Kevin_A very frustrating hobby when mother nature has the last word!

After 60 years in this hobby, I don't really care about clouds anymore. I just do something else than take TikTok pictures (which I can now do with my iPhone, ha ha).

I just started gathering the parts I have (OAG, tube mounting rings, camera angle rotator, etc) to see if I can assemble a laser jig for measuring camera tilt. Clouds would be noo problem.

The only precise part needs to be the camera angle adjuster, which must have no tilt (if it has tilt, it cannot be used on an OTA anyway :-).

It really is just a fun project because I have always wondered if I can use two cameras facing one another (one is device under test, and the other to record the result, instead of using a pen on a piece of paper -- so 19th century :-). And with the laser beam injected with a small mirror (like a Newtonian diagonal, used backwards). In this case, the prism of an OAG. I have a couple of (Askar and Orion) OAG sitting around. Each time I am temped to use one, I'd reject the idea because a separate guide scope is so much closer to ideal (as long as there is no differential flexure). Heck, with the sloppy helical focusers on these OAG (especially the ZWO ones, which I won't buy even to test), a separate guide scope is probably more solid. OK, that is my daily rant on autoguiding :-).

Chen

Kevin_A A good RTV silicone may work too

Ah! That is a good idea. Just a small drop of it.

Chen

Kevin_A OAGs were never good quality stars in my opinion since you have to use an inferior part of the flattener when using a bigger sensor camera for imaging.

How true that is.

If the part of the image plane where the OAG resides is good, why not use it for a larger image, instead of wasting it on an OAG :-).

Except for 16" SCTs, I have never understood why people like using OAG. Nor the "Duo" cameras.

SBIG gave up after inventing the "duo" scope and sold as the ST-7. ST-4 was the first and the last camera to have two chips. ST-4 was the first camera that had the guide pins, that is why those pins are called ST-4 today.

http://www.company7.com/sbig/products/st7.html

I still have my SBIG 8300; that one once had the reputation that the Sony IMX571 based cameras enjoy today.

I regret sending Sam an email about the two camera ST-7 a year or two ago. He got one of his flunkies to jury rig one up (guy used to do product support, not engineering). But I'd warned him about all the shortfalls of a dual camera system, which he of course does not include in the Duo documentation. To ZWO, everything is a rush job to get the next bundle of money. And then abandon support to go the the next money making scheme. Nothing is studied properly.

I also now regret telling Sam about how well the RST-135 worked as a mount. He went off and copied that too, but without really studying the minutiae of the RST-135. It puzzles me (... it is a puzzlement... The King And I) the Chinese blindly copy things without first trying to understand how they work. The leaky tantalum capacitor was a good example, and exploding LiON batteries is another. Both were missing a critial ingredient that stabilized the chemistry. The copied (stolen IP) ones lacked them cause fires, and I think the expolding batteries caused lives to be lost.

At least the ZWO mounts won't kill anyone, and it serves them right, now that the Chinese are copying themselves, and with more stable mounts. At least two companies now have starin wave mounts with much better measured guiding -- only one of them recently distributed in the US by Agena. They all use the same strain wave gears; so we know the tracking glitches are how ZWO is incorporating the gears, not the Chinese gears themselves. In addition to real EE who can design USB interfaces, ZWO also needs to hire real ME who can design mounts.

After those two things, I have now sworn off telling Sam about neat ideas to implement. I was expecting him to start some real investigation, but he simply took off and got his people to copy things superficailly. Unfortunate for the ZWO mount customers, I don't think there is a way to get rid of the mount glitches short of a full recall (like they did with the leaky ASI2600), and I don't think they will do that.

Chen

Kevin_A I need a few good clear nights to retest my old Rokinon and see how it looks focussed to neutral and to retest for aberrations at that setting.

The 40mm Sigma is really well QA'ed.

After spending hours (more than one night) adjusting backfocus. What I eneded up is a backfocus that caused the line in the focus window to point precisly at the center of the infinity symbol!

I.e., the backfocus is at precisly where they have designed it.

The key for me is to first remove tilt, and only then tune backfocus. I could have save a lot of time if I had started doing that in the first place.

I had a set of larger (102" diameter) tilt plates made and it is heading my way right now. This next design will clear the entire cylinder of a 90mm ZWO camera, and will allow me to adjust tilt with a very long 2mm hex wrench from way back of the camera :-). It should make adjusting tilt and backfocus a breeze (like the two-plate-tilter allows you to adjust the tilt of an ASI2600 from the back). It is worth the money for the new tilt plates to prove this to myself. Paying money for knowledge is the most worthwhile thing one can do with money.

made by Samyang optics so maybe I will have better luck with their house brand than rebrand.

I'll bet they both go through the same fingers and hands at Samyang. I don't think they have different factory buildings for the two labels.

Every internal element that I have looked at are identical between two Samyang 135, and a Rikonon Cine. The only difference I have found so far is the number of brass shimming washers for the front lens group.

Even the three stupid small black screws are the same :-)

But... there is a different Rikonon 135/T2.2:

https://www.adorama.com/rkx135c.html

Much more mullah, and with huge lens hood -- 114mm. It touts close focusing -- if that was their design goal, it is a complete waste for astrophotography.

But (ha ha) it has 11 iris blades, so 22 pointed spikes.

Chen

Kevin_A E

By the way, Kevin, have you thought about spreading the payload of your ZWO mount sideways (e.g., using a side-by-side mounts, like I do with my dual saddle plate) instead of stacking stuff taller, and thus further away from the RA axis?

All the way back to the RST-135 (and perhaps even further back to Hobym), stain wave mounts have emphasized the need to keep the moment arm short. Emcan claims that they have designed their mount so the declination plate just 80mm away from the RA axis. I just measured my dual saddle plate, and it is 75mm away from the RA axis .

My dual saddle plate itself adds another 10mm (flexture? whats that?), and a PrimaLuce Losmandy dovetail clamp another 25mm or so).

The more I look at the Emcan mount, the uglier it is (although not as ugly and with gaudy colors as the ZWO mounts). Just compare to what the RST-135 and RST-135e look like. The Chinese mounts look like they were designed by Neanderthals. Years at Apple has taught me that well engineered stuff don't have to also be ugly.

Chen

Kevin_A but on my cooled camera with a filter was closer to the 3’ mark…. Hmmmm, i will have to recheck that as maybe the zwo sensor back spacing on my 183mc pro camera is off a bit or my stack was off.

That is almost for sure wrong backfocus.

I just checked the Samyang 135 that I fooled with backfocus the other night -- the middle line of the depth of field scale points.... right at infinity mark.

When I get the larger tilt plates in, I am going to keep the EAF and focus right were it currently is, and start tuning the Askar backfocus adjuster for best focus with a Bahtinov mask.

Chen