First Light with new ASI585MC Pro.: Page 5

First Light with new ASI585MC Pro.

w7ay

Kevin_A is there a good brand of Electroluminescent light panels?

I use the Gerd Neumann Auroras (two sizes, one for everything, and a larger one for >5" OTA tubes). Works pretty well. DHL gets here just as fast as UPS :-). Just be sure to get the 12 V ones instead of the 220V ones.

I top it off also with the Daystar FlatCaps to further diffuse the light -- I have one for the Baby-Q and one for the FOA-60Q. Definitely not neccessary, the panel is way out of focus already, but I am OCD :-).

Chen

w7ay

Kevin_A

Kevin, I found a good combo to get f/4 from the Samyang/Rokinon.

The cheap Tiffen 39mm filter with the glass removed has a clear aperture of 33.7mm, which givesreally close to f/4 for 135mm.

https://www.amazon.com/gp/product/B0BBQ4GM9C

So, step down the 77mm filter thread to 39mm thread, and insert the glassless "filter." The aperture appears quite clean and round.

Since getting my Rokinon, I have increased my existing collection of step-down/clear-filter combos to these:

The 37mm and 39mm needed two steps of step-down, since I could not find any 77mm-39mm and 77mm-37mm step-down rings. So I chose 55mm as the intermediate; 77mm-55mm is one of the existing step-down rings.

Darn lens should be clear as a bell at f/4. (Sorry for mixed metaphores. :-)

I am always nervous about placing an iris at some place other than a nodal point of the lens. But so far have not found that it increased the vignetting. And using a glassless filter gives a very clean aperture with no diffraction spikes. I have not seen anyone else mentioned using apertures from camera filters, by the way, but just using step-down rings themselves, which do not have a clean circular aperture due to the filter thread. $10 to get rid of nuisance diffraction seems worth it.

Chen

Kevin_A

w7ay I cannot wait to see the results… you have peaked my curiosity!

w7ay

Kevin_A I cannot wait to see the results… you have peaked my curiosity!

Here you go...

halfway clear enough skies so the diffraction spikes don't get smeared by the clouds. Just out of astronomical twilight. I used Alioth (magnitude 1.75, and far from the full Moon) with an ASI6200MC set to an APS-C ROI. Gain of 10 dB (HCG), and 60 seconds, and cooled to -10ºC, with ASIAIR. like so:

I centered Alioth as well as I can, so there is no infuence by lens aberrations.

Here is a crop at 1:1 scale (1 camera pixel == 1 pixel in this image) with the lens set to T=4. Recall that my Rokinon is a cine version of the Samyang, so everything is calibrated in transmission values (wide open is T/2.2, so there is a loss of 10% through the glass elements -- should be the same with the photo version of the Samyang f/2.0). Also, to comply with cine folks' usage, the T ring does not have click stops, so I am just approximating by eye where T/4 is.

Next image is the T-stop set to wide open (T/2.2), and I added a 77-55mm step down ring, followed by a 55-39mm ring, with a (cheap) Tiffen 39mm UV filter that has the glass removed. I.e., I am just using the metal filter holder as an aperture. The clear aperture of the Tiffen's retaining ring is about 33.7mm, so that gives me the f/4 at 135mm (my Rokinon actually plate solves to 129mm focal length). Some other brand's 39mm filter may (and probably will) give a slightly different clear aperture.

Neat, eh? Practice agrees with theory. I don't know why no one has thought of cleaning up step-down ring filter threads this way.

The Tiffen UV filters are pretty cheap, and I chose it because all of the Tiffens' retaining ring that I played with seem to be loose enough to use a simple lens spanner -- there are some high end filters that really torqued down the retaining ring.

Be sure not to destroy the fragile retaining ring as you remove it (don't even try it, if you don't have a lens spanner; I assume all astrophotographers owns at least one lens spanner), since that has to be mounted back, without the glass, to form the actual circular aperture.

