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Piero

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Posts posted by Piero

  1. 5 hours ago, John said:

    Out of interest, if you had to dispense with paracorr, how would that affect your choices ?

    That's a good question.

    It took me a bit to answer the OP's question, because the whole selection of 5 eyepieces somehow depends on the telescope.

    The eyepiece set I mentioned cannot be used with my TV60, so that telescope would go.

    As you noticed, the paracorr2 has an impact too. I'm not sure whether I would use an f4 dobson with a set of <= 50 deg AFOV eyepieces because a coma corrector is not available. Therefore that fast telescope would go too.

    The two remaining telescopes I have are a 12" f6 dob and the 4" tak f7.4. With these telescopes I very much like the minimal set: 30mm APM UFF, Docter, Zeiss Zoom 25.1-6.7mm and Baader VIP Barlow. The OP said barlows are not allowed though (ouch)! Mm.. I could go for the 30mm APM UFF and the 4 short focal lengths of Pentax XW. However, the limit of 5 eyepieces does not play well as a medium power is left out.. therefore, between the two telescopes the refractor would go. With only the dobson left, I could just use the 30mm APM UFF, docter, Zeiss zoom and skip the Barlow.

    A completely different approach could be to use 24 Pan, 13-7-5-3.5mm Nagler T6 and keep all telescopes apart from the f4 one or accept views "full of comets". Planetary observing would be a bit sacrificed with this set though..

    Somehow all this reasoning shows that if one has more telescopes, more eyepieces are likely needed too.

    • Like 1
    • Thanks 1
  2. I recently received a kineoptics HC-2 focuser (https://www.kineoptics.com/HC-2.html) shipped from the US to replace the Antares helical focuser currently fitted on my 12" f6 dob (aka Phoenix). This new focuser should allow me to use non-parfocal eyepieces more efficiently - my plan is to use my 30mm APM UFF and Zeiss zoom +/- VIP in addition to my small set of 24Pan and 3 naglers T6.

    I will soon receive an aluminium plate so that I can create a custom base for this new focuser (to shorten the story, I cannot change the UTA plywood panel and the new focuser mounting holes kind of overlap with the current drawtube hole on the plywood panel so I need to make a custom base).

    I also received a couple of metal bars to update the counterweight system so that this will be neater and a bit more flexible. These bars are removable and will be attached to the mirror cell directly. They will also compensate for the extra weight of the 2" eyepieces I intend to use.

    After fitting the new focuser, I will need to cut the truss poles, slightly decreasing the height of the telescope.

    • Like 6
  3. I use a 2" Howie Glatter (650nm) with 1mm aperture stop attachment and collimate both focuser and primary, also matching the 1/4" white triangle mark on the primary against the concentric rings projected by the laser onto the attachment surface.

    The tublug is used sometimes, when I am tired.

    Other times, I use a 2" Catseye telecat for both focuser and primary alignments. This tool is also used for positioning the secondary correctly. This is easy for me as I don't value the theory of "benefitting" from an excessively small central obstruction at all.

    • Like 1
  4. Well, I'm probably the black sheep here but I much prefer modern designs with more eye relief and wider fov for all targets TBH.

    For observing more details, I use a telescope with larger aperture.

    I used to have a few Vixen HRs and they were excellent, although, as said above, 42 deg afov feels too restrictive to my eye.

    • Like 4
  5. For a while I had both 2" Baader BBHS mirror and prism diagonals. They were used with my 4" tak f7.4.

    None of them showed more details than the other, but the prism showed some chromatic aberration on that telescope and so I let it go.

    The prism also seemed to take a considerable longer time to cool down.

  6. 40 minutes ago, PeterStudz said:

    Where did you buy yours from? I think that like me you are in the UK. I looked for embossed FRP but the places I found didn’t offer small-ish sections like and/or charged a lot for postage. Eg here…

    https://www.whitecladding.co.uk/product/white-embossed-frp-fibreglass-wall-protection-panel/

    The smallest size is £55.99. But add VAT and “economy” postage of £30 the total comes to £103.

     

    I bought it from http://www.beplas.com/ and it was a sheet larger than 500x500mm. The extra material has been used in other projects. Considering the cost of building a dobson + mirror, this felt rather cheap. It is also something I don't want to compromise.

    • Like 1
  7. Agree with Gerry (jetstream) above.

    "Bright DSOs" is used for "the brightest DSOs given aperture and sky darkness".

