiffen as an aperture mask on the Rokinon 135 (creating a 50mm astrograph).

Shock of my life when I saw the iron cross (4 notches in the halo of the star) on just a not very bright star, Alphecca (M 2.22) about perhaps 8mm away from the optical center, with 180 second, gain 100 exposure on the ASI6200 (used in APS-C ROI).

By the way, after all these years, this is the first time I encounted an iron cross. Usually, I see aperture vignetting, and that causes only two notches, instead of 4.

Thinking it is a 55mm lens hood that I use, I removed the hood, went back to the large Rokinon hood (same as Samyang), and the iron cross was still there.

I then went to my alternative: a 40mm astrograph equivalent -- using a 46mm Tiffen as the aperture mask.

Guess what? No more iron cross! Same star, same location, same bat channel.

So, either there is some flaw in the Rokinon that gets suppressed by stopping down, or the 55mm Tiffen itself is causing the 4-notch diffraction pattern.

If you want the iron cross gone with your Samyang, try using a 46mm Tiffen aperture mask -- 40.4mm true aperture, thus f/3.3.

The difference from a 55mm Tiffen is not horribly huge... f/3.3 vs f/2.7, or just a tad over 1/2 f-stop. (factor of 1.22; 1/2 stop would have been 1.19 [i.e., sqrt( sqrt( 2 ) )].

Looks like my usual "stop down by 1.5 f-stop" holds for the Rokinon too.

Chen

Kevin_A

Looks like f/3.3 is still too wide for the Rokinon 135mm.

This is one 180-second capture of Corona Borealis, APS-C frame, at f/3.3. Alphecca (CB alpha) is the brightest star. North is bottom of frame.

At the same location, Alphecca had exhibited an iron cross at f/2.7 ("50mm astrograph"), but as seen here in an expanded scale, no iron cross at f/3.3:

However, Coronae Borealis Beta (star close to left hand edge of the first image) at the same scale, still shows an iron cross:

I will probably try and go one more step down (with the Tiffen masks that I have) to 40.5mm Tiffen (35mm clear aperture, and equivalent of f/3.85 lens). If that still shows an iron cross, the next step is a 39mm glassless filter with 33.7mm clear aperture, giving a f/4.0.

Other than having to slow the lens down, the darn lens is still very nice (f/4 is still quite fast; the Baby Q with its 0.73x reducer gives 330mm focal length and f/3.9).

Chen

Kevin_A

OK, even at f/4.0 (39mm Tiffen filter with glass removed), the Rokinon 135 shows the iron cross inside an APS-C frame.

I start seeing iron cross when the star (albeit, bright star) is 4.2º from the optical axis.

Now, this is significantly better than at f/2.7, where the iron cross starts appearing at just 1.5º from the optical axis.

Note: the corner of an APS-C is about 6.4º from the optical axis. The long axis is 3.54º from the optical center, and the short axis is 5.3º away from the center.

Chen

Kevin_A

Looks like the iron cross is coming from the glassless filter.

Here is the typical iron cross from a step-down ring + glassless filter:

The next image is with the filter removed, leaving only the step-down ring as the aperture:

This latter image is more random -- probably caused by the rougher threads of the step down ring. This is what the smooth retaining ring in the glassless filter is trying to solve.

I am beginning to suspect that the filter body is too thick. I.e., if you hold up the filter and tilt it even slightly you can see irregular shapes. Perhaps what we need is some aperture that is made from a thin piece of metal.

When it is daytime, I am going to try to see if I can epoxy just the retaining ring of a filter to a step down ring. I don't think we will be lucky enough that the retaining ring will thread into some step down ring.

Another possibility is one of the metal spacers (like from Blue Fireball)

https://agenaastro.com/blue-fireball-9-pc-fine-tuning-spacer-ring-set-for-m54-threads-0-1-to-1-0-mm-s-set9.html

I think I have ones for 54mm, 48mm and 42mm. They are larger than 54mm, 48mm and 42mm, of course. But the "42mm" may be a reasonable aperture, and good enough for testing purposes. The shims are very thin (the thinnest is 0.1mm).

Centering the shims and the retaining ring may be hard to do. But, from the Fourier Transform, a slightly shifted aperture should not cause an iron cross. Diffraction patterns (like 18-way spikes and iron crosses) are caused by an aperture that is not perfectly circular.

Chen

Chen