jetstream

2 hours ago, Ships and Stars said:

Sorted! 

so, how are the views?

5 hours ago, Piero said:

Here we go 

Seeing as your good at spread sheets and graphs and I'm good with pictures how about a graph?

2 hours ago, John said:

My Orion Optics 12 inc dob mirror cell is so basic compared to all this. I'm amazed that it does a reasonable job at all !

If it works it works lol!

My VX10 is vg too but the 15" with "worse" specs, a wooden cell and a sling handily outperforms it in mag/inch.  Both scopes really do well. I think when it comes down to splitting hairs scopes that take unlimited mag, like your APM/LZOS have all their ducks in a row, optically and mechanically. Personally I have 2 of these now, the TSA120 and the 15" dob, with the 24" hindered by seeing/cooling, so far only 500x razor on the moon. The 15" goes 762x with the 2.4 HR and could take more.

I believe our goals in the thread is not only to fix and maximize our telescopes but to show others the many paths that can be taken to do it, hats off to @Piero for having the guts to expose a set of problems with a brand new telescope and also for coming up with some VG solutions.

Mind you he's pretty smart

56 minutes ago, Piero said:

 

 

John, here is a photo of an original Glatter sling attachment. As you can see, it is attached to a roller which can slide up and down according to the mirror plane. Together with the velcro strips, the sling remains in place and in the correct position. It's a great design really. The sling does the job, not the person who needs to assess the correct mirror height. Also, that design prevents the sling to squeeze the mirror (and therefore astigmatism). 

My telescope also works, but this just because of the wood platform holding all the components (mirror, sling attachments, etc). Still in this case it is necessary to find the exact height of the sling attachments and mirror so that these are in agreement. Once this is done, it is done though.

I wonder if this design can "lag mirror movement"? ie the angle of the sling as it approaches the post.

6 minutes ago, Piero said:

I don't think it is linear. I suspect it is sin/cos, but haven't thought about this properly.

I visualized that it was not linear but this is just a dream until math proves it lol!

Just now, Piero said:

Yes, please

You are probably going to do this already, to try. I think that narrow nylon tips triangles need very accurate placement to get the abstract support circles in the right place. I also think furniture pads with a much larger dia are much more forgiving. When the pad is placed on the nylon tip it does not have the same effect as glued directly on the triangle- a flat mounted pad might catch the mirror on its top edge for support during transition.

If I needed more height I would first try a double pad and if it works maybe get a few circular pieces of nylon machined- mount to triangles and then the pads to these.

what is that mounted under the sling in the vid- a pad?

1 minute ago, Piero said:

Sling and triangle supports play orthogonal (=perpendicular) forces AND they should do so, otherwise the result is astigmatism (at the least).

Triangle support is 0 at 0 deg altitude and 1 at 90 deg altitude.

Sling support is 1 at 0 deg altitude and 0 at 90 deg altitude.

what happens when transitioning- is it linear?

8 minutes ago, Piero said:

Quick solution,for the time being they are on top of the nylon pins. See video.

May I make the humblest of suggestions? just to try?

2 minutes ago, Piero said:

The problem is the difference in support that the mirror receive from the bottom.

I was always puzzled by and interested in exactly how the transition from full triangle support to increasing sling support worked and the possible 'potato chipping' etc. I'm glad your gonna figure it out and explain it to me lol!

5 minutes ago, Piero said:

Adding 1 pad reduced this gap, although there is still a little gap left. 

where did you add it?

27 minutes ago, Piero said:

At what angle are these two cruces attached to the mirror?

these pieces of velcro strips are 90deg to the mirror surface, straight up and down separated by a sling width and in 2 places. I'll measure the deg apart tomorrow when light.

21 minutes ago, Piero said:

If you raise the mirror too hight or low, the mirror will slide a bit on the triangle supports a

yes the curvature of the eyebolts will pull the mirror one way or let it slide the other. In reality it doesn't have much effect because of the furniture pads and the vg movement of the triangles- also a thicker mirror is less susceptible to all this astig anyway- at least within the limits of the sling/cell design.

1 hour ago, Stu said:

What you say makes total sense John. I can't recall what they do on my Sumerian, will check.

@jetstream Gerry, yes they do have velcro on them. Presumably in that case it is best to position the mirror towards the top of the range so that it slightly presses against the velcro?

 

Yes, as long as the mirror can slide around with light finger pressure alls well, same goes for where the edge rests on the clips, velcro is good here too. Might be good to check the triangle points to see if they disengage like Pieros, the more horizontal the angle the less they support but there will still be some needed until 90 deg.

8 minutes ago, Stu said:

Related to John's question, with my Sumerian the clips which prevent the mirror from falling forward are fixed ie they do not move with the mirror cell when collimation. This means the distance varies depending on how compressed the springs are. Is this normal? I think not from what I can remember of previous scopes but I'm far from a newt expert (understatement)

It seems to give problems with the mirror flopping forward when at extreme low altitudes, one of the reasons I didn't observe some of the low down Sagitarius object when I was in Pembroke last year and something i would like to fix.

Should really take some pics and start another thread.

Stu, no expert here but do your clips have velcro on the underside? the velcro can be compressed a bit but still offer no restraint allowing for a higher collimation height possibly.

23 minutes ago, John said:

Very interesting Piero - it's like detective work, eliminating one cause after another

Just out of interest, does your whole sling mechanism, attachment points included, move with the primary cell and primary mirror when collimation adjustments are made ?

 

Good point John. There is a sweet spot for triangle height in relation to the sling,but... I eyeball this, nothing fancy when setting collimation position up. My scopes use "eyebolt"  sling attachments where the sling can slide up and down a bit following the center (well lower a bit) of the mirror. No restraint in the Astrosystems cell and the sling does track well.

