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digital_davem

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Everything posted by digital_davem

  1. I've never managed to find M31 despite hours of looking. I suspect it's a hopeless cause from London.
  2. The threaded rod, nuts and washers have arrived. The centre column is safely dispensed with and the head is bolted directly to the tripod legs. First impressions are that it has made a difference. The telescope rings, arca-swiss mounting plate and quick release plate-to-head connections seem solid. The head-to-legs connection is now very solid without any detectable wobble. The only obvious source of flex is now the legs themselves. 30mm tubes are reasonably hunky for a portable field camera tripod but distinctly anorexic by the standards of astronomical tripods; can't do anything about that without investing in a 'proper' tripod. Anyway for now, I shall play around with it as it is while I await the arrival of the longer threaded rod and the steel pipe. Even if I have to go for the astro legs in the end, I'll still have got myself a pretty solid camera tripod....
  3. A quick look on ebay and I can get a 60mm diameter 3mm thick wall stainless steel 30cm tube for £11.00 incl postage. 60mm is the limit for my tripod. I'll try it first without the tube and if dispensing with the centre column improves things, I'll give this a go. Cheers Dave
  4. That looks pretty much what I was thinking of. Nice set up! I think i'll try a similar arrangement with my tripod first. I have this table leg from Ikea that is basically a steel tube about the right diameter to fit the legs casting and the head baseplate. I just need to cut it to the right length and pass a threaded bolt through it. The question is, how long dare I make the tube without compromising stability? If the tripod isn't hefty enough after all that, I think finding the EQ6 would be a plan - where would I get one from though? Thanks very much for the suggestions - in line with what I was thinking
  5. Hi It's a 3" 1250mm FL long-tube metal OTA refractor. The OTA weighs about 3.2Kg without accessories. I agree that a skytee or an Az4 would be more appropriate but they are lots of money. The scope cost me £27. I had initally hoped that it would be the bargain of all time and I would not need to spend any additional money. I was disappointed (naive?) in that hope in that I so far I've bought tube rings, a dovetail, a new visual back, new diagonal, three new eyepieces, a t mount adaptor, a moon filter, a bahtinov mask, vixen finder bracket and a red dot/reticule finder (and wasted another £20 on a 0.965 - 1.25" adaptor that serves no purpose). I've only managed to look at the moon a couple of times so far and I want to stop spending now before my £27 bargain costs me £1000! Sorry about the poor cameraphone snap. edit: The circle dangling from the mounting bar is the lid from a tin of finnish licorice I'm using as a lens cap. I like it ;-)
  6. I tried mine with a ball head and found also that it worked best on it's side. But my pan and tilt is more solid and easier to point and clamp accurately so I've switched to that.
  7. The head is a manfrotto 141RC - it is a reasonably hefty 3 way pan and tilt with substantial locking knobs. I couldn't say what it was made of but a variety of metals would be my guess. I don't think the head is the weakest point, the centre column is. It's thin walled aluminium and it's telescopic - there's another tube inside the main tube. It makes sense to me to do away with the column and bolt the head directly to the casting. I don't think it would be easy to fit wooden legs because it doesn't have the fork and bolt arrangement of astro tripods. Instead there are short metal "cups" or sockets fitted to the casting using a clamp arrangement. Metal tubes them slide into these cups/spockets and are secured by clamping. Replacing the aluminium tubes with single piece steel would seem the most likely option but maybe expensive.
  8. Just thought of another solution: mount the two section tripod on some kind of platform or fashion some kind of tall "feet" (like fence post supports?) the tripod can rest on...
  9. I have a classic 1970s long tube refractor that didn't come with a mount or tripod. I do own an aluminium manfrotto O55c camera tripod fitted with a R141 pan and tilt head. This is a substantial tripod for small cameras but I hadn't quite appreciated how much stress a long tube scope puts on the tripod. If I can avoid it, I want to spend no money on this hobby (!). I've tested the weak points of the tripod as best I can and they seem to be the centre column fixing (a spring clamp that is applied to one side of the tube only) and the lower skinny section of the legs. - The plan is to remove the centre column entirely. I'll fix the head directly to the legs using a length of threaded rod passing through the hole in the casting at the top of the legs and bolted in place underneath using a large washer and nut. - the legs start off as 30mm diameter aluminium tube but by the feet sections are only 20mm. - The telescopic sections are clamped with spring loaded alloy "flip lock" clamps. There was definite flex at the clamp positions and while attempting to tighten the nuts on the lower clamps, I managed to break two of them - so I'm now left with a tall one legged tripod or a shorter, sturdier 3 legged tripod! I'm pondering what can be done about the legs now. - I could leave it as a short two section tripod and try and raise the height by adding some wide steel tubing between the head and the leg casting - I could replace the legs in their entirety leaving me with just the casting to which I fit replacement one piece steel tubes What do you think?
