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Lukehurst-Nichol classic dobsonian modifications


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@BGazing

Thank you for the info. I was not aware of the ADC, and it seems interesting. Will certainly study it a bit more. 

For the time being, I decide to postpone the purchase of a coma corrector. After removing the sling due to its incorrect installation which caused a massive amount of astigmatism (it squeezed the mirror because it passed through the mirror edge supports and also the wrong installation did not allow it to remain parallel to the mirror plane), I noticed that there was still some astigmatism, this time due to the mirror edge supports which tightened the mirror too much. To fix this other issue, I had to cut off 1mm from the padding of each of the 3 edge supports. Now the mirror can move and rotate more freely, without being trapped by its supports. The primary axial alignment is still retained after using the telescope, so no collimation issues thankfully. Hopefully the glue used to attach the secondary mirror will not cause astigmatism this winter, otherwise I will have to fix that too. As astigmatism is more visible than coma (square vs linear), its solution had a priority. Said this, I haven't abandoned the idea of a coma corrector, but I'm not that keen towards the one sold by Televue. In addition, I have postponed the decision about selling my 20mm Lunt as I don't get along with 100 deg eyepieces very much (I find the huge field quite distracting). Generally, I move from the 30mm UFF to the 12.5mm docter and coma is not too much annoying. It might be that I get the Baader MPCC and use it with the 30mm UFF only. 🙄 

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Very happy with it! Wish the sky were clear, but as we all know, when we buy astro equipment, it's cloudy! Argh! The telescope was collected from David Lukehurst at noon and then we travelled bac

Last night the new dobson saw its first light!  In summary, I really enjoyed using it, it surpassed my expectations, and I will have a learning curve with it due to many new variables to me.

Yesterday, I managed a quick observation of Jupiter before the clouds covered the sky. Despite the average seeing, it was clear that the astigmatism was gone. The telescope works as when the cable sli

Posted Images

A while ago, I decided to spend some time in re-design the mirror cell supports of this telescope, in order to fix the issues of astigmatism and spherical aberration once for all. I have tested this new design on a few sessions in the field, with very positive results. The telescope delivered very crisp and details views of the moon up to 300x (limit caused by the seeing, really). Stars were puntiform and uniform. A lot of small faint stars were visible. Off-axis views have improved massively too. In addition, the telescope behaviour has become consistent between sessions and within the same session.

The main issue was due to the design of the mirror edge supports: too tall, thick, and wide. In particular, they:

  1. tightened the Glatter sling cable which squeezed the mirror, causing severe astigmatism. See Figure 1, but please, keep in mind that that installation of the Glatter sling is incorrect, as the sling should not pass through the edge supports;
  2. tightened the primary mirror too much, squeezing the mirror, causing astigmatism;

The second issue was due to the screws at the edge of the triangles. In particular the screw threading could block the triangle vertical movement. 

Here is a picture of the original installation:

IMG_20190728_162307.thumb.jpg.246b4d4271ba76695cee8019ad9895e8.jpg

 

 

My first intervention to solve the severe astigmatism issues was the removal of the Glatter sling. This largely reduced the astigmatism, but did not solve the issue. Therefore, I decided to cut 1mm from each edge support pads. While this reduced astigmatism a bit more, again, the problem was not yet solved. On the other hand, the reduction of astigmatism, highlighted the moderate presence of spherical over-correction. I spotted this spherical over-correction before as the diffraction rings were distinct with a bright external one outside focus, but not visible inside focus. Said this, the impact of this spherical over-correction on the views was overshadowed by the severe astigmatism before the support adjustments were applied.

In order to tackle the remaining astigmatism, on Gerry's suggestion ( @jetstream ) I removed the screws at the edge of each triangle. These screws were installed to prevent the triangle from rotating. However, the threading could cause the triangle to get stuck in a certain position causing mirror support impairment. Therefore, I design a simple structure from a milk carton and applied to the triangles. Milk carton is great because is rather strong, but still flexible. The triangles can move more freely now. The central nut was slightly loosened to allow even more vertical movement (which is now almost to the underneath wood plane). 

This work left some marginal astigmatism, but did not affect the presence of spherical over-correction.

IMG_20191026_182850.thumb.jpg.31d02374e4517841c638795c0123bfb0.jpg

 

IMG_20191208_163012.thumb.jpg.7dc014356ec4f9c3e208e6efaf99a33b.jpg

 

 

For a few months, no other modification was applied, as it was not clear to me what could have caused that visible spherical over-correction. The views were not satisfactory unfortunately. On-axis, bright stars appeared rather bloated, and faint stars were essentially not visible. Off-axis the views were much worse. Not being a telescope maker or an expert in optics, the only thing I could do was studying. One morning on the bus heading to work, J. Sidgwick gave me the information I needed: poor mirror central support can cause spherical aberration, due to the mirror sagging on itself.

