6" Refractor vs 10-12" Dobsonian

[John]

9 hours ago, jetstream said:

APM contract with LZOS says bubbles are OK. At least they are telling us, most (all?) other manufacturers dont.

"5. Bubbles and inclusions requirements
Permissible dimensions of bubbles and inclusions:
For systems of diameter up to 152mm
In lens material bubbles and inclusions of no more than 0.3mm in size are allowable , and a number of bubbles of
diameter 0.03mm to 0.3mm in each lens shall not exceed 3 pieces.
For systems of more than 152mm up to 254mm diameter
Number of bubbles and inclusions of size up to 0.3mm - not more than 20 pieces. - up to 0.5mm - not more than
3 pieces.
Bubbles and inclusions in material exceeding the requirements above shall be coordinated on the stage of optical
blanks selection for the manufacturing of each lens"

https://www.apm-telescopes.de/en/telescopes/apochromatic-refraktors/apm-lzos-apo-refractors/apm-lzos-telescope-apo-refractor-130-780-3.7-inch-focuser.html

Yes - I was surprised, but pleased, to find this spec in the public domain when I bought my LZOS refractor.

 

[Deadlake]

6 minutes ago, John said:

Yes - I was surprised, but pleased, to find this spec in the public domain when I bought my LZOS refractor.

 

If people have to much information they can make an informed choice....

Manufactures publish Strehl, where is the PV measurement??? 

Edited May 24, 2021 by Deadlake

[John]

2 minutes ago, Deadlake said:

If people have to much information they can make an informed choice....

Or start to quibble about the spec as they do on another forum that I can think of !

 

 

[Deadlake]

37 minutes ago, John said:

Or start to quibble about the spec as they do on another forum that I can think of !

 

 

You mean where someone get a better number and they send the scope back? Which forum could that be on. 😁

However as you know from another thread on this forum, you can publish RMS, PV and Strehl and those scopes are still third division due to Abbe number for the glass. 😉

 

[jetstream]

13 hours ago, jetstream said:

The 20% (or so) missing from the primary mirror due to obstruction is not missed...

I say this with regard to both the loss of contrast with respect to MTF and also the loss of light due to the "bullet hole" effect 😀

I'll take a near perfect 20% obstructed scope over an un obstructed scope with 1/4 wave SA...

The nice thing about large aperture newts is the ability to use small central obstructions and still get a nicely illuminated field.

 

[jetstream]

4 hours ago, John said:

Yes - I was surprised, but pleased, to find this spec in the public domain when I bought my LZOS refractor.

 

Yes full disclosure is a sign of honestly and allows for an informed consumer. All (?) optical glass contains bubbles and inclusions. For those interested Schott publishes the data

[PeterW]

Perfection doesn’t exist…. The closer you get the higher the price. Not sure where I came across it, but an interferogram can turn a good mirror into a worthless one if the numbers are slightly different. Sometimes people can be too picky for their own good.

Peter

[Rusted]

It would be incredibly easy to bung a bit of fine sawdust on a dry mirror to confirm optical degradation.
Get Schroedinger's Cat to blow it off afterwards and you have a double blind test.

[chrispj]

Returning to this, we had good clear skies in Exmouth last night and I rounded off a session at half past midnight with viewing of reasonably convincing gray smudges in the appropriate locations for M94, M63 and M51. A good start, and considering I can improve my viewing environment (seating & some sort of observing hood for starters!) as well as investing in shiny wide-field glass to help show open clusters to best effect there is certainly potential for improvement. The black paint on the inside of the tube showed some reflections under the garage lights and from the moon on the first brightly lit night I had it out so I have flocked the tube and also blacked the sides of the secondary which I choose to believe has helped somewhat.

 

Practically, the size of the 10" scope is as large as I would like, it is two easy trips to carry through the house to the garden and any larger would be a squeeze to fit the base through interior doorways. I like to tinker so I am toying with replacing the teflon pads/mini roller assembly azimuth bearing with a large radius lazy susan as it is a little sticky to start moving even after cleaning. The altitude adjustment works very smoothly (though I don't have any very large eyepieces to test it with... Collimation has been straightforward with only minor adjustment of the primary needed after driving back from collecting the scope. In the future as a treat next summer I'm also contemplating sending the mirrors off to Orion Optics for a recoat, if I'm likely to see any visible difference? (having had the mirror cell out for flocking the tube to my eyes the primary does appear in very good condition).

 

Overall I would have to say I'm a happy punter! and also looking forward to autumn when I don't have to stay up so late for decently dark skies (as the kids make sure I'm awake early no matter what...).  

 

 

[Louis D]

Both of my Dobs use large Teflon pads riding on Ebony Star Formica (which is discontinued, I believe).  Any pebbly, glossy laminate will work well to provide just the right amount of stiction.  Lazy susans accumulate crud over time, ruining the motion.

Edited June 1, 2021 by Louis D

[chrispj]

Ah,  fair enough,  I might look at larger/ increasing the number of teflon pads then. Movement is smooth once it's going, but after being stationary for any length of time the initial movement jerks just enough to be annoying at a higher power...

 

[John]

1 hour ago, Louis D said:

Both of my Dobs use large Teflon pads riding on Ebony Star Formica (which is discontinued, I believe).  Any pebbly, glossy laminate will work well to provide just the right amount of sticktion.  Lazy susans accumulate crud over time, ruining the motion.

Agreed on both counts. If the teflon pads are correctly positioned, the motion against the formica-type surface can be very smooth and yet controlled. My 12 inch dob uses just that approach.

 

 

[Louis D]

3 hours ago, chrispj said:

Ah,  fair enough,  I might look at larger/ increasing the number of teflon pads then. Movement is smooth once it's going, but after being stationary for any length of time the initial movement jerks just enough to be annoying at a higher power...

 

Is the surface smooth or pebbly against which the Teflon glides?  If perfectly smooth, it will have too much static stiction to overcome when it comes time to start it moving.

Make sure you're using virgin Teflon and not mechanical Teflon.  This web page does a good job describing how to modify your bearings.

This material might also make a decent bearing surface for the azimuth motion.  I've seen similar used in some Dobs.  Just make sure the pebbly surface is glossy and not matte.

[Piero]

8 hours ago, chrispj said:

Ah,  fair enough,  I might look at larger/ increasing the number of teflon pads then. Movement is smooth once it's going, but after being stationary for any length of time the initial movement jerks just enough to be annoying at a higher power...

Increasing the size of the pads will give you more friction, not less.

Do you have a fourth pad where the pivot bolt is?

A quick and cheap solution is to cut an area from a milk plastic bottle and add it to the fourth pad located at the centre. If one is insufficient, you could add another one.

Milk plastic bottles can have many applications in ATM, as the plastic is soft, but strong enough. Another application is to improve the secondary mirror collimation by adding one layer between the plate of the secondary mirror holder and the collimation knobs. These two mods are by JasonD on cloudynights.

Another application is the structure connecting the triangles in a mirror cell.

[chrispj]

22 minutes ago, Piero said:

Increasing the size of the pads will give you more friction, not less.

Won't larger pads spread the weight of the top assembly better?

 

22 minutes ago, Piero said:

Do you have a fourth pad where the pivot bolt is?

 

There is a mini-roller assembly in the centre, I'm slightly unsure if it's a benefit or a bit of a gimmick. In general it's all smooth running, just need to reduce the initial stiction, especially with the scope at higher angle. Probably just a very minor change needed.