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Piero

Lukehurst-Nichol classic dobsonian modifications

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1 hour ago, Piero said:

They support the primary mirror. They look identical to those installed on the triangles of Orion Optics mirror cells.

I meant the metal ones- one on each triangle- is this what you meant?

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9 hours ago, jetstream said:

I meant the metal ones- one on each triangle- is this what you meant?

The screw at the centre attaches the triangle to the underneath platform. The lateral screw prevents the triangle from rotating more than a certain amount, which I haven't measured (5mm -ish?). Vertically, they can move much more (possibly 20mm). Again, I have not measured this.

Edited by Piero
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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 sling was loosened. Good!

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Do you think it was the sling itself or the way that it was implemented that was causing the astig issues Piero ?

Either way, I'm glad you have got to the bottom of it.

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8 hours ago, John said:

Do you think it was the sling itself or the way that it was implemented that was causing the astig issues Piero ?

Either way, I'm glad you have got to the bottom of it.

Good question. 

In my opinion, the way it was installed would have caused astigmatism independently of any adjustment.

In my limited knowledge about this, a requirement for a sling to work correctly  is that it must be parallel to the mirror plane. This is the basis. Then there are different configurations as Gerry showed some post above (e.g. 180 Deg, 90 deg..). In my case the sling was passing through the two mirror edge supports which are attached to the mirror cell and don't necessarily follow the mirror plane at different altitudes. Therefore when the telescope changed altitudes, those supports tightened the sling which eventually sequeezed the mirror edge, causing astigmatism. To work, the sling should have been detached by those supports and a gap of a few mm should have been left between sling and edge/bearing. 

I could have done this, but didn't want to risk with other potential problems related to the sling that I haven't yet foreseen. Knowing that it was not required for the correct functioning of this telescope, I thought it was safer to simply use the current edge supports and remove the sling.

Edited by Piero
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Mate, thats a schmick classy looking scope.. enjoy... of course you're looking at weeks of cloudy weather from now on....

 

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The new finder arrived!

I didn't know it was so versatile. Out of curiosity, I swapped the RACI prism with my TV everbrite diagonal and 24 Pan. It reached focus without any issue using the helical focuser near the objective! I tried in daytime and optically speaking, it seems excellent for a sub F4 acromat refractor. I'm curious to test how much magnification this finder can take...

This will be the finder for both my Dobson and Tak, as I plan to attach it to the other arm of my Aok Ayo2.

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Last night the sky was good here in Cambridge. 

I took the telescope out quite late (at about 11pm). After a couple of collimation rounds, I pointed at Saturn. For the whole session, the fan was on. It was incredible how Saturn was getting better and better at the eyepiece as soon as the mirror was cooling down. Seriously, the improvement was observable. After about 15 minutes, Saturn was showing some lovely details. The Cassini division was visible almost all over the rings. To be honest, it wasn't just visible, it was a nice black lane. In the lateral part of the rings ("wings"), some differences in contrast were also detectable. The planet showed a nice and distinct equatorial belt with again faint differences in colour in the north hemisphere. For sure, the visible moons were: Titan, Dione, Rhea, Encelados, and Thetys. These were checked with the Sky and Telescope website. However, there were a few other "objects" which were not so dissimilar from those moons. Without a map for those, I cannot confirm whether they were faint stars or some of them was a moon.

After Saturn, I observed M15, Double Cluster, NGC 7331 (C30 - galaxy in Pegasus), and one of my favourite open cluster: NGC 457 (Cassiopeia). 

It was the first time I observed these targets with this aperture. Absolutely impressive views. Very happy about this. :) 

 

---

Astigmatism? A memory of the past! Removing the sling really solved that issue. :) 

New finder works nicely. I particularly like its long dew shield and the fact that I can rotate the eyepiece.

Edited by Piero
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Thanks Jeremy! :)

it worked really well last night. 

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Great to hear, Piero.  Sounds like the fans made a big difference; I’ve never used mine, but I wonder whether it might be worth plugging them in.

How did you find the Antares? 

I’m up north this weekend so unfortunately missed the clear (and finally dark!) skies!

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2 hours ago, FenlandPaul said:

Great to hear, Piero.  Sounds like the fans made a big difference; I’ve never used mine, but I wonder whether it might be worth plugging them in.

How did you find the Antares? 

I’m up north this weekend so unfortunately missed the clear (and finally dark!) skies!

Hi Paul, :)

the big difference was due to the lack of cable sling. The fan helps this telescope cool down and keep constant temperature very well in my opinion. I believe this is due to the wood structure and the mirror cell that is not much exposed. In a split tube dobsonian, possibly the fan is less effective, but I don't have experience with this really. 

The new Antares finder works really well. Thank you for the suggestion. As I said previously, the longer dew shield is certainly an advantage in delaying dew formation. I could easily spot M15 as well as other DSOs in the finder directly. :)

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Replaced the black (why black?!) doughnut with a 1/4" Catseye white triangle.

It is considerably easier to see and the triangle silhouette allows me to align the edges to the primary mirror collimation bolts.

After carefully removing the central spot as described on the Catseye website I attached the triangle using their template (image 1 and 2).

Then I collimated the focuser (image 3) and the primary mirror axes. The latter using the tuBlug (image 4).

Alternatively, both the axial alignments can also be checked on the 1mm aperture stop attachment (image 5 and 6).

Finally, I checked the 2" Howie Glatter laser collimation with my new 2" Catseye Telecat XL (image 7). :)

The Telecat was also used for checking the secondary mirror tilt before starting collimation.