    The vast majority of targets in this hobby require a certain amount of "study" and "re-visits" to be appreciated. It takes dedication and patience, a bit like fishing.. 🙂

    "Bright targets" is also used for objects that can be spotted relatively easily, but they cannot really be observed. For instance, I spotted a few planetary nebulae with my TV60 under moderately polluted bright skies with or without filters. However, excluding a very limited number of bright and large PNs (e.g. M27, M57) this remains a tick exercise (and a few members have this kind of approach with this hobby) using small apertures. I really started appreciating them using at least moderate powers (at least 250-300x and much more) with my 16". That is when some structure starts showing up and therefore the target can be studied.

    • Like 3
  8. On 28/05/2023 at 13:49, Mutley said:

    The unpaited plywood pier you see is fixed to the back of the moving plane of the mirror cell, just so I had somewhere to mount the brackets for the sling support. The sling itself I bought pre-fabricated from here : https://www.gsproducts.co.uk/

    Thanks to all of you for all the helpful advice and information. Now off to do some painting and think about how to implement a cooling soultion.

     

    Thanks for sharing the photos of your telescope and mirror cell.

    My thought:

    • Looks like you need a spanner to collimate the primary mirror. In more recent versions, David uses metal wing nuts which work on that kind of cell. Of course this is not required but it would allow you not to use an additional tool.
    • The first thing I would do to improve the cooling is to drill, using a hole saw, where you have that marking point at the centre of the triangle plane of the mirror cell. To make a clean cut at the back, I would clamp some wood to the other side so that the wood does not split. Following that, I would install a fan (80mm is fine if it runs at good speed - you could also opt for 100mm, depending on how much space you have, considering the collimation bolts). Having said that, this requires some thought on where to install this fan as it does not seem that you have much space between the triangle panel and the bottom panel of the mirror box. One way could be to install it to the back of the triangle panel, but this depends on whether the gap between the two panels is sufficient. A possibly neater way is to make a groove to the bottom panel of the mirror box (outside side) and install it there. It looks like that panel is quite tick. Doing so, the gap size between this panel and the triangle panel does not really matter. I have a feeling that your fan positioned at the front of the mirror box improves the views, by cooling down the mirror. As it is positioned to front of the mirror, it will blow away the boundary layer, but my feeling is that this is a secondary effect really. How long does it last? I haven't done the maths, but I would imagine a mirror of that thickness takes about 1h to cool down to ambient temperature. I keep the fan of my 16" on all night as the temperature decreases. However, I reduce the speed once the mirror is at ambient temperature, so that the views are not affected by the fan being on, whilst the mirror temperature continues dropping following the decrease in ambient temperature.
    • If possible, you should consider painting the additional wood panel supporting the sling. The sling angle brackets should also be sanded coarsely, primed and painted matt black to avoid reflections coming back towards the secondary mirror. This will improve contrast a bit. I would do this later once everything is fixed though.
    • was that black ring structure at the edge of the mirror installed to fix a turned down edge issue?
    • Is your telescope f4 or even faster?  In the third photo, the centre of the sling cable seems quite low. Now, I would imagine it to be visibly lower than half mirror thickness as your primary mirror is fast, but from the photo it seems lower than what I would have imagined. Just to double check, is the centre of the cable really at the COG of the mirror? Unfortunately, that installation of the cable follows David's design and the biggest downside is that the height of the cable is fixed. Therefore, to regulate it to the COG of the mirror, the only way left (maintaining the existing installation) is to trim/raise the pads. This would allow you to decrease the mirror height and therefore raise the position of the cable - assuming that the cable is positioned a bit lower than the COG. If the cable is positioned below the COG of the primary mirror, the effective mirror figure will not be parabolic as it should and this will cause spherical aberration. Depending on whether it is position higher or lower than the COG, you will have an overcorrection or undercorrection effect. You might also see astigmatism if the cable is positioned at slightly different heights around the mirror from the back of the mirror.
  9. 6 hours ago, Littleguy80 said:

    Thanks Piero. I felt sure I’d read somewhere that the Taks do better with prism diagonals. That was one of the reasons I tried the prism. I wonder if getting the BBHS mirror version would remove the CA on Venus. I’m not sure why Venus seemed more prone to CA than even the moon. 

    I wouldn't base all my CA tests on Venus. It is a challenging target: it is low (so the atmosphere plays a role in the views), incredibly bright, incredibly white, low contrast and smallish.

    The best views of Venus I have had, are still with my TV60 + single polarising filter..

    The Tak fc-100 is a doublet refractor. It reduces CA at super low levels, but it cannot fully correct it. I also have some doubts about fixing this using a prism as prisms slightly overcorrect, so whilst "potentially" fixing the blue, the introduced overcorrection would affect the other two colours.

    I cannot quantify how much CA you have detected in your tak based on a post here, but I would observe a range of targets to asses this properly.

