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Lukehurst-Nichol classic dobsonian modifications


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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

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I am still experimenting with this Dobson. Last night I slightly loosened the glatter sling as my suspect was that it was too tight. Still early to confirm this, but the result was noticeable.

On Friday, I will receive the socket to be installed at the end of the new cable. With this, the fan can be turned on and help reduce temperature differentials on the primary mirror.

Last night, I collimated the focuser and primary mirror axes using the 1mm attachment. This made collimation much more precise in my opinion. This attachment is meant to be removed when collimating the primary mirror axis. Dunno.. I think the device is more precise with it attached....

Result: a superb view of Jupiter!

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10 hours ago, Piero said:

Last night I slightly loosened the glatter sling

Just curious, why loosen? was the mirror pulled up contacting the stops at times?

Do you have velcro on the mirror edge to keep the sling positioned in the center of the edge? If you rack the focuser in and out with the laser installed does the laser dot move ?

Awesome you are getting great views Piero. Man, I just counted my questions...sorry lol!

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5 minutes ago, jetstream said:

Just curious, why loosen? was the mirror pulled up contacting the stops at times?

Do you have velcro on the mirror edge to keep the sling positioned in the center of the edge? If you rack the focuser in and out with the laser installed does the laser dot move ?

Awesome you are getting great views Piero. Man, I just counted my questions...sorry lol!

No, it was not touching the clips and no, I do not have velcro on the mirror edge. The sling stays in position pretty well. The laser dot remains in the correct position when racking the focuser in and out. 

My concerns are more due to astigmatism which I saw in the previous sessions. I am tackling the potential causes, e.g. imperfect collimation, too tight sling, air fluxes, light shroud, mirror cooling. 

In the last two sessions I made very good progress. Yesterday there was no astigmatism at all, and Jupiter was really gorgeous to observe.

When the fan will be used, I will have another bit of improvement. This due to the current star pattern showing minor cooling issues on the outer circle. 

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I believe the sling was just too tight, but have to check this better. Possibly it was just air turbulence. In any case, being rather loosened last night it does not seem very critical for this 12" scope. Still happy to have it though.

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Just now, Piero said:

I believe the sling was just too tight, but have to check this better. Possibly it was just air turbulence.

Unless the sling wraps the mirror instead of 1/2 or so cradling it there should be no issues with restraint causing astig. Another possible cause of transient astig can be secondary mirror restraint and as you know astig during cooling. I was told that centering the sling in the center of the mirror edge eliminates potential issues, but this may or may not apply always.

Does your sling "squeeze" the mirror?

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1 hour ago, jetstream said:

Unless the sling wraps the mirror instead of 1/2 or so cradling it there should be no issues with restraint causing astig. Another possible cause of transient astig can be secondary mirror restraint and as you know astig during cooling. I was told that centering the sling in the center of the mirror edge eliminates potential issues, but this may or may not apply always.

Does your sling "squeeze" the mirror?

 

Oh man, so hot today! 

So....

The sling seems installed correctly to me and it is placed right in the middle of the mirror edge.

In the first two sessions, I could see astigmatism on off-axis and on-axis stars also at low power (60x, 90x). In the second session, the star pattern was a bit better, but astigmatism was still visible in the lower part of the diffraction rings (about 1/3 of the circle). That lower part coincides with the sling position and the main entrance at the bottom of the mirror box where air can flow into it. In both these two sessions, the light shroud was used and closed completely. Seeing was average. Collimation was done without the 1mm aperture stop attachment.

In the third session, I did not use the light shroud and the astigmatism at the bottom of the star pattern was noticeably reduced. Seeing was still average and collimation was performed like before.

