Lukehurst-Nichol classic dobsonian modifications

[Piero]

6 minutes ago, jetstream said:

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.

Absolutely! 

 

 

6 minutes ago, jetstream said:

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.

The problem is not really the lack of contact between 0 and 5-ish Deg of altitude.

The problem is the difference in support that the mirror receive from the bottom. The point is that even if the mirror is supported by all pins of the triangles (all pins touch the mirror), this does not guarantee that the mirror is well supported. If the pins touch the mirror at the same time at the same altitude (0deg ideally), then the support is expected to be identical or close enough.

[Piero]

5 minutes ago, jetstream said:

where did you add it?

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

[jetstream]

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!

[jetstream]

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?

[Piero]

1 minute ago, jetstream said:

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!

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.

[jetstream]

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?

[Piero]

10 minutes ago, jetstream said:

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

Yes, please

[Piero]

Just now, jetstream said:

what happens when transitioning- is it linear?

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

[Piero]

It's late here Gerry! What's your humblest suggestion? I'm curious!

[jetstream]

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?

[jetstream]

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!

[jetstream]

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.

[John]

Thanks for the responses Piero and Gerry.

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 !

I think if I was going to replace it, I would have to put the replacement together myself now to be confident that the design was really up to the job.

This thread is becoming a great resource that will be valuable to those considering a reasonable sized newtonian either DIY or through one of the smaller manufacturers

 

[jetstream]

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

[Piero]

Gentlemen, you are too kind!

I haven't come up with new products or design, but just put some information together and been trying to analyse / address some issues which are often ignored.

Hopefully, this thread will become a useful resource for those of us having to deal with these issues or want to improve their telescope performance. That would be great!  

 

I feel the thread title should change. Currently, it is not really informative and this might affect searches.

Probably something like "Lukehurst-Nichol classic dobsonian mods" would be more appropriate? If happy with this, could @John or @Stu change this, please?

 

Edited January 30, 2020 by Piero

[Piero]

7 hours ago, jetstream said:

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?

Thank you, Gerry. Very helpful.

Placing a pad on top of the pin is not a final solution, but only a quick way to test the idea. I certainly need to consider either taller pins or taller pads, once I get the the measure of the exact height. If this is exact there is no friction caused by pins or pads.

[John]

3 hours ago, Piero said:

I feel the thread title should change. Currently, it is not really informative and this might affect searches.

Probably something like "Lukehurst-Nichol classic dobsonian mods" would be more appropriate? If happy with this, could @John or @Stu change this, please?

 

Change made Piero

[Piero]

18 hours ago, jetstream said:

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!

 

Here we go 

You don't need the actual W vector for this, so it can be assured to be the unit vector. What really matters are the sine and cosine functions at different angles alpha. The alpha angle is 90 - telescope_altitude.

Sine and cosine receive angles in radians, so you need to covert: 1deg = pi/180 = 0.01745 rad.

[jetstream]

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?

 

[Piero]

7 hours ago, jetstream said:

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

 

Hey Gerry,

Discard the z function (green) as not necessary here.

What we care here is the part of the functions where x is from 0 to pi/2 rad (90 deg). This is the movement of the telescope regarding altitude.

Basically, depending at 0 deg telescope altitude the weight force is all on the sling (see cos function) and this decreases as the altitude grows to 90 deg (pi/2). Follow the sin function for studying how the weight force works on the triangles.

Note that the intersection is not at 0.5 y.

 

Source: Wikipedia.

[Piero]

On 30/01/2020 at 00:35, jetstream said:

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?

 

I ordered a set of felt pads of different sizes and 5mm thick. After taking all the measurements, I need to stack two of these in order to replace the pins. Will do the work tonight or tomorrow morning.

What's the diameter of your pads installed by Randy? 1inch, 20mm, or more?

[jetstream]

Just having an exquisitely brewed cup of dark roast coffee, out to measure in a bit.

[Piero]

I worked another bit last night and this morning:

• replaced temporary nylon pins + 2mm pad on top with a stack of 2 x 5mm thick pads (20mm diam). This raised the mirror 1mm. 
  • the milk cartoon structure to maintain the triangles in position without affecting their movement was moved underneath
• marked the position of the sling (COG) on four points. These are about equidistant.
• cut velcro strips in two segments and placed them under and above the COG where the sling will pass. The external border of the velcro strip (which is slightly flat) was placed towards the sling cable. Advantages:
  • the velcro strips do not attach to the sling. Therefore they cannot stretch it.
  • they remain attached to the mirror. Therefore there is no need to remeasure the correct position of the sling
  • removing the sling is much easier
  • as the velcro strip do not attach to the sling, it is possible to rotate the mirror when placed on its mirror cell. 
• one velcro strip working as a bridge to make sure that the sling remains in position

 

Here are the photos

 

 

Mirror cell:

 

 

 

 

 

 

COG marking (see previous posts for weblink on how to calculate this). The number is the sling position from the bottom of the mirror

 

 

 

 

 

Velcro strips and placement:

Make the sling tight, but blocking the mirror position using two felt pads placed opposite to the sling:

 

 

 

 

 

 

 

Add velcro strip to keep the sling always in position. Note, these attach to the velcro strips on the mirror. The sling can slide through these strips. Therefore the glue on the velcro strips does not cause any issues (e.g. stretches).

 

 

 

 

 

 

Mirror on top of the triangle pads: 

 

 

 

 

 

 

Mirror cell put back into the mirror box. Gap between sling and lateral support pad. View from the bottom

 

 

 

 

 

Video showing how the mirror goes back to the mirror cell increasing the mirror altitude. There is still a minor gap, but I will leave it as it is. This is probably 0.5mm or so.

[Piero]

Last night I had a chance to try the telescope following the last work on the mirror cell. 

The sky conditions were clear, windless, and rather stable temperature between 8.20pm and 9.20pm. After that the temperature started dropping. The telescope was left outside with the fan on since 6.30pm. The light shroud was also fixed in order to reduce internal and nearby turbulence.

Both telescope focuser and primary mirror axial alignments were adjusted with my 2" Glatter laser 650nm.

Several stars were used for star testing: Betelgeuse, Procion, Rigel, Pollux, and Aldebaran. This was done in order to have a range of colours and different altitudes.

I could not spot any trace of astigmatism. Spherical aberration was highly corrected too. This was at about 300x.

The rest of the time was spent observing the moon which was outstanding. Copernicus at 533x (zoom plus VIP barlow) was really impressive, showing fine and minute features (e.g. tiny craters inside and ground detail) that I didn't even imagine were possible to see. 

Very happy about this result.

 

From sky and telescope lunar map (at the eyepiece I could see far more detail that this):

[Piero]

Last Thursday night the seeing was great with very good transparency.

Observing Aristarchus crater and plateau from 160x to 630x was amazing. In particular, I was impressed by the kind of "river" approaching the crater to the left and how the light played fantastic shades inside.

 

From Wikipedia: Aristarchus plateau (NASA).