Lukehurst-Nichol classic dobsonian modifications

[Piero]

The telescope has consistently been working very well for a while now. Stars don't show traces of astigmatism. 

Last night I had another nice session. The sky was rather steady and clear, with very low humidity. The telescope cooled with the fan on for about 2h and then collimated with my 2" Glatter laser (without tuBlug). Eyepieces: 30mm APM UFF (large targets), 12.5mm Docter (all targets), Zeiss zoom (all targets). 20mm Lunt was used for the galaxies, but didn't show anything more than the Docter. Lumicon OIII was used for the Eskimo PN.

Main targets were:

• Monoceros: M50, NGC 2264 (Christmas tree cluster), Plaskett's star (giant O-type binary SE of 13 Mon)
• Leo: Leo triplet, NGC 2903 (W from epsilon Leo)
• Gemini: M35, NGC 2158 (interesting particularly above 250x), NGC 2129, NGC 2392 (Eskimo PN - what a sight at 400-500x!), Wasat (delta Gem)
• Cancer: M44 (Beehive) M67 (King Cobra cluster)
• Canes Venatici: M51 (faint spiral, but no connection due to LP) 

I must say that I was impressed with all the targets I've seen. The views were really rich in colour and intensity, with "star dust" visible in the background sky.

Beautiful.

[Piero]

Yesterday I received 4 new pipe insulators for the truss poles. These reduce vibrations to nearly zero, are much more comfortable to touch in the cold, and minimise internal light scatter.

They are also rather cool! 

 

 

 

Edited March 18, 2020 by Piero

[niallk]

Nice - looking good.  I've them on my truss poles too: they really are the job on a cold night especially when it comes to tear down!! Good for transport too - the poles just dont rattle and bang off each other.  Simple & effective

[Piero]

@niallk I agree! They make a substantial difference on a cold night. 

[Piero]

This past weekend I took care of the telescope coatings. Certain parts were minimally coated, whereas others (e.g. UTA, wheelbarrow handles) didn't seem to be coated at all. Also, the internal was gray / black. After ordering the same products used by David from Rustins, I applied 4 coats of clear lacquer to the outside and 4 coats of matt black to the inside. Now the telescope looks so robust and protected from any kind of humidity! The parts in the light path are really black now. 

Also, before doing this work I removed the "button" screws used for attaching the light shroud to the UTA and mirror box. So many! Once removed the holes were all filled and the sanded. This was done a couple of weeks ago. I also removed the usual label that is attached to the UTA as the specs were wrong. 

in addition, I attached 4 X 2mm felt pads to each wheelbarrow handles so that there is a small gap between these and the rockerbox. This should have been done a long time ago, but for a reason or another I always forgot 🙁. Anyway, there are two advantages with this little mod: 1) the wood does not touch, therefore no scratches, cracks noises, etc; and 2) if the telescope is left out overnight or during a week of clear sky (I use a telegizmo cover when this happens), this gap allows the wet surfaces to dry properly.

I also decided to apply the matt black on top of the mirror box, to optimise contrast by reducing any stray lights or reflections. Will post a photo later! 

Finally, Rob Teeter let me know that the Heather completed and just dispatched my new light shroud! It will arrive soon, hopefully.

This evening I will put back the primary mirror and re-collimate the telescope. Very happy with the results. Now I am confident that the telescope can really last for several years!

 

 

The first two external coats (..the UTA looked already so different!):

Edited April 20, 2020 by Piero

[Piero]

Mounted! 

Just re-collimated the focuser and secondary. Therefore, I collimated the focuser and primary axial alignments and now it's ready!

Here are a few photos after painting:

[Piero]

Just returned from a session outside. Venus was superb, the best view I ever had. Faint white - grey patches were visible between 250x and 400x. Really good view!

I then moved to the classic open cluster in Auriga, Gemini, Cancer, and eventually ended up to the bright galaxies in Leo.

The new black paint certainly makes a difference. On Venus I believe the top of the mirror box now painted black also increased contrast, as the sky appeared noticeably darker than usual. 