Therefore, I took the mirror cell out again, and after a few tests, it was noticeable that despite being reduced in thickness, the edge supports could still prevent the mirror from returning to its original position after the telescope altitude was changed. In addition, if the mirror sagged towards the two edge supports at the bottom, more than half of the top mirror and most of the centre was not supported at all, despite of the triangle pins being very close to the glass.

Therefore, I decided to get some packs of felt pads generally used to reduce gaps underneath chairs/tables legs, and replace the current edge supports completely with these. The less padding surface was applied the more the mirror was equally supported by the underlying triangles. This raised other two problems: how much padding was necessary and at what height of the mirror the padding should being applied.

One of the features I wanted in this telescope was that the primary mirror was supported by a Glatter sling cable. On Google, there are many photos of the original Glatter sling design. The one originally installed on my mirror cell is different, in particular, the sling attachment do not move vertically upon mirror collimation. In most mirror cells, this could cause severe astigmatism as the sling would squeeze the mirror edge if too much collimation travel is applied. Thankfully, in my mirror cell, the triangles are fixed on a plane. Upon collimation, this plane moves all together. As the sling attachment are attached to this plane, the sling cable always horizontal to this plane which is pretty much horizontal to the mirror plane. 
Therefore, I decided to re-install the Glatter's sling cable, making sure that:

  • the sling was installed slightly under half mirror thickness and attached to the mirror edge with some velcro strips;

  • the sling does not hold more than half mirror circumference;

  • the distance between the mirror and each edge support was equal (mirror completely centred);

  • 2mm were left between the edge support pad and the mirror or sling able (mirror was able to freely move on the cell and that the edge supports as well as the top clips were only used for precaution, rather than actual support); 

 

IMG_20191208_162848.thumb.png.85a622ba6b62e8dff9025cda43d488fa.png
 

new_edge_supports.thumb.jpg.efc437a47c24040eede3a06db04c8b8a.jpg

 

 

Here is a photo after the clips have been put back on the mirror edge supports, and the mirror cell back into the mirror box.

 

IMG_20191207_153429.thumb.jpg.aeb03d3c992db76513d3195029601bbd.jpg

 

 

 

After a few tests, the mirror is adequately and proportionally supported by the underneath triangle pins. Primary mirror axial alignment is well retained during a session. I never felt the need to recollimate after changing azimuth and altitude several time. Most importantly, stars have shrunk in size massively and as a consequence have become much brighter. The secondary mirror spider spikes are now prominently visible on bright stars. Detail has significantly improved. In particular lots of very minor craters popped up easily on the lunar surface, as well as the contrast between shadows and bright surfaces was well defined. Light scatter control seemed also improved. In particular, the contrast between the Moon and the background sky was striking with the Docter at 150x.

I am very glad that the views through this telescope have largely improved. In the last few sessions, I have got some of the very best observations with plenty of details to keep me busy on few targets.

 

On a side note, I measured the laminated diameter of my mirror and this is ~300-301mm instead of the 305mm on the specs. Not a big deal really, but this makes my telescope a 300mm F6.04 instead of 305mm F5.94. Signature changed accordingly.

Hope you enjoyed this update and found it useful.

Piero

IMG_20191208_162848.jpg

Edited by Piero
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Great to hear you have got to the bottom of this @Piero. Seems a shame you've had to go to these lengths, though. I wonder how many people without your perseverance and expertise might have been left with a sub-optimally performing instrument. I fear I would have been one 🙂

 

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Very interesting read Piero. I agree with Jeremy that you have shown great determination to sort out these issues in a systematic way.

Is it possible to feed some of what you have learned during this excercise back to the maker of the scope so that they might consider some revisions to the primary cell design and execution ?

Many other owners, I suspect, would not have got to the bottom of these problems and might have either lived with mediocre performance or moved the scope on.

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Agreed, I think your feedback would be very useful and would hopefully lead to some design changes. Kudos to you for working through all the problems and getting them sorted.

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4 hours ago, Piero said:

In order to tackle the remaining astigmatism, on Gerry's suggestion

You mirror cell is looking more and more like my Astrosystems :grin:

I only related to Piero what I knew worked and why- the Astrosystems cell has many nuances that can be overlooked but careful examination reveals a well thought out cell that works in the real world.

Piero, Kudos to you for figuring out the mirror sag!

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Thanks guys for your comments. Yes, I spent a lot of time trying to figure out the source of the problems and this effort has been quite frustrating at some point. 