Last session (yesterday), the light shroud was not used, seeing was very good, collimation was done using the 1mm aperture stop attachment, and the Glatter's sling was slightly loosened in order to exclude sling-dependent astigmatism. The astigmatism at the bottom of the star pattern was completely gone. The views were very good - I could see the GRS hollow with the Docter 12.5mm + VIP barlow ~2x (~300x). Only a minor residue was detectable (an inconstant flickering) on the outer border of the star pattern. To me this also excludes secondary mirror-dependent astigmatism due to strong glue otherwise I would have seen this on axis. The visible residual astigmatism (flickering) seems due to mirror cooling in my opinion. The fan will test this. Air turbulence can also be excluded due to the lack of light shroud. 

 

This Friday I will receive the socket for the fan, so that I can test its effect. My prediction is that the fan will reduce that flickering as well as improve air circulation. This is somehow expected considering that warm air is almost trapped by the wood structure (very different from a steel / aluminium tube!). With the fan on, I want to try to tight the Glatter's sling as it was before, and see whether this causes astigmatism or not. If it does not, it means that that astigmatism was mostly due to mirror cooling and air turbulence. Regarding the light shroud, there are many advantages in using one - in particular it prevents something from falling onto the primary mirror. Thankfully, I can still use it and just leave it partially open on the bottom of the dobson (my shroud has some kind of "buttons" which can be left open). Anyway, this will be tested. 

Thoughts?

Edited by Piero
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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...

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41 minutes ago, 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...

 

Thanks for your reply, Gerry.

I haven't rotated the mirror nor take it out from the cell. 

In the first two sessions, the astigmatism was the classical tangential on one side, sagittal on the other side. 

In third session astigmatism was skewed towards one side, a kind of half astigmatism. Of course in these three sessions, the degree of astigmatism increased off axis towards the edge. 

Yesterday, there was no astigmatism in the star pattern. Just a minor variable flickering on the external circle. The thickness of this flickering was about like one ring. 

After the view I had yesterday, I am not worried at all, to be honest. Actually, I'm quite excited to see how the views will be when the mirror is uniformly cooled with the fan. I only need more sessions to get acquainted with this telescope and learn how it works under different conditions. :)

Do you still use your Catseye tools or just the Glatter's?

I'm working out my way on the many excellent posts by Jason D. A new book is also coming..

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5 minutes ago, Piero said:

Do you still use your Catseye tools or just the Glatter's?

I use Glatters laser for secondary alignment on the primary dot and in general the TuBlg. There can be an increase in accuracy with the Catseye cheshire over the Tublug - the cheshire is more consistent. For critical lunar/planetary the cheshire goes in and also the autocollimator at times.

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The socket arrived today and I've already installed it at the end of the cable.

The fan can be turned on, very silent and vibration free. :)

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Interesting reading this.

Your scope seems quite "particular" Piero. But maybe these are just teething issues ?.

When I think about how little care I take over collimation (above a quick tweak at the start of the session) and the rather basic primary cell with my 12" F/5.3, it's slightly embrassing :rolleyes2:

I have a fan behind the primary but I never use it. No sling either - just the older OO 3-point cell.

When you say that you are seeing astimatism, do you mean an elongated airy disk / diffraction rings on one side or something else ?

 

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20 hours ago, John said:

Interesting reading this.

Your scope seems quite "particular" Piero. But maybe these are just teething issues ?.

When I think about how little care I take over collimation (above a quick tweak at the start of the session) and the rather basic primary cell with my 12" F/5.3, it's slightly embrassing :rolleyes2:

I have a fan behind the primary but I never use it. No sling either - just the older OO 3-point cell.

When you say that you are seeing astimatism, do you mean an elongated airy disk / diffraction rings on one side or something else ?

 

Hi John, 

I would say that there are a few additional variables compared to a solid tube dobson, plus my level of inexperience. What I am trying out is to isolate each potential issue and maintain an unbiased attitude (as much as possible) to the causes of the problem.