Chen

w7ay

Kevin_A

By the way, Kevin, since the Moon is too bright to do anything serious, I started to look at guiding the encoder version of the mount (RST-135E), and it looks like there is some improvement in guiding at 1 second than at 0.5 sec. ASIAIR's multistar centroid algorithm sucks, since with that many stars, longer exposures should not improve guiding so much.

Chen

Kevin_A

w7ay my Rokinon plate solves to 129mm too! Nice results. Can you post a photo of what it looks like without the lens hood on with the ring setup on? Thanks!
I still think I would do the setup at close to f2.8 for 2 reasons. I think it is like buying a fast car but only driving it slowly…. So I would still want it close to max speed but a bit better handling.
F4 is sharper but a bit more abberations than f2.8 and why buy a fast lens and use it 2 stops slower… 1 stop would be my setting, but that’s just me! Haha My lens spanner is toast as it was a cheap one so if you have a recommendation of a better design then let me know what you have been using.

w7ay

Kevin_A Can you post a photo of what it looks like without the lens hood on with the ring setup on?

OK, first the stepdown rings by themselves. This first image is for the case of a single stage step-down (77mm-40.5mm):

The large disc is the stepdown ring, and the protrusion is a 40.5 UV filter with the glass removed. Nice thing about camera filter sizes is the lens caps that are available; shown on the right of photo.

I could not find any 77mm-39mm stepdown ring, so the next photo shows a 55mm-39mm step down mounted on a 77mm-55mm stepdown, with a 39mm UV filter mounted on the 55-39mm ring, with its glass removed:

Now, the clear aperture of the 39mm filter is not 39mm of course. In the case of the Tiffen, it is 33.7mm (thus the f/4).

OK, finally, here is the ring mounted on a Rokinon. The camera band is from Astrodymium, and I had drilled and tapped the 3D material and also the body of a ZWO EAF to directly mount the EAF on the tube band. Notice my temperature probe hack on the EAF :-)

The lens hood works as normal (since photographers need a lens hood when they use filters),

By the way, did you notice something in the Alioth image? It completely lacks any diffraction spikes. This means there is no aperture vignetting at f/4, when done this way. If there is any vignetting, you will see two "negative diffraction spikes" around a star, as discussed here (last post there is the best one; I have a similar program for macos as the Maskulator that I had used to find the inverse filter for non-blind deconvolution; my program can actually also control the intra-focus and extra-focus positions too):

https://www.cloudynights.com/topic/547028-anyone-know-what-causes-this-artifact-on-bright-stars/

So, it is perfectly safe to do f/4 aperture control this way. I will need to test the other aperture for vignetting.

Again, here is the clear aperture (and 135mm f-number) for various step-down rings and Tiffen filters:

"Thread" is the stepped-down thread, i.e., "67mm" indicates a 77mm-67mm step-down ring. For an f/2.8, try either a 77mm-55mm step down or a 77mm-52mm step down, together with the 55mm or 52mm Tiffen UV filter. The nice thing about the cheap Tiffens is that they seem to make it for pretty much any size.

BTW, be careful when you buy from Amazon, that the ring is a step-down ring. A step-up ring (filter that is larger than camera lens) will not work.

Chen

w7ay

Kevin_A My lens spanner is toast as it was a cheap one so if you have a recommendation of a better design then let me know what you have been using.

My Amazon records say that I had bought this one in 2015 (no longer available). These tools last forever if you take care of them. Lots at Amazon with the same style, though.

https://www.amazon.com/gp/product/B00J5F6O92/

However, after mentioning lens spanner yesterday, I saw this one at Amazon:

https://www.amazon.com/gp/product/B0C4P5L9PC

Can save a lot of time with adjustment, so I could not resist, and ordered one :-). If it lasts 10 years too, it will be well worth the money.

Chen

w7ay

Kevin_A I still think I would do the setup at close to f2.8

I normally use between 1 and two stops down with camera lenses, but the Rokinon appears to be quite good one stop down from the initial images I took.

The MTF graph that you posted still shows better MTF (smaller stars) at f/4 though.