Looking forward to use the new light shroud, which landed in Heathrow airport this morning!

[niallk]

Looks great & glad to hear it's working well

[Piero]

43 minutes ago, niallk said:

Looks great & glad to hear it's working well

Thank you! It is getting there! 

The positive thing of all this process is a good learning curve. There are a few things I would like to build..

[jetstream]

On 22/04/2020 at 02:19, Piero said:

Thank you! It is getting there! 

The inside of the rocker box also needs a coat of the black paint...stray light needs to be absorbed here. Espc the bottom.

Edited April 23, 2020 by jetstream

[Piero]

34 minutes ago, jetstream said:

The inside of the rocker box also needs a coat of the black paint...stray light needs to be absorbed here. Espc the bottom.

 

Thanks for your feedback, Gerry. And yes I agree.

To do so, I would have had to sand it first, which was even more work. On the other hand, I recently built a wood panel to cover the opening on the lower part of the mirror box. This mod has given positive results, although there is still a bit of boundary layer that can be blown away. For this I will study the use of a fan gently blowing on top of the mirror surface. 

Anyway, because the opening is semi-permanently closed, I painted this wood bar instead. Therefore the is no chance to get stray light from the rocker box. 

And to be thoroughly, I would have preferred to stain and varnish the telescope rather than using lacquer on the outside. This would have required sanding the whole telescope enough so that the previous coat given by David did not interact with the stain and varnish.. I didn't want to risk, so I simply apply 4 coats of the same products.

All in all, another bit of leaning curve for my future project if it will ever come up to life! 

 

--------

The wood bar. It's light and attaches with 4 velcro stripes. Two side openings allow some air flow.

 

 

 

After lacquer and painting:

[jetstream]

52 minutes ago, Piero said:

For this I will study the use of a fan gently blowing on top of the mirror surface. 

I would source Bryan Greers knowledge on fan ie micro blur.https://www.fpi-protostar.com/bgreer/sep2000st.htm

My very best views using the 15" are with no fan, it will go over 1000x on the moon using the barlowed 2.4mm HR. My fan reduces this ability considerably.

You might find a bit more opening on the bottom to be a benefit...

[Piero]

On 23/04/2020 at 14:16, jetstream said:

You might find a bit more opening on the bottom to be a benefit...

Not necessarily. I've tested this quite a lot and I am more and more convinced that what really matters is the symmetry (or almost) of the air flow around the mirror. 

• In you astrosystems dobsons, there is a low opening on the bottom front and two circular openings at the bottom back. To me, the purpose of those two holes is to balance the air flow of the opening at the bottom front of the mirror box...
• In Kriege open design, the symmetry is present by definition
• John Dobson's design was fully closed, preserving the symmetry of the air flow.
• In David's design, this symmetry is not present instead. The flow is at the bottom front, but not balanced at the bottom back, causing unbalanced cooling. The problem is that David used that opening as a way for sliding the mirror cell in / out, whereas the real purpose of that opening (Kriege's design) is to decrease the height of the rocker box of 1-2 inches.... anyway.

 

It isn't just the air coming from the bottom... It is also the air coming into the tube from the UTA. If the design of the bottom of the mirror box allows a symmetric / balanced airflow, the mirror will be balanced in terms of temperature. In David's design the air flow can 1) go from the top UTA to the opening at the bottom front of the mirror box (therefore exiting) OR 2) hit the mirror and the wood behind the mirror, rotate and come up... 

In line with the points above, I found a significant improvement when the opening at the bottom of the mirror box is closed, basically replicating John Dobson's design. This guarantees a symmetry of the airflow. By significant improvement I mean that there is less turbulence in the tube, the mirror temperature is more stable and balanced across the body, and the views are better, noticeably better as I can reach 300-400x quite frequently without having bloated stars.

Edited December 9, 2020 by Piero

[Piero]

BTW, I now use this dobson with a TV paracorr2. Despite the fact that a few people suggest the requirement of a coma corrector on sub F4.5 Newtonians only, I can say that I do notice an improvement in the views of this F6 telescope.