Whilst I think that every telescope is not perfect and can require some adjustment, it should still be functional. Unfortunately, this was not the case for this telescope. There are other things that could have been implemented in a different way, more in line with most of the other designs. For instance, the truss pole attachments, the plate holding both the focuser and the finder, the wood plane making the primary mirror cell, etc. At the time I asked for these changes, but these requests were simply refused.. Anyway, the issues here that compromised the function of the telescope could have been easily sorted out if the telescope had been tested before consignment. It is a shame because I offered to pay more and wait more time to make sure that the telescope was working fine. Again, this was refused.. Thankfully, these issues have been solved. 

I'm glad that Sidgwick wrote that page in his Amateur Astronomer's Handbook (a bit old, but still a great reading. Highly recommended like Suiter's star testing).

As the whole book can be read online, I think it is okay if I report that page here. If this is not possible, please, let me know and I will remove it.

IMG_20191009_182703.thumb.jpg.eeaf94a055bb6a78a3a423f018df60cd.jpg

Edited by Piero
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  • 1 month later...

Further tests on the Glatter sling cable. :)

There are several ways to install a Glatter sling. Here I will focus on the 180 deg (or "U-shape") installation. 

IMG_20200129_195643.thumb.jpg.98f1121daab2b5b7ef2c5aef03eac325.jpg

 

 

There are a few important aspects to pay attention: 

  1. the sling must be placed slightly below half of the mirror thickness (see "h" in figure). The exact value can be obtained here http://www.cruxis.com/scope/mirroredgecalculator.htm . For my telescope, this is 17.75mm.
  2. the sling must be attached with some vecro stripes, so that it remains in place. These stripes should be minimal in order to avoid mirror distortions.
  3. the mirror must exert the same force (see f1, f2, f3 in figure) on each bottom mirror support (e.g. triangles) and that this force is parallel to the mirror axis.
  4. the sling must be parallel to the mirror axis (see f4 in figure) or perpendicular to the force exerted on the bottom mirror support.
  5. the sling should not wrap the mirror more than half otherwise it exerts extra forces, which will result in astigmatism

 

The original Glatter sling solves (3) automatically, adopting rollers where the sling is attached. This and the velcro stripes guarantee that the sling remains parallel to the mirror plane. 

In my Lukehurst dobsonian, the sling attachments do not have rollers, but are fixed on the same wood platform where the mirror lays. This platform is regulated by the primary mirror collimation bolts. As a side effect, the sling attachment do not need to have rollers, but it becomes crucial that the sling attaches at the same height "h" (see figure), in order to remain parallel to the mirror plane. In order to check how the mirror lay on its back support after the sling is placed at the correct height "h", I placed the telescope horizontally and I laid down underneath the mirror box. A neckless can be helpful to evaluate that the mirror is exactly vertical. By increasing and decreasing a tiny bit the mirror altitude, the mirror moves towards and backwards its back supports (the pins of the triangles). In my case, it was clear that the pins on the top triangle did not touch the back of the mirror at the same time as the pins of the two bottom triangles. This meant that the mirror was not equally supported by the three triangles, and in particular less supported by the top triangle (about 4-5mm gap). To improve this, 1 adhesive pad (2mm) was added to the pins of each triangle so that the sling became more parallel to the mirror plane. 

The outcome was a noticeable improvement to the views, with further reduction of spherical aberration and astigmatism. Sirius B was visible at 250x without difficulty. E and F in Orion's Trapezium were also easily visible. Background stellar dust was noticeably more apparent. Even at low power the contrast was improved (e.g. Orion nebula). In addition, a few times while observing stars of medium-low brightness, I had the feeling to observe through a refractor rather than a Newtonian telescope.

Some traces of astigmatism and spherical aberration were still present, but I really had to look for it. This was expected though as I estimated that there is still 1-1.5mm gap left between the mirror and the pins of the top triangle when the former started touching the pins of the two lower triangles (see video).

 

In general, I am very happy with this result and the mirror and telescope are working better than my expectations! The next step is to calculate the exact gap and either replace the existing nylon pins with longer ones or add another pad to the pin to raise the primary mirror a little bit more. No doubt, I am going to fix to completely! 

 

 

Edited by Piero
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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.

Edited by jetstream
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That is excellent info Piero. My Sumerian has a similar sling and I'm sure it is not setup correctly. I need to get it out and have a good look at it. Will refer back to your thread! Thanks.

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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.

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

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 ?

 

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23 minutes ago, John said:

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

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.

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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.

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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.

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I was just thinking that with the positioning of the sling around the primary being so critical in terms of where it sits around the mirror edge, the pressure it exerts on the support points beneath the mirror, remaining perpendicular to the optical axis etc, etc, ideally the sling would move together with the mirror and mirror cell if the primary collimation is adjusted to avoid changing the position or angle of the sling :icon_scratch:

But perhaps such movements are so slight that they would make no meaningful difference ?