A bit of info- my 15" has 2  small strips of velcro, one on top of each other separated by the slings gap (2 sets) where the gap position is set as in Cruxis . My 24" does not have these (will soon), maybe they fell off in transit or something. These 2 sets of 2 strips with positioned gap allows a piece of velcro to fasten on them with the sling positioned correctly all the time.

Great info Piero!

Out of all the issues a new mirror cell expert has emerged! and with such a strong math background, perfect. I'm going to check mine for the heck of it- at what approximate altitude does the pins lose contact? I'm going to check my sling position too, just used Cruxis.

1 hour ago, Charles Kirk said:

https://www.assa.org.au/resources/variable-stars/observing-variable-stars/

Experienced visual observers can see 0.1 mag differences under ideal conditions, and 0.2 mag more typically.

 

Interesting, this is something I've never done or considered but am puzzled from a statement earlier on in this site.

"How many variable stars are there?

Nobody knows. Seriously! Defining a star as variable requires at least two reliable magnitude measurements, at two different times, which disagree with each other. How much of a discrepancy qualifies as a "real variation" is a matter of opinion. Anything below 0.2 magnitudes is virtually undetectable to the human eye; and visual observers trying to follow a star with less than 0.4 magnitude variation are wasting their time. They should use a CCD or photometer instead."

2 hours ago, Charles Kirk said:

To make comparisons between variable stars and comparison stars in the same field you would need to have 100% field illumination for the circle containing the two stars. For that reason you would want a larger central obstruction than you would in a reflector optimised for observing fine details on the planets and/or moon.

This is technically true- but what point source magntitude drop can be detected visually by an astronomer? ie .10mag, .20 mag etc?

48 minutes ago, pez_espada said:

why wouldn't one plug a CC for visual then? 

It will work nicely, try to find a used CC like S&S did- the ES or possibly a Baader .

3 hours ago, pez_espada said:

So there's something I'm missing or maybe the role of CO on contrast is not that great as common knowledge implies?

He runs relatively small CO for the aperture and with a PCII in specialized little dobs- big ones now too. Lockwood likes 12"+ aperture for the ultra fast newtonians because you can reduce the CO, at least when I talked with him. I have used 37% CO in my 200mm f3.8 but much prefer 31.5% and I observe many of the things he does. There are theories, ideas and then what we each prefer- and I know what I prefer.

Point sources such as stars do behave differently than extended objects, but one thing is certain- to view near the edge of the FOV you must control coma and astig IMHO.

Have you heard of Suiters "wobbly stack"?

29 minutes ago, pez_espada said:

It seems, or at least I think I remember reading something along these lines,  that in the reflector world the size of secondary mirrors scale inversely proportional to the focal length, which is  independently of the focal ratio , meaning that, for instance a 5" F6 would need a proportionally larger secondary than a 10"F6 scope (?).  I have seen from a popular brand a 6"F6 with a 46mm secondary (~30% of the primary),  and from the same manufacturer an 8" F6 with a 50mm secondary (25% of the primary). Both, they claim , optimized for visual..

If that's true then for a wide-field Newtonian with good contrast (i.e. proportionally smaller secondary) one necessarily hits a trade-off region, as the larger the scope so it can have  a proportionally smaller CO, the larger the focal distance that negates wide fields...  would that be a sound reasoning?

Maybe my best bet is to go the fast achromat refractor route?

This calculator from Mr Bartels will show you exactly whats happening re CO IMHO.  Yes the larger the aperture the smaller the CO you can use,fully illuminated field considered. No experience with achromats but I do like triplet APO's.

http://www2.arnes.si/~gljsentvid10/diagonal.htm

33 minutes ago, pez_espada said:

Many thanks and your opinions and insights are very welcome

You might want to look for a scope with less CO, 35% is getting right up there. A Paracorr will increase perceived contrast (fo me anyway) but the large CO might not be your friend here. 30% CO is my max, 25% and less preferred and my best scopes are 21% or less.

1 minute ago, Ships and Stars said:

No I sure haven't, how do you check that? I'm familiar with screw-on rotating polarizing filters for camera lenses, but not anything astro-related. Can you simply look into the binoviewer body and see if the mirrors have a certain type of coating? Another facet I wasn't aware of! Haha.

Try looking at an LED computer screen which is polarized light- I use a white screen, and look through each barrel for brightness differences. Denis in Croatia is critical of polarized prisms, which can effect image brightness on the moon etc.  Honestly I think it is no big deal unless very severe. You can test your screen with a polarizer filter, I use a Baader single polarizer.

36 minutes ago, Ships and Stars said:

I'll have to ask the fellow in Croatia who supercharges BVs

He is the one who says the Binotrons are less than 27mm and also that they are strongly polarized- not so in either case. Not that all this matter much- have you tested your binos for polarization?

I use a Binotron 27 with a 45mm OCS for use with steeper light cones. Some say the clear aperture is not 27mm... well the prisms are but the powerswitch is a bit restrictive in 2 modes. Using the center (IIRC) position there is no aperture reduction.

So any 1.25" eyepiece is not vignetted in the center powerswitch position.

Will this translate into real world DSO gains over S&S's system? I would like to think so but would not bet on it....

I can say this- the Binotron 27's on the moon using the 15" give the most fantastic, jaw dropping views I've had and the jaw continually drops- for years now. I believe the Binotron 27's approach if not equal Zeiss Mark 5 optical quality in a much user friendly package- and as we all know its the system that counts espc with binoviewers- a couple of niggles here and there will drag them down fast. ie eyepiece alignment, focuser sag, bino collimation, etc etc the there is the typical GPC issues.

When I purchased the Binotrons I put them in thre focuser, merged for the first time in under a minute and had fantastic views, right out of the box.

S&S has achieved a superb observation with his set up- the HH.