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rfdesigner

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Posts posted by rfdesigner

  1. For reference, Multi-coated means the anti-reflectance works well over a range of wavelengths.  single coated means it will work nearly perfectly at one wavelength then deteriorate away from that

    i.e. green is excellent, deep red and deep blue rather less so.

    • Like 1
  2. Nice capture, M33 is hard.

    I always fractionally unbalance and misalign the polar so as to maintain a very slight drift and keep the gears meshed.

    There appears to be a little colour speckle noise when viewed at full resolution, is this hot pixels or just the top of the background noise?..  though even printed A4 you're not going to see this, only if you wanted to print bigger than that...  which if fully polished I'm sure this could do.

    well done

    Derek

  3. what do you expect to do?

    Travel / only use from home

    Planets / Deep sky or both

    Image / View  (Imaging is going to cost much more than £150)

    I would normally advise a basic lightweight newt..  generally you get best bang for buck with them, don't bother with putting much cash into the mount, you'll end up throwing away any mount at that price level (usable deep sky imaging mounts start near £1000)

    A nice portable scope will get used and used, even if you end up with £10k worth of imaging rig.

    Derek

    • Like 2
  4. Here's a stretched master dark frame from my KAF8300 based camera, as you can see it's a touch snowy..  but that's OK,   it doesn't really matter if you have just one hot pixel or hundreds, you should be running some kind of hot pixel mitigation, either a hot pixel repair algorithm on each sub prior to stacking or a statistical stack or both.

    snowstorm.jpg

    post-8988-0-27047000-1407746322_thumb.jp

  5. Harvey

    Thanks.. in fact I have played with PLOP..   and with my mirror, due to the thickness I can just about get away with just a 3 point cell (which is just as well as that's what it has at the moment).. but I'll go for a 9 point if I'm making...  the key as I see it is to use enough material to avoid excessive flexure and to make it sufficiently accurately to get the pressure even.  I'm guessing half a dozen pixels of image shift from a meridian flip would be about the right level of stiffness to aim for.

    Very good news on the worm and crown..   I will look forward to seeing your post.

    Derek

  6. When you say you're concerned about weight, are you referring to not enough weight of too much? If too much weight you could remove considerable material from the back plate without losing strength Derek :)

    I actually find the ally parts of my cell are pretty light, it's the mild steel that quickly adds considerable weight.

    I want to keep weight down.  The primary mirror is 50mm thick and the window is 18mm thick.. being a 12" scope that's a lot of glass.

    I'll have to do some stress analysis and find out how much I can get away with before I see noticeable image shift as the scope flips.

    Derek,

    No, but a stepper motor is probably the best solution. it works by taking an image and measuring the Half Flux Radius of the stars in the whole image.

    HFD..  ah.. is it based on the Focusmax software?

    The focuser I have is a JMI crayford with the motofocus option.. swapping out the motor for a stepper is as much work as adding some kind of a rotary encoder (for me the mechanics is the hard bit, I find the electronics is relatively easy)

    JMI do offer some kind of a position sensor setup, but again I can see the bank account suffering...........       thinking about it though, it'll still be cheaper than a Carbon Fibre tube.  hmmmm.

  7. You won't need a mill or a lathe to make a mirror cell. I built mine from aluminium and mild Steel :)

    I could do an ally one, I was concerned about the weight though.. especially if I have a bottom plate that is solid except for the fan holes, I suppose I could use a thin frame then bolt on a thin sheet to seal it.

    My current favourite program, Sequence Generator pro, will automate focusing during imaging runss. You can set it to re-focus every x number of subs, on filter changes or on temperature changes.

    With regards to the worm, are you not auto-guiding? Auto-guiding will correct for periodic error in the worm, especially if it's nice and slow. You could also investigate PEC training and correction?

    I'm auto guiding but the worm's a bit rough and my focus is drifting, so where I should be getting 2 arc sec FWHM round stars, I can get nearer 4"x3", which is about 3" of defocus and about 2" of drive "thrashing" (I have had 2"x2" but only at very high declinations, near the equator it's pretty bad.  I've tried programming in a PEC algorithum, but I only have a 4 minute main frequency, no faster ones as my steppers drive the worms directly, all the rest of the thrashing is worm/wheel roughness.