The potential issues causing astigmatism (described here: https://stargazerslounge.com/topic/337091-new-dobson-arrived/?do=findComment&comment=3682534) I can think of are:

  1. poor seeing
  2. thermal issues due to primary mirror cooling
  3. zone of warm air trapped around the primary mirror, instead of dissipating (including light-shroud)
  4. secondary mirror attachment (glue)
  5. poor collimation
  6. Glatter's sling
  7. primary mirror support (mirror cell)
  8. three clamps preventing the primary mirror from falling 
  9. mirror figuring

I can exclude

  • (4) because last time the telescope worked really well as testified by a great view of Jupiter and star testing on Vega 
  • (5) because I checked the position of the donut on the primary mirror (it's precise) and I paid considerable attention on this. Result: see (4)
  • (7) for the same reason stated in (4)
  • (8) because I checked that there is a small gap between the clamps and the primary mirror. 
  • (9) for the same reason stated in (4)

My current thought, but I try to be open minded as possible, is that the real causes are mirror cooling (2) and trapped warm air (3). I am not convinced that the Glatter's sling is one issue. After checking this more carefully, It seems to me that it was tight in terms of supporting rather than squeezing the primary mirror.

As far as I can see (2) and (3) work differently between wood truss and solid tubes. In a solid tube, the whole tube cools down, and the mirror is separated from the back environment by its cell only. In my telescope, the whole wood structure "wraps" the primary mirror considerably and wood is a bad conductor. In the 2 photo posted in my first thread, you can see that there is an open area at the bottom of the mirror box. My thought is that area helps the mirror cooling down, but the part of the mirror which is opposite to it will cool down more slowly. Hence, thermal differentials around the mirror surface. I read a fair amount of comments on CN by people having classic dobsonians and the problem seems very similar to mine. They solved thermal issues using fans, also left on through the whole session. It would make sense as fans would not only cool but flow away warm air trapped around the primary mirror.

Edited by Piero
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@Piero I would still try rotating the primary to see if it (astig) rotates with it, just a thought...

Your mirror is thin and should equalize right away so the chance of cooling astig is small unless there is residual strain in the glass, but the chance of this is minute.

What does a very high power view look like that opens up the airy disk?

Edited by jetstream
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On 23/07/2019 at 17:56, jetstream said:

Unless the sling wraps the mirror instead of 1/2 or so cradling it there should be no issues with restraint causing astig. Another possible cause of transient astig can be secondary mirror restraint and as you know astig during cooling. I was told that centering the sling in the center of the mirror edge eliminates potential issues, but this may or may not apply always.

Does your sling "squeeze" the mirror?

should be set at the c of g gerry not the centre of the mirror edge 😁

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Just now, faulksy said:

should be set at the c of g gerry not the centre of the mirror edge 😁

Yes,great point Mike! and I'm very thankful that my mirrors aren't fussy to need the extra precision!

How about OOUK cells that let the mirror rest on the clip (supports!) your former Whiffletree cell is optimum I think, but a well adjusted cable sling works well with thick enough mirrors IMHO.

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1 hour ago, jetstream said:

@Piero I would still try rotating the primary to see if it (astig) rotates with it, just a thought...

Your mirror is thin and should equalize right away so the chance of cooling astig is small unless there is residual strain in the glass, but the chance of this is minute.

What does a very high power view look like that opens up the airy disk?

Hi Gerry,

I will rotate the mirror by 90 degrees and see if this has any effect. 

The thickness is 1.45", not very thick but not thin either.

I haven't yet tried a very high power observation due to seeing conditions and limited time due to work commitments.

Last time the airy disc and diffraction rings looked very good as described previously, but this was only at 150x.

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2 hours ago, Piero said:

.... In a solid tube, the whole tube cools down, and the mirror is separated from the back environment by its cell only. In my telescope, the whole wood structure "wraps" the primary mirror considerably and wood is a bad conductor. In the 2 photo posted in my first thread, you can see that there is an open area at the bottom of the mirror box. My thought is that area helps the mirror cooling down, but the part of the mirror which is opposite to it will cool down more slowly. Hence, thermal differentials around the mirror surface. I read a fair amount of comments on CN by people having classic dobsonians and the problem seems very similar to mine. They solved thermal issues using fans, also left on through the whole session. It would make sense as fans would not only cool but flow away warm air trapped around the primary mirror.