    Said that, my tak FC 100 DF (now FT!) shows a touch of CA on very bright targets, but this is well controlled. I do not have a problem admitting this - perfection does not exist, but it is very often reported as people desire, even ending up believing, that their tool is perfect and can do better than the rest. Similar story about members claiming that their tak 100mm shows views which can be observed with at least 10-12" telescopes.

    • Like 3
    • Thanks 1
  10. At some point I had both 2" bbhs prism and mirror diagonals.

    I sold the prism as it showed CA, whist the mirror did not. The mirror version also cooled down a bit faster. In terms of details, again I prefer the mirror as the views seem more natural.

    Lots of members prefer the prism though.. 

    • Thanks 1
  11. :thumbsup:

    Some observations:

    - nylon bolt vs felt pad. Felt pads can stick to the back of the mirror as well as get squashed. I used heavy duty felt pads on my 16" dob and they don't stick. Also the back of my 16" Lightholder mirror is smooth, so any friction is minimal. I used nylon bolts as back supports for my 12" Nichol mirror (which is not back smooth). In the original cell, I replaced the nylon bolts with pads and did not find any difference. Probably, where I live, it is not particularly humid. Dunno.

    - the reason for the "cut" at the bottom / front of the mirror box is avoid that the box hits the central bolt connecting the rocker box to the ground board. Therefore that cut enables to reduce the height of the RB side panels, and therefore the telescope, by a couple of inches. This is well documented in telescope making books. This "cut" is not particularly high. Unfortunately, this other maker ignored the whole principle and used that opening for sliding out the mirror with its cell. The idea is crazy and a bad one too. It is crazy because it is much easier to access the mirror and its cell by rotation (2 locks, 2 pivots - see Astrosystems mirror cell). It is a bad idea because it exposes part of the mirror to colder air than the the internal side of the mirror. A minor breeze can cause astigmatism. Before redesigning my 12" (new MC, etc) I installed a panel of plywood closing that bottom / front cut on the MB. The astrosystems mirror cell does not suffer from this issue because the bottom front opening is still shallow (so air does not hit the mirror side) and it has two holes at the back. This enables a steady flow of air.

    - side holes / fans are not added for cooling the mirror down, but to prevent the formation of warmer air above the mirror surface. To cool the mirror down faster, the most effective way is to install a fan to the back of the primary mirror. I would imagine that your mirror is F4 or similar, so the mirror box is not particularly deep. Therefore, lifting up the bottom of the light shroud might be sufficient to prevent the formation of air boundary layer (BTW my original light shroud was made by him and broke down within 6 months). I would recommend to get a copy of the book "the dobsonian telescope" by Kriege. There have been many advances to those ideas, but the design is simple and effective. It contains lots of way to redesign components. 

  12. 16 hours ago, Mutley said:

    Yes, pointed the dob up at about 45 degrees and hooked the fan over the front of the mirror box so the airflow was over the mirror face. Left it there running for about 2 hours.  It seemed to do some good - much better star test. Practically speaking, easily split Izar, which was just a mess before I came across your original thread, and copied some of your improvements (part of the reason I started down this route was discovering the dob was being outclassed by my 4" refractor on this particular double).

    I could but you probably wouldn't see much. The 2008 Lukehurst dob was quite a bit different from your model. There were no fans anywhere. The mirror cell is surrounded by solid ply, apart from the slot at the front of the mirror box for getting the mirror in and out.  Also, there was no wire sling. Lateral support was from 2 posts at 45 degrees either side of the centre line. Cork lined. This was for a 20inch mirror, 31mm edge thickness. It didn't work great.

    I'll probably take it to bits again after I've had a look at the 1st quarter moon this coming weekend. I'll take some photos of the mirror cell then so you can view my handywork. No laughing.

     

    So, if I understand this correctly, the bottom of your mirror box is fully closed with a plywood panel which forms the support of the mirror cell. How do you take the primary mirror out (e.g. for cleaning)?

    Note that in the original MC/MB of my 12":

    - the fan at the back of the mirror was ineffective as too distant from the back of the mirror and only blew on one part of it.

    - the 3 additional ventilation holes were ineffective (I closed them for testing and there was no difference)

    - the bottom/front opening, using his words "for taking out the MC" caused a lot of thermal issues and the side of the mirror was exposed.

    - his sling is implemented incorrectly

    - same for the 3 side mirror stops

    - the triangles were nearly paralised and asymmetrical between each other.

     

    Your mirror is particularly thin, meaning that it requires excellent support (back and size) to work well. What I don't understand is whether you have replaced his MC with your own, as I did, or, if not, what changes to the MC you have applied.