Even more.. if one wants refractor-like views with a Newtonian telescope (AND I DO!), a coma corrector is needed, IMHO..!

[jetstream]

1 hour ago, Piero said:

is the symmetry (or almost) of the air flow around the mirror. 

Totally agree and I find adjusting the truss shroud up a bit can improve things sometimes. Sometimes closed works, open a bit works... maybe interaction of scope to changing ground thermals? and or puffs of wind?

Getting truly high power views is a process IMHO.

[Piero]

6 hours ago, jetstream said:

Totally agree and I find adjusting the truss shroud up a bit can improve things sometimes. Sometimes closed works, open a bit works... maybe interaction of scope to changing ground thermals? and or puffs of wind?

Getting truly high power views is a process IMHO.

Yup! Agreed 👍

[faulksy]

On 27/07/2019 at 10:53, Piero said:

For now my cable sling is loosened in such a way that it does not touch the mirror. 

In the coming months, I will think about whether taking it off completely. For a mirror of this size and weight, the current mirror edge supports do work well and do not cause any astigmatism nor pinched optics. This was proved in my last observing session.

I like the Whiffletree with rolling bearings proposed in Lockwood website. To me it seems a much nearer approach than having a sling.

 

Being this a public forum, I think it is also fair to state that the whole telescope works really well, and I am more than happy with it. We all can be picky at the details, and being critical is a good thing in my opinion as it allows us to improve things and make them work better. I did have a problem with the sling but this can be solved easily and in any case a sling is not required for the correct functioning of the telescope. When observing planets at low altitude I always collimate the telescope at the altitude of the planets before observing anyway. In my opinion, this is the most accurate way.

In my last session (with loosened cable sling), this telescope gave me the best view of Jupiter I have ever had. Considering the intricate and delicate details on that planet, I think this is a sign of very good optics and telescope structure.

i made my own wiffle tree support mirror cell. look in my build thread

[Piero]

1 hour ago, faulksy said:

i made my own wiffle tree support mirror cell. look in my build thread

Yes, I remember your thread. It is among my browser bookmarks! 🙂

[CraigT82]

Have you got any pics with your Teeter shroud fitted?  I have just made my own from a big sheet of thick black lycra and it looks awful.

[faulksy]

On 23/07/2019 at 22:11, jetstream said:

Does the astig rotate with the primary?

What kind of "pads" does the primary sit on in the cell? I had a case of restraint with my 15" causing astig when the mirror froze to the cell pad supports. Any large friction etc here can cause issues. It disappeared when I freed the mirror by moving it.

Uneven cooling can cause astig, my 24" likes the fans off for the final cooling as the 4 fans cool the center faster than the edges but I have never had an issue using a shroud with either truss dob. I normally leave the shroud pulled back at the top a hair to let the rising warm air out, until equalized.

Whats the astig look like- flipping oblongs in and out of focus?

Hot here too, just got back from a walk, not as hot as there though. Time for a nice cooling swim...

hi gerry. why did you ask how his astig looked, with reference to oblongs in and out of focus ?

hope you dont mind me asking this piero 😁

[jetstream]

5 hours ago, faulksy said:

hi gerry. why did you ask how his astig looked, with reference to oblongs in and out of focus ?

hope you dont mind me asking this piero 😁

Hi Mike, if astig is very bad it will look like ovals or oblongs that flip on either side of focus at lowish mag. Minor astig is harder to detect but does the same thing, seen at high mag. My 24" when its cranky will exhibit minor astig detectable at high mag. The big secondary caused astig before I repacked the holder for more even cushion and freedom.

Sling position can cause issues and I imagine a Whiffletree can too if the support roller distances are off or there is restraint in the cell system somewhere.

[Piero]

On 14/12/2020 at 15:57, jetstream said:

Hi Mike, if astig is very bad it will look like ovals or oblongs that flip on either side of focus at lowish mag. Minor astig is harder to detect but does the same thing, seen at high mag. My 24" when its cranky will exhibit minor astig detectable at high mag. The big secondary caused astig before I repacked the holder for more even cushion and freedom.