There could be something in that, I agree.

The rear of my primary is very open due to the simple cell design so it cools evenly I guess:

 

oo12cell.jpg

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I agree with the idea of leaving a fan on to break up the air above the mirror... you can switch the fan off and watch how the image quality changes without it. I've read of some scopes having fans blowing across the face of the primary for this reason.

I also wonder whether option 4 secondary issues could still be a contender if the figure of the secondary may vary with temperature or rate of change of temperature.

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Just a thought Piero - are the triangles in the mirror cell all freely pivoting/floating?

I had one 'acorn nut' on a pivot point which was just a little too tight, and stopped the triangle floating.  I loosened it a fraction of a turn and voila!

In my case i had noticed a tiny bit of collimation shift because the mirror wasn't resting right.  When i loosened the nut just a smidgen, all was well.

I am just wondering with a thinner mirror (mine is 2") could something like this cause a little deformation in the mirror - like astigmatism...?

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27 minutes ago, Paz said:

I agree with the idea of leaving a fan on to break up the air above the mirror... you can switch the fan off and watch how the image quality changes without it. I've read of some scopes having fans blowing across the face of the primary for this reason.

I also wonder whether option 4 secondary issues could still be a contender if the figure of the secondary may vary with temperature or rate of change of temperature.

Thank you for your reply, Paz.

I'm not an expert, so my answer should be taken with a grain of salt. Yes the figure of the secondary mirror can change with temperature, but the mirror is considerably smaller and thinner than the primary mirror. I doubt this requires a long time to stabilise. As far as I know secondary mirror can cause astigmatism in the optical system when they are glued to the holder by a large amount of glue or silicon rather than three small points. The issue arises because in a large amount the outer part of the glue gets thicker much earlier than the inner part. In the process the glue can slightly deform the mirror, causing astigmatism. This astigmatism is visible on axis and in the whole field. If this happens, the mirror must be taken apart from its holder, removing the glue and repeat the attachment.

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16 minutes ago, niallk said:

Just a thought Piero - are the triangles in the mirror cell all freely pivoting/floating?

I had one 'acorn nut' on a pivot point which was just a little too tight, and stopped the triangle floating.  I loosened it a fraction of a turn and voila!

In my case i had noticed a tiny bit of collimation shift because the mirror wasn't resting right.  When i loosened the nut just a smidgen, all was well.

I am just wondering with a thinner mirror (mine is 2") could something like this cause a little deformation in the mirror - like astigmatism...?

Thanks Niall,

I haven't taken the mirror cell out of the mirror box, but I think yours is a good point. 

As far as I know, if the mirror gets somehow stuck in its mirror cell (e.g. due to excessive heat, frost, or same position for a long time) this can cause astigmatism as the mirror is essentially deformed. The primary mirror should be able to rotate easily without friction in its mirror cell. If the problem won't be solved with the fan, I will take the mirror cell out and see.

Regarding the collimation shift, I don't see how this would cause astigmatism though. 

Apart from the autocollimator technique which is for very fine adjustments, the three things to collimate are:

- secondary mirror position and tilt so that it faces the focuser

- focuser axial alignment to the center of the primary mirror 

- primary axial alignment to the secondary.

In case of miscollimation, the first one causes an uneven illuminated field, the second one causes differential focus positions across the field, and the third one shows on-axis coma.

In star test collimation, one aims to remove that residue of visible on-axial coma using a high power eyepiece. This because the airy disc is larger and so the comatic tail is more apparent.

 

Astigmatism is due to optical deformations or issues in the optics themselves.

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