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Piero

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Posts posted by Piero

  1. Just now, Ken82 said:

    I think the 1mm attachment comes as standard ? Is that the white disk fitted to the front ? 

    Yes, that is the one. See: https://www.firstlightoptics.com/howie-glatter-collimation-tools/howie-glatter-single-beam-2-inch-laser.html 

    There are other attachments for more exotic collimations, but the 1mm is the one you need really.

    I have the 650nm version which is fine for collimation in the night. You can use the 635nm which is fine for day time collimation, or night time using a brightness reducer. 

  2. I would recommend a 8" f6 Dobson as an all-around telescope.

    The skywatcher version is good and rather cheap, but if you can stretch it a bit, the Bresser version is mechanically much better.

     

    Orion optics UK... I would only consider it in the second hand market. Even then, I would probably go for something else.

    • Like 1
  3. My minimalistic 2" set.

    As I am moving fast, a paracorr2 was added a few months ago. Flo sent me the 3" version (that's a beast!) by mistake, but then all was sorted. It might be that I will let go the tublug at some point as I collimate with the laser only. Thoughts for the future.

    Left to right: Zeiss zoom, APM UFF 30mm, Docter, 2" HG laser 650nm + tublug. The VIP barlow is above the Docter. I use it with 0mm, 15mm, or 40mm spacers. The two boxes include 2" Lumicon OIII and 2" NPB filters

    IMG_20210109_122932.thumb.jpg.46261fdf44e959363afbba0690a5d61c.jpg

    • Like 7
  4. 16 hours ago, Space Hopper said:

    Back to the topic of the thread : my favourites ?

    Still my Tv Panoptic 24's. The nicest eyepieces i've ever used. My 'Goldilocks' pair !! 😀  One of them was purchased used as well.

    Even when i used them with an F4 300mm Newtonian i could see very little Coma, they are so well corrected.

    Coma is an aberration present in telescopes using paraboloid mirrors. It is not present in modern / half decent eyepieces. An F4 Newtonian has a coma free linear radius of 0.022*4^3 = 1.408mm . This defines the area in which the Strehl of a perfect mirror is 0.80 or better.. Also, variations of the mentioned formula generally use a more stringent factor than 0.022. Considering a field stop of 27mm for the 24mm TV Panoptic, coma is expected to be present and easily detectable in an F4 newtonian telescope. Also, the fact that Televue eyepieces are highly corrected to the edge, will make the detection of coma even simpler as this is less likely to be mixed with other aberrations present in the eyepiece, like astigmatism (although a poorly supported and / or unevenly cooled mirror can show astigmatism, spherical aberration, and have other side effects..).

    TBH, I can see coma in my 12" F6, when observing off axis using eyepieces with more than 50-60 deg AFOV. In this telescope coma is not terrible to ruin the views but it is there. For refractor like views, a coma corrector is necessary in a fast / medium Newtonian using paraboloid mirrors. Without coma corrector, faint stars at the edge can just be invisible, or better said, visible but the presence of coma smears their faint light on a larger surface which make them undetectable.

    • Like 3
  5. In `Observational Astronomy for Amateurs` Sidgwick mentioned magnifications of at least 400x for critical lunar observing. Assuming crisps views, his comment can offer an idea of what kind of minimal aperture he meant!

    Again, I think we all have different tastes on this. At mags over 400x, one can seriously study the moon for years, due to the small image scale. Said this, some of the very best views of the moon I had were when it was full, slightly above the horizon, and using my TV-60 at 15x. In particular, the Moon was coming up from bushes and tree branches with a bird or two crossing the FOV time to time. Yes, it was 100% artistic and 0% scientific, but oh man, how beautiful it was to my eye! :wub:

    • Like 5
  6. Binoviewing with two docters is a rather expensive experience.. I don't binoview though.

     

    I compared my docter against a Delos 12mm and I preferred the former one. You can find that thread on CN forum. So, for me it does justify the cost. Besides, I bought mine in the second hand market from a member of this forum.

     

    Anyway, I don't have many eyepieces as a large set doesn't fit with my way of observing. For others, it is the opposite and owning an eyepiece between two magnifications (or FOVs) is important. For others, 100 deg afov eyepieces are cool, for me they are a distraction, whereas others love orthos. Regarding the docter specifically, some dislike the angular magnification distortion (AMD) that this eyepiece shows, whereas others (like me) like the fact that it balances some RD with AMD. That's why we have so many favourite eyepieces, and this kind of topic is quite hot on astro forums. P.s. just realised that I managed to write all this without even mentioning the black and green's! LOL

    • Like 4
  7. My favourite eyepiece is the 12.5mm Docter UWA. Combined with my modified Baader VIP barlow I get an equivalent ~7mm or ~5mm, depending on target and mood.

    Another eyepiece I love and use quite a lot is the 30mm APM UFF. 