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Very impressive collimation tools Piero and a very exacting approach to getting things spot on :smiley:

Makes me a little embarrassed with my cheap plastic cheshire approach :rolleyes2:

 

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1 hour ago, John said:

Very impressive collimation tools Piero and a very exacting approach to getting things spot on :smiley:

Makes me a little embarrassed with my cheap plastic cheshire approach :rolleyes2:

Thank you, John. :)

I'm still in the process of getting acquainted with this telescope. Due to many new components, there's a bit of learning curve, but it's getting there, I feel. 

I have just come inside from a session with this telescope. Just a few open clusters in Lacerta and the blue snowball nebula in Andromeda. :)

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2 hours ago, John said:

Very impressive collimation tools Piero and a very exacting approach to getting things spot on :smiley:

Makes me a little embarrassed with my cheap plastic cheshire approach :rolleyes2:

 

Maybe thats why you didn't find the Leica zoom/VIP better than your Ethos!:grin:

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19 hours ago, jetstream said:

Maybe thats why you didn't find the Leica zoom/VIP better than your Ethos!:grin:

If thats the case it undermines most of the eyepiece reviews that I've posted on here because I've used the 12 inch for most of them :rolleyes2:

 

 

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Lovely looking ‘scope. Hope you enjoy using it!

That ‘wheelbarrow’ arrangement for moving it around is a nifty idea.

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6 hours ago, Tomjo59 said:

Lovely looking ‘scope. Hope you enjoy using it!

That ‘wheelbarrow’ arrangement for moving it around is a nifty idea.

Thank you!

That certainly helps. It would be very difficult to move the telescope around otherwise!

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I found out the "correct" size of my secondary mirror! 

Minor axis: 57.15mm
Major axis: 85.725mm

It was supposed to be 62mm minor axis.

Anyway, with the new correct measure, the central obstruction is 18.73% . On a positive note, contrast should be better.

Using the laser I could also notice that the secondary is currently not offset along the mirror axis and that it's about 2mm titled. Not a big deal as my larger ep field stop is 38mm, but I will adjust it when back home.

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secondary_mirror_rotation_2.jpg.e4371ca6e5cf2563150816c3c4598897.jpg

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Does your secondary holder allow for adjustment of the secondary offset?

Just interested - I've an Astrosystems one, and it doesn't!

For my f-ratio (4.5), the compromise involved, and the fact that it runs at a low 17.3% CO are small beer for visual according to Dave Kriege.  I've seen several threads on CN discussing it in depth.  For better or for worse, I don't worry about it;)

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Bryan Greer of Protostar opinion on secondary offset. 

“When you get to the final collimation step of tilt adjusting the primary, you are pointing the primary back at the optical center of the secondary. Once again, the light cone will be intercepted perfectly symmetrically. The only consequence of not offsetting is that the primary mirror is no longer pointed exactly down the middle of the tube. It will end up oh-so-slightly tilted up towards the focuser. But this causes no optical problems at all, as there is nothing sacred about having a perfect 90-degree reflection at the secondary. Collimation, vignetting symmetry, and concentricity of the optical axis within the focuser drawtube are all still perfect.”

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53 minutes ago, niallk said:

Does your secondary holder allow for adjustment of the secondary offset?

Just interested - I've an Astrosystems one, and it doesn't!

For my f-ratio (4.5), the compromise involved, and the fact that it runs at a low 17.3% CO are small beer for visual according to Dave Kriege.  I've seen several threads on CN discussing it in depth.  For better or for worse, I don't worry about it;)

I can move the sec mirror closer to the primary mirror along the axis, but cannot slide the secondary mirror along the 45 degrees axis. I would not do the latter anyway.

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48 minutes ago, johninderby said:

Bryan Greer of Protostar opinion on secondary offset. 

“When you get to the final collimation step of tilt adjusting the primary, you are pointing the primary back at the optical center of the secondary. Once again, the light cone will be intercepted perfectly symmetrically. The only consequence of not offsetting is that the primary mirror is no longer pointed exactly down the middle of the tube. It will end up oh-so-slightly tilted up towards the focuser. But this causes no optical problems at all, as there is nothing sacred about having a perfect 90-degree reflection at the secondary. Collimation, vignetting symmetry, and concentricity of the optical axis within the focuser drawtube are all still perfect.”

If it's not offset, the downside could be an unbalanced field illumination. This is generally more critical in imaging than visual.

There's nothing perfect in this job (collimation), just a requirement to remain within tolerances.

Edited by Piero

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Congrats on your new scope, I have only now stumbled upon this thread. Seems like we caught the dob fever in succession. :)

It also looks like you held off Paracorr, which makes sense given the f/6 mirror. Just don't borrow one, you might like the look, I was told.

I see that you are observing planets, and they happen to be quite low at the moment. I found dispersion to be objectionable to the point that I could not enjoy the views with planets so low without the ZWO ADC. Effects of dispersion get progressively worse with aperture, it is always present, at 100mm at my latitude the views with or without are night and day, SCT 8 the difference is astonishing. I compared C8 with ADC with 12 inch dob without and it was a no-contest. I would say that even a 100mm refractor with ADC would show more than a big dob under effects of massive dispersion. ADC even makes seeing look better because it cleans up the smear.

So to enjoy planets in the coming years to the best extent, Jove and Saturn in particular, I suggest getting ZWO ADC. Its use is a bit more tricky with a dob (see online tutorial on lining up with the horizon), and you would have to use a 2.5 powermate to bring your f number higher and avoid astigmatism. But trust me, you need it for such massive aperture with planets so low. Just make sure to have 1.25 eyepiece in 2.5x more focal length than you would otherwise have...(e.g. in my Tak DF I use PM 2.5, ADC then 13mm instead of the usual 5mm for Jupiter). You may use it for your Tak as well, although you might need to buy additional EP or two...

 

Edited by BGazing
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