BTW, take a careful look at the two f/4 images of Alioth. Even the dimmer stars from the artifical aperture shows a tighter spot size. You cannot see the diffraction spikes directly (too small and dim), but the stars appear smaller and "rounder."

Chen

w7ay

Kevin_A

Ooops, sorry about the vignetting comment. Alioth was at the center of the frame, so of course no aperture vignetting.

So I still need to test vignetting by placing a bright star at the corner of the frame to look for those diffraction notches.

Chen

Kevin_A

w7ay they were the two spanners I was looking at as well. I noticed the lack of difraction spikes… that’s what interested me the most. At f4 mine were wicked so your stepdown design would work well with me.

w7ay

Kevin_A At f4 mine were wicked so your stepdown design would work well with me.

The step-down rings have been used as aperture control by camera-lens astrophotographers for since I could remember.

But I myself have never been happy that the actual optical aperture is created by the threads on the step down rings.

Adding a glassless filter as the aperture gets rid of that concern for just $10 (what in astrophotography cost less than $1000).

So, I did not come up with using step-down rings, just the addition of a glassless filetr to clean up the aperture.

You can try it without a glassless filter; there still might be small random diffraction spikes from the step-down thread, but it would still be better than that horrible diffraction spikes from the Rokinon's iris. The step-down by itself will produce an order of magnitude smaller (and random) spikes since the Rokinon only has 9 blades, I think -- an odd number of blades will cause twice (I think I counted 18) number of radial spikes.

Rokinon may have deliberately chosen an odd number of blades. There are twice as many spikes from an od--symmetry, but they are smaller than using an even number of iris blades like 8 or 10 blades.

Without a glassless filter, the aperture of say, a 77mm-52mm step down ring is larger, of course than the 47mm with the Tiffen filter. But the actual diameter (whether it is 52mm or not -- probably not) depends on how they machined it -- typically, an M6 screw is a tenny bit less than 6mm, for example, so I suspect a 77mm-52mm would produce a 52.3mm aperture that is slightly irregular (due to the threads).

Good luck, Kevin. May your diffraction spikes woes be over (there are people who actually add them during post-processing; imagine the mentality of such folks).

Chen

w7ay

Kevin_A they were the two spanners I was looking at as well.

Received the V spanner and it worked really well. Saves a lot of time adjusting the jaw size. I used it to remove the 46mm K&F filter, in fact (they really torqued the ring -- not sure I could have opened it with the parallel bar tool).

However, be aware that the V will only open so much (86mm filters). For 105mm filters, you still need to use the parallel bar one.

Chen

Kevin_A

w7ay thanks I will see where it leads me!
Yes some people add spikes and some people remove all the stars too…. Neither for me please and thank you. I prefer what the heavens look like the most and not a fan of some fake Hubble palettes either! Where did you get the 77 to 55 stepdown? I know Amazon has it as well as 77-52 and 77-49. Thanks n cheers mate!

Kevin_A

w7ay one question… actually 2. I take it that you leave the lens set wide open? And if I got a 77-52mm that should work well with my empty 52mm filter with 47mm clear apeture.

Kevin_A

w7ay I just ordered a 77-52mm step down ring as I have a 52mm filter without glass already and it will give me a f2.9 so in the ballpark of where I want to be. Thanks!

w7ay

Kevin_A Where did you get the 77 to 55 stepdown? I know Amazon has it as well as 77-52 and 77-49.

This is the one I got:

https://www.amazon.com/gp/product/B0BNN476PM

Be sure to pick the right one -- that page has lots of sizes. But I see you have one ordered already.

I take it that you leave the lens set wide open? And if I got a 77-52mm that should work well with my empty 52mm filter with 47mm clear apeture.

Yes, I leave the Rokinon at T2.2 (same opening as F/2.0 on a non-cine Samyang). Same with my other lenses.

You might be able to tweak to get better flare protection by using an F-number just a little larger than the artificiall aperture -- I don't know.

And if I got a 77-52mm that should work well with my empty 52mm filter with 47mm clear apeture.