  13. Did you direct the fan to the front of the mirror (e.g. from the top of the mirror box)? 

    Would it be possible for you to take some photos of:

    - mirror box

    - mirror from the top of the mirror box

    - mirror from the back of the mirror box (ideally showing the mirror cell)

    - some photos of the mirror cell as fitted in the mirror box (no need to take it out)

     

    Fans do help in my opinion, particularly the one fitted at the back of the mirror. The side fans are not for cooling the mirror down, but for avoiding the formation of 1-2" of slightly warmer air released by the mirror as this cools down. Wiping this out improves the views.  

  14. 21 hours ago, Mutley said:

    Thanks for the suggestions, I'll have a read of the S&T article Merlin.  I have a David Lukehurst dob, circa 2008. Piero I read your thread about the extensive modifications you made to your Lukehurst. So much of it was familiar to me ( In a bad way). I've since made some similar modifications to the mirror cell, and converted to a sling support. My 2008 model used 2 posts for lateral support. The star tests were awful in the unmodified scope, and to be honest I had no idea why. I even contacted the mirror maker to confirm it was genuinely one of theirs (Oldham optical -it was genuine). Better now with the sling/ furniture pads/removal of those bolts for restraint of floating triangles (see Piero's thread). But still much room for improvement and I'm wondering if remaining problems are thermal in origin. Like the passive cooling idea for boundary layer. Certainly worth a try before fitting fans.

    Piero if we ever meet I think I owe you a pint.

    I see now..

    Sorry to hear that you have been experiencing similar issues to mine. I really wonder how many of his telescopes share the same problems - my guess is many unfortunately.

    Like you, I also contacted the maker of my mirror (John Nichol) to confirm that it was really made by him. It was before doing the large telescope redesign as I wanted to be sure that the mirrors were authentic. It is a shame that a customer has to go through so many issues. It isn't just about testing and some improvements. It is about real fixes needed due to poor and incorrect design choices. I feel a lot of people don't understand this.. these are the problems that can ruin a hobby because the whole time is invested dealing with tricky issues (a few of them unresolvable unless the component is redesigned), instead of enjoying observations.

    I am not a telescope maker nor a wood/metal worker, but I spend quite a lot of time reading and understanding issues and solutions. The work I did on my two dobsons is nothing of innovative, but contains some of the best designs from talented engineers that I could implement with my basic skills and tools.

    For unknown reasons (I raised the suggestion but no reply), SGL does not allow one to have more than one link in the signature. Here is the link to the album containing the redesign of my 12" Lukehurst dob (now called Phoenix as the bird reborn from its ashes..). I don't even called Lukehurst dob because a) it is significantly different, b) it works now, whilst the original telescope did not, and c) I don't want to remember the nightmare I experienced with it due to that maker. 

    https://photos.app.goo.gl/qPdsWRQUiqxmm9ec9 

     

    Whether in this thread or via PM, let me know if you would like me to share details about the changes. Happy to help.

     

    From your original post, I didn't understand you had a Lukehurst telescope. I don't know in detail how far you have gone with improvements. Issues caused by boundary layer are very minor compared the rest. I would make sure that all the other issues are fixed before investing time and resources on improving the boundary layer.

  15. In my 8" skywatcher dob the lock bolts were removed when the new springs were installed.

    Originally, my 12" truss dob had heavy duty springs and they held collimation well. That mirror cell was replaced by a new one due to other issues and the latter does not have springs at all. Collimation is done via rotation of bolts which push up / pull down the mirror.

    Old mirror cell with springs:

    IMG_20200201_121514.thumb.jpg.ed9169ff93ebda5bb38ec4e69f95d2f8.jpg

     

    New mirror cell during telescope making:

    image.thumb.jpg.10e9ffd6d4ac28127340998ba6d85def.jpg

    IMG_20220430_124706.thumb.jpg.8925ae8d601f8d99bdcdad5ff0e5160e.jpg

     

    Same cell design (but more supporting points) for my 16":

    IMG_20200809_124656.thumb.jpg.47c4852301ce639f73f1f2a43d3ace13.jpg

    image.thumb.jpg.ca7c5cd431986e026eb9de8060f784f2.jpg

    • Like 2
  16. 8 hours ago, Ratlet said:

    What pickles my brain is that they use cheap spring then they add lock screws to help it keep collimation.  From my understanding, with decent springs you don't need to lock screws.  Surely the springs cost less than an extra 3 bolts?  Or am I wrong on that?

    Correct. With good springs, the lock bolts can be removed.

    I agree with the posts above that commercial telescopes should be fitted with decent springs from the factory. 

    • Like 2
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