    • Like 5
  8. I think it all depends on the kind of objects one wants to observe.

    With my TV-60, I've spotted/observed about 250-300 targets. I intentionally wrote spotted/observed, because a few of those targets can only be spotted on that telescope given the limited aperture. Said this, it has been a great learning curve. In contrast to a larger telescope, a small wide field refractor can let one find many objects, which means the knowledge of the sky increases considerably. 

    There are so many classes of objects that that telescope doesn't / cannot show well though. My 100mm Tak is a bit better on this, but it is still rather limited. I don't want to sound dismissing about small apertures though. They do have a lot of potential. 

    To me, the class of telescopes whose future is uncertain is the long focal length / small aperture, like the classic 60/70mm F10-F15 acromats, which were popular in the past. I see these more like piece of nice furniture, but nowadays quite limited in the field. In contrast, a 60 ED short tube shows similar optical quality, but it is much more portable, possibly lighter, and can show a massive (!) FOV with normal 1.25" eyepieces.

    • Like 3
  9. Indoor, I enjoy using Interstellarium and Bright Star Atlas 2000.0 . Outdoor, I generally use SkySafari Pro or Stellarium apps on a tablet. The only "paper" tool I use in the field are the Herschel 400 guides by Alvin Huey (see: http://www.faintfuzzies.com/DownloadableObservingGuides2.html, but a printed copy can be purchased and delivered at home) . When using these guides, I focus on one single constellation for the whole session. 

     

    • Like 3
  10. 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)..

    • Like 1
  11. On 07/12/2020 at 09:05, CraigT82 said:

    I'm considering installing a glatter sling in my 12" f/6 dob.  Looking ta the actual Howie Glatter product it comes with two posts and sliders that the ends of the cable attached to, which I believe is to allow the cable to move with the mirror during collimation, so that the cable always stays perfectly parallel with the flat plane of the mirror as it tilts.

    On my dob the posts for attaching the cable ends to could be attached to the base board of the cell, and the whole lot would move during collimation, hence negating the need for the sliders on the posts? Am I right in thinking that?

    Yes, you can do that. You could install a metal bar on the that mirror wood panel to provide a support for the two shafts where the sling cable can be installed. On a 12" mirror, a 1/16" cable is fine. 

    The difficulty is that you need to make sure that the height of the sling cable is at the COG of the mirror. 

    The bearings in the Howie Glatter's design are there for this reason and because they are installed on the mirror cell of a Kriege's design, which is different from the David's design. 

    Here is a tool for calculating the mirror COG: https://www.cruxis.com/scope/mirroredgecalculator.htm .

     

    On 07/12/2020 at 09:40, markse68 said:

    Hi Craig, I’d have thought it’d be better to put the posts on the main mirror box so it takes all the weight of the mirror. If you do it like you’re thinking all the weight of the mirror would cause a lateral force and bending moment on those collimation studs which could cause lateral movement of the mirror as the scope moves in alt axis.

    Mark

    That's a reason why I don't particularly like the collimation of the mirror panel, instead of the mirror itself. 

    In any case the problem you mention is not caused by the sling. As it is right now, the problem is present. Said this, if the bolts are large enough and remain firm when threaded, it is fine. My dobson (another Lukehurst) is very similar and retains collimation well (tested this with Glatter's laser at 2x and Catseyes tools).

    • Thanks 1
  12. 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.

    • Like 1
  13. 19 hours ago, CraigT82 said:

    For anyone interested found some really good info on Howie Glatter slings in Piero's thread about his Lukehurst dob...

     

     

    Just to clarify, David Lukehurst installed a sling cable which is not the Glatter's design. 

    David's sling works ONLY IF it is installed on the same plane of the mirror and if the cable is mounted parallel to the COG of the mirror. If these conditions are not satisfied, astigmatism will be visible. 

    In the telescope in that thread, the first condition was present, whereas the second was not (the cable was placed at half mirror height..). I reached the second condition by changing the back mirror supports, effectively raising the mirror height as much as necessary. The other issue that I found is that David installed 3 strong edge supports (unnecessary really as the sling is meant to be the mirror edge support). These additional edge supports tightened the mirror, causing astigmatism and spherical overcorrection as they also affected the way the mirror was supported by the triangles... The description and solution is in that thread. Thankfully it works well now.

     

     

    • Thanks 1
  14. To me most of the time, the "atmosphere" 1 inch on and above the mirror related to improper cooling, uneven mirror temperature in general, poor mechanics (e.g. mirror back and lateral support, the way the secondary mirror is mounted, the way the focuser drawtube moves, etc), often so and so optics, and poor collimation are at the basis of limited magnifications in larger telescopes, rather than the actual atmosphere.

    To the OP. That's a great project. One warning though as you mentioned finances. Even if you make the whole thing, including the mirrors, the overall cost will still be noticeable.

    • Thanks 1
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