 

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9 minutes ago, John said:

I was just thinking that with the positioning of the sling around the primary being so critical in terms of where it sits around the mirror edge, the pressure it exerts on the support points beneath the mirror, remaining perpendicular to the optical axis etc, etc, ideally the sling would move together with the mirror and mirror cell if the primary collimation is adjusted to avoid changing the position or angle of the sling :icon_scratch:

But perhaps such movements are so slight that they would make no meaningful difference ?

 

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?

 

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1 hour ago, jetstream said:

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.

Thanks Gerry. That's an interesting idea as it minimise the amount of strips, and therefore potential side effects from these. I should do the same. At what angle are these two cruces attached to the mirror?

 

 

1 hour ago, John said:

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

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 ?

Thanks John. 

Yes, in my telescope the collimation bolts adjust the alignment of a wood base on which lay the triangles, mirror, sling attachment, sling and edge supports (for safety). 

This is not generally the case though. In almost all mirror cell designs I've seen, the collimation bolts move the triangles up or down. I think the range can be up to 1 inch. The sling attachments are fixed (they don't move with the mirror), hence the rollers invented by Glatter so that the sling always remains parallel to the mirror plane independently of collimation adjustments. The clips on the lateral edge supports are for safety. The edge supports are for safety too. They don't touch the sling - there is actually a gap of 1-2mm between them.i copied this last setting in my mirror cell.

 

 

54 minutes ago, jetstream said:

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.

In the original Glatter sling, there isn't any sweet spot because the sling height follows the mirror height (and most importantly the mirror plane).

If the sling is attached with eyebolts like in your mirror box, yes, there is a sweet spot for sure. To check this you need to make sure that the cable is parallel to the mirror and perpendicular to its attachments when the mirror is collimated. If you keep the mirror height like that, it should work well. If you raise the mirror too hight or low, the mirror will slide a bit on the triangle supports and could not be equally supported by these. There is also a chance that astigmatism is introduced due to friction of the pins of the triangles on the mirror.

Spherical aberration is also likely. If you raise the mirror too high, the sling position will go slightly down on the mirror side. Therefore it will squeeze the lower part of the edge, making the mirror figure hyperbolic (overcorrection). If the mirror height is too low, the sling cable can slight a bit too high, changing the paraboloidal mirror figure to ellipsoidal (undercorrection).

 

 

45 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.

 

As I said above, I don't think this is a problem at all. Just make sure not to ease the mirror so that this touches the clips. How are the sling supports attached to the structure of your telescope? Can the sling move up and down following the mirror plain upon collimation or are they fixed? Also is there a bit of gap between the mirror edge supports and the sling?

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1 hour ago, John said:

I was just thinking that with the positioning of the sling around the primary being so critical in terms of where it sits around the mirror edge, the pressure it exerts on the support points beneath the mirror, remaining perpendicular to the optical axis etc, etc, ideally the sling would move together with the mirror and mirror cell if the primary collimation is adjusted to avoid changing the position or angle of the sling :icon_scratch:

But perhaps such movements are so slight that they would make no meaningful difference ?

 

 

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.

post-5464-0-32050900-1514139093.jpg

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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.

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3 hours ago, jetstream said:

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.

 

Very low altitude, probably 5 degrees. You need to raise the telescope very gently. 

I laid down with the mirror box above me, and then pulled it towards me very gently, raising the telescope. The video shows this. The mirror touched the pins of the 2 lower triangles, but the telescope needed to be raised another bit before the mirror was supported by the pins of the top triangle. Adding 1 pad reduced this gap, although there is still a little gap left. 

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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.

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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.

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      Next pieces to get will be WA 6mm and 15mm.
      There's a small mark on the main mirror, but it doesn't seem to affect the view, and at that price I can't complain. The base also needed some attention. Vertical movement is smooth, but horizontal rotation was hindered by the bottom board being slightly bent, and rubbing the corners against the telescope base. I have added some washers between the teflon pads and it's much better. Not buttery-smooth, but I feel like that would make it too sensitive to movement, In the current condition it lets me track Jupiter easily. I guess the real test will be when I get the 6mm piece
      Collimating laser is also coming to make sure the mirrors are aligned after haling it home for 40 miles.
      Other than that it doesn't seem like it needs any serious work, so for a 20-30 year old telescope this is amazing.






    • By Harshn
      Hi all
      I purchased a skywatcher 200p dobsonion from first light optics a few weeks back. During assembly I found that the package containing side handle and the tension control handle is missing. I have contacted the first light optics but they are not sure when and how they are going to make it available for me.
      Does anyone of you have these to spare??
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