    I've had a quick look at Sequence Generator Pro.. does it require a position sensor on the focuser?..  it seems to do pretty much everything else, nice program.

    Derek

  8. No takers?

    hmm..  an update on my own efforts

    Q1: Materials: I've looked at nylon and carbon fibre, both have drawbacks...  nylon is much easier to work with but will end up heavier and the last thing I need is extra weight, CF is a pain to fabricate.. I'm after lightweight but workable, any other ideas?

    Q2: Venting: I've realised I have another option, keep it sealed, but install a fan to keep the air in the tube well mixed..  thus I accept the air could be somewhat hotter than ambient, but at least it's all the same. (within 1C)

    Q3: Autofocus: the more look into this the more I realise I need a position sensor, a rotational encoder on the focusser knob looks like the way to go..   does no one do autofocus?

    Derek

  9. @John, I certainly didn't imply that members here lacked experience, far from it. My comment was that only one respondent had used this particular approach.

    @ Dan, thanks. I am planning to drill the hole. I've been around equipment most of my working life. Subject to change, however.

    Right now, I'm starting to consider secondary support designs. I've seen several designs in articles about DIY curved spiders. Not too fond of any, yet.

    I'd like to make the window support fairly easy to remove, so the whole assy could be removed for cleaning and service if needed. Wish I knew someone with a 3D printer...;-)

    After reading reviews of the MN190, I'm going to diligently stay on a weight saving regime.

    The window itself will most likely be a pre-made pre-multi-coated item off the shelf from Edmund.

    Can I suggest starting by just trying the proposed window and leaving your spider in place, just cap the tube..no centre hole, KISS.

    Cutting the hole could release a strain, and give you a false negative.  As an engineer I like to take single steps, especially on "research" projects, which arguably this is.  One unknown at a time..   Just take the piece of glass, hold it over the top and see if you still have a telescope or a light scattering tool, also means you can do a with/without test under identical conditions.

    If it succeeds, cut the hole, and test it..  if that succeeds, mount the secondary in the hole, and so on..  

    One step at a time..

    In my case David Sindon ground mine..  I knew it would work, this is cheaper but you're taking a risk, if it doesn't go right it's worth knowing exactly what caused the failure in case you want a second go.

    Good luck

    • Like 4
  10. Since ball bearings were mentioned somewhere as a possible alternative to slip rings, I tested one of mine.  The resistance varied from around an ohm up to 4 ohms.  Could be a possibility for signals but too high for power.  Myth busted :D

    When I had my own lab I used to measure the resistance of the ESD hand straps to confirm they were working.. sometimes I'd get an open circuit as the connectors were not too good, so I used a spot of graphited grease on them and the contact was much better, perhaps with ball bearings using a graphite grease would also help to improve consistancy.

    just a thought

    • Like 1
  11. Nice project.

    Have you considered getting the tracks gold plated.  Gold plating is commonly used in the electronics industry as it aids soldering and prevents oxidisation so maintains contact impedance over time.   There are different grades available with some being specifically for contacts, not just soldering.   It needn't be expensive.

    Derek

    (electronics R&D by day)

    • Like 1
  12. Additionally, I just remembered, Glass refractive index varies across a piece, when grinding you are trying to make the optical thickness an equal number of wavelengths at all points in the window, that is not necessarily a constant thickness, although you'd never notice just by looking.

    point is: making sure both sides are flat isn't enough.

  13. Just to to all this.

    I own a 12" windowed newt.   The window was not cheap.

    I had the window ground to special order many years ago and it was a little more than a comparable mirror, but not as heavy as it can be thinner.  (mine is 18mm thick, I wouldn't want thinner as it would warp and bow causing optical problems)..   the biggest advantage is that any surface eddies in the air at the top of the window have only half the optical effect as those above a mirror as the light passes that point only once.  With the rest of the scope sealed and settled this can improve high resolution image quality..  but if it's all warm it takes much more time to cool down.

    If I was starting over again I would put all the money into a better/bigger OTA..  bang for buck conventional newts are very hard to beat.  That said, it does work fairly well on controlling tube currents once cooled down so it's best to leave it in an obsy but suffers like most SCTs from dewing (unlike newts).