Yes, that would give you something like an F/2.9, which might well be the sweetest spot for that lens. I am thinking the same myself.

I might enjoy this Rokinon 135 a lot. I don't have wide enough opening between trees to be able to use even 50mm focal length gainfully. During a meteor shower, I don't even try to track the sky with a 50mm -- I just point at any opening between the trees that I can find; usually at the Meridian :-). The Samyang XP50 that I have sucks though, at f/1.2. Not a good purchase, but I was tempted by the 1.2 number. Perhaps useful as a f/2.8 lens at best. But, for capturing meteors, you don't really care about star shapes :-)

Chen

w7ay

Kevin_A I take it that you leave the lens set wide open?

It is going to be cloudy and rainy for at least a week, so no star testing for vignetting.

So, I will have to spend some time with a (photo) camera pointed into the exit pupil of the lens and change the angle to see if there is any aperture vignetting at the max FOV of a full frame (and APS-C frame). It can be tricky since the EOS back focus is so short -- we'll see if it is too hard to do. This should also tell us if the true iris (presumably at the lens nodal) setting affects vignetting and if different apertures affect vignetting. Always go back to science, eh?

Chen

retman

Kevin_A

i too have a Rokinon 135 and am very happy with it, with the exception of the diffraction patterns. at F/4 it's pretty bad (at least to my eyes). i tried at F2.8 and the pattern is almost "pleasing" but i would still prefer no spikes, or very minimal. (i'm using the ASI2600MC Pro at gain -15 (at F2.8) but am very curious to hear of your results with the step down ring. thanks for the link. i've seen conflicting comments on other sites re the introduction of vignetting and other reflections when using these "rings" so your experience will be much appreciated.

w7ay

Kevin_A I

OK, not as bad as I thought. I had a scaling error. I think this iscloser tothe aperture vignetting one expects at the corner of an APS-C frame (for 39mm Tiffen filter):

Now, this might be really good news... this is with the Tiffen 52mm (47mm actual aperture) and f/2.9at APS-C diagonal:

And Tiffen 49mm (f/3.1):

40.5mm filter at APS-C corner:

It is counterintuitive that the wider the opening, the less the error. We need to do star tests. Darn aperture sure looks round, though.

Interestingly, the filter location has a pretty large magnification factor, but there are fewer photons on average :-).

Chen

Kevin_A

w7ay I only use APS-C 2600 on my Rokinon so I will check this weekend when my ring arrives on Friday. I can usually see in my flats how much vignetting will be with this lens. I have reference data already for non stepdown at f2.8. I can deal with vignetting if my stars are better!

w7ay

Kevin_A I only use APS-C 2600 on my Rokinon so I will check this weekend when my ring arrives on Friday

OK, first numbers are coming in... not bringing good news. I did a quick lashup that allows me to measure the angle of a ray at the sensor plane.

Very severe vignetting with a sensor diagonal of 33º (just a little larger than APS-C corner) with a 52mm filter aperture.

Next is with the aperture removed, with the Rokinon's iris set to about f/4.

Notice that you can see the iris blades (thus diffraction spikes), but no vignetting

This is Rokinon's f/4 iris with 40mm diagonal, still no vignetting:

I need to make sure my measurements are kosher.

Chen

w7ay

Kevin_A I can deal with vignetting if my stars are better!

Unfortunately, this is not the kind of vignetting that is a simple light attenuation. The almond shaped optical aperture is going to create the two diffraction notches on bright stars. See that Cloudy Night page that I linked to earlier.

So, you go from one type of diffraction (radial spikes for all stars) to a different kind (notches for bright stars that more than 1/3 from the center of a APS-C frame). At least that is what I expect.

Depends on which kind of diffraction you prefer :-). I actually prefer the notches (I hate spikes from spiders), if it is not too severe since you only see them when the filter has halo-- I get it often mild ones on the FSQ-85. Here is Alnitak being notched (IDAS NBZ filter; the very ugly halos caused me to stop using it when there are bright stars, otherwise it is a fantastic filter):

Chen

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