Sling position can cause issues and I imagine a Whiffletree can too if the support roller distances are off or there is restraint in the cell system somewhere.

  

The main reason for detecting astigmatism at high magnification (e.g. <=1mm exit pupil) is because any potential astigmatism present in the eye is minimised or even absent. An observer's astigmatism must be really severe to show effects at high power. In general, most of the eye imperfections are off axis of the pupil. Reducing the exit pupil, reduces the use of that part of the pupil. On the other hand, any astigmatism in the telescope is still present, whether at high or low magnification. 

Of course, one could also discriminate eye astigmatism against telescope astigmatism at low magnification by rotating his/her head. Said this, it is difficult to quantify in this way because both astigmatism will still be overlapped somehow, meaning that the observer can see 4 spikes instead of two in / out focus.

Incorrect sling position can cause astigmatism. Mine did in the beginning when it was not at the COG.

Astigmatism is quite easy to detect and even to fix somehow, unless it is present in the optics. The reasons behind spherical abserration are trickier to analyse to me, instead. That is more noticeable at medium / high magnification, but there are some many factors which can play a role or mimic it (e.g. cooling/warming (dynamic), mirror support (static and / or dynamic), air turbulence (dynamic), surface roughness (, static, !=spherical aberration but the effect on the view is not so different), unbalanced temperature around the mirror (dynamic, causes astigmatism + spherical aberration), etc. 

In my opinion, a lot of people stating that only mags up to 200x can be used in the UK with their newtonian telescope, actually have issues with spherical aberration (and potentially astigmatism too)..

Edited December 17, 2020 by Piero

[Piero]

On 14/12/2020 at 09:06, CraigT82 said:

Have you got any pics with your Teeter shroud fitted?  I have just made my own from a big sheet of thick black lycra and it looks awful.

 

Sure and sorry for the delay

Heather Teeter's light shrouds are excellent in my opinion. Mine improved the telescope a lot!

 

[Piero]

UPDATE

After building my 16" f4, the future for this telescope became somehow uncertain. It was put on sale for about 4 months, but apart from a couple of potential buyers, it did not attract much, despite of the large discount. Therefore I decided to remove it from the second hand market and redesign it in order to make it more portable and compact. I will soon move out to a new location at some point this summer, so the 16" will be used at home, whereas this 12" will be left at ambient temperature in the garage, ready to be picked up for short trips to the darker countryside.

Changes.. well many. I addressed all the components of the original telescope that I did not like much or that I would have liked to be be different. In contrast to the previous pages in this thread, in which I applied small but critical changes in order to fix the telescope making it functional, these new changes are radical..

Here we go: 

[Piero]

Mirror cell

Well, this is the part I disliked the most. Here's the original: 

 

When dismantling it, I also found out that the triangles are quite asymmetric with each other. Quite embarrassing really.   It is not that difficult even for an amateur to cut aluminium and make precise holes at the same position.

Apart from aesthetics, these errors affect back mirror support..  

See here:

 

Despite of being advertised as an excellent mirror cell, here is a summary of the problems for this design: 

1) sling cable positioned at 1/2 mirror thickness instead of at mirror COG. Also, cable was 1/4" thick, whereas it should have been 1/8" thick

2) sling cable passed through side stoppers, squeezing the mirror

3) triangles were asymmetrical (points in inner ring were 10mm off from what they should have been!)

4) triangles locked by screws whose threads blocked triangle floatation

5) triangle barycentre (holes of collimation bolts) not optimized by PLOP. Likely determined arbitrarily (looks like to be 1/2 radius)

6) 3 side stoppers blocked the mirror so tightly to cause serious aberrations

7) 2 panels of 18mm thick plywood caused the mirror to cool down incredibly slowly and in an unpaired manner. Also, the cooling fan was almost ~65mm away from the back of the mirror. This was too distant to be effective. 

 

 

Needless to say that his horror mirror cell had to go. Literally, I removed the metal from it and it went down the green bin as that is the place it belongs to. 

Edited June 3, 2022 by Piero