    Derek

    • Like 1
  14. Hi

    I'm hoping a few here might have seen my problems in the past and know of suitable solutions.

    I've been analysing my telescope woes and finally managed to identify the order of problems affecting me.  I'm measuring performance in imaging terms, so I don't just care about resolution, but also about ability to hold focus, refrain from dewing up suffering image shift etc.  Also my OTA is fairly unique, a Windowed Newtonian.  Whilst it has performed fairly well there are drawbacks which I'm trying to fix.

    Problem No.1:  Dewing. 

    The window is isolated from the tube wall by about 10mm of rubber.  A dew heater is going to have a jolly hard time so I've never bothered, preferring dew shields and hair dryers...  additionally a 14" diameter window is probably going to respond differently to dew heaters compared to say an 8" SCT.

    Ideally I want to unseal the tube, pump filtered air through the whole thing, around the sides of the window, thus eliminating the hair dryer and maximising air temperature homogeneity.   To do that I need a flat plate across the bottom, with fan holes and so on, but that's extra weight so I'm thinking of replacing the rather weighty cell, then I can support the cell parts on a flat plate thus combining parts so saving weight.

    I'm thinking something along these lines but not in wood due to water absorption (not mine, just a helpful pic I found on the net to illustrate):

    cell.jpg

    So QUESTION 1:  what could I make it out of, and where do I get that material?  (Mirror is 300mm x 50mm thick)..  the only thing I've seen commercially is the Orion Optics mirror cells but they're £280+vat..   PS whilst I'm pretty handy at making things, I don't have a milling machine or lathe, just hand tools and a pillar drill.

    QUESTION 2:  Window Cell.  Given I'm venting the tube from the bottom I need to change the window cell to allow air past it, any ideas?..  just supporting the window on three blocks won't do as the tube will warp allowing the window to drop through, I've been thinking of using 6 supports in 3 pairs, each pair fixed to the tube by a mid point pivot, but that's going to restrict airflow along the length of each pair of supports, unless someone can suggest a way of getting the air past them

    Problem No.2:  Holding Focus. 

    Analysing my subs I'm getting roughly 100ppm tube expansion over an imaging session..I'm guessing that's over a 5C temperature change which is only 20ppm/C which is pretty good..   but not good enough (it spreads my stars over 3x the area they should be losing sensitivity)  The real solution is a carbon fibre tube but that's serious money.  An alternative would be some kind of autofocus solution, something that could check and correct focus between subs.  I have a JMI crayford with motor drive on the scope already, but it doesn't have a position counter of any sort..   so QUESTION 3: Is there a way to do autofocus as part of an imaging session?   is this Maxim territory and what other hardware am I looking at?..   again without breaking the bank.

    Problem No.3:  Worm drive unevenness

    I know I have a certain degree of high frequency thrashing going on with the worm, hardly a surprise with a 40 year old mount.   But it is a bronze worm wheel and stainless steel worm, any worth in polishing this (I'm assuming using something somewhat smoother than silvo as the polish)

    I'm still working on the main problem I've had:   no obsy, and it's really taking all my time at the moment as I'm making something to last longer than I do:

    here's a photo of the latest:

    Frame.JPG

    That's the south face, nearest beam is the sill.  This was just making sure all the joints would fit properly.   Foundations and brickwork are done.

    anyway thanks in advance for any suggestions.

    Derek

  15. Hi

    I use Phd2 when imaging.  With both Phd and Phd2 they only accept the lodestar camera in 1x1 binning mode, which makes download times around 200ms.

    If like me your mount is a bit thrashy a short guide time is advantageous, so a 500ms integration time becomes 700ms once download is included.  The shorter the guide time the more this is important (ie for AO).  So I raised a request to have Phd2 able to do hardware binning:  2x2 would give roughly 50ms download time, which should be a noticeable advantage for both AO and thrashy mounts like mine.

    Today I received an email that my request has been accepted and so we can expect this to be implemented at some, as yet undefined point.

    I am of course deeply grateful to the Phd2 guys...  

    Derek

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