  10. Personal experience from a newbie with just a couple of observing sessions under the belt... My second hand 1970s classic 3" refractor came with 3 EP: a 4mm ortho, 6mm and 20mm Huygens all in 0.965 barrel sizes. I've upgraded the eyepiece holder to 1.25" and purchased 12.5mm and 25mm Plossls. These are branded Antares and cost a princely £17.99 a piece. The difference between the stock EP and the new ones is ridiculous. The stock 20mm with a barlow screwed into it at least provides a view of the sky, without the barlow it vignettes. The other two serve no use at all, I might as well look through a toilet roll. The Antares give lovely views that are much brighter, have a wider field of view and are sharp to the edges. I also acquired a Celestron Xcel 7mm 6 element EP this week. It's huge, like a camera lens. It's also very usable compared to the short stock EPs which really don't work properly at all. It appears to have a wider field than my plossls despite the extra magnification. It also provides a comfortable viewing position, you don't have to screw it deep into your eye socket to see something. Quality-wise, at £70 new (I got mine for £35), you would expect it to be superior to the plossls but in the brief time I have tried it I didn't notice it to be so. This may simply be down to the challenges of focusing at 175x on a less than solid tripod or the seeing conditions at the times I've tried it. Going from practice to theory, I've read that the benefits of top end EPs really kick in when you want well corrected wide fields with fast (low f-number scopes). My scope is f16.4 so very slow and in theory the view through the end of a coke bottle ought to be reasonable with this scope (which makes you wonder about the useless view through the stock EPs!). I suspect that even cheap eyepieces supplied with modern scopes are going to be better than the junk that came with mine...
  11. I received my camera adaptor today. Just tried it out on the moon. It doesn't focus with the diagonal but it is fine straight through. The 99% moon doesn't quite fit in the m4/3 field of view, with the top slightly clipped. here's a snap: Interesting that sharpness isn't consistent across the globe. Why is that do you think? Even a quick snap demonstrates that I need to do something about the tripod. If i can get some 3/8" threaded rod and some nuts and washers, I should be able to directly bolt the head to the legs without the flex of the centre column.
  12. Is there an easy way to know the diameter of the optical circle produced by the objective? And what percentage of it is usable? My f/16.5 refractor seems very gentle on EPs. My £17.99-a-piece Antares plossls are pin sharp out to the field stop as far as I can tell. But those exotic wide field EP do seem to have an remarkable reputation. I'm wondering how large an optical circle they need to do their stuff. If I wanted something with a wider view than plossls, how much wider a view will the optical circle of my scope support before I'm just looking at the fuzzy edges? Is this only possible with fast, short focal length scopes? I've been doing some calculations based on formulae I found on a web site that, if correct, suggest the max true field of view of my scope with any EP is 1.2 degrees...
  13. Does anyone have 40 years of experience with digital cameras yet? Possibly. My experience only goes back to 1999. Before that I was a large format silver halide kind of guy ;-) Don't get me wrong, I wouldn't pretend to know the first thing about astro-imaging - I've taken precisely one usable picture of the moon so far. And as I have only a 3" F/16 refractor sitting on a standard camera tripod it's been made plain to me on this handy beginners forum that that's as far as I can expect to take it. I have no pretensions to making a contribution to astro-photography, no worries about that!
  14. Photographically, they can be connected in a vague way, in the sense that full frame sensors tend to have the highest pixel counts and if you use the whole sensor image you can usually make the biggest prints. Not always true, of course. That's a lovely picture. Which is a funny co-incidence actually, because in my very short experience as a stargazer, M31 is the only deep sky object I've looked at. Looked for,actually. I can't find the damn thing! They say find the pointy bit of casseopeia, work you way over towards Andromeda and at the end of that string of fainter stars you will find M31 6 times the size of the full moon. I've looked with naked eyes, with binoculars, with my telescope and with the telescopes I made from telephoto lenses, eyepieces and gaffer tape and I simply cannot find it.... I suspect living in Bromley mighty have something to do with it being as just about the only thing I can see is Capella.
  15. I'm familiar enough with the effects of pixel size on S/N ratio, dynamic range and high ISO performance. I'm also aware of the impact of sensor generations - for example the impact of the low read noise of current Sony sensors; the light and resolution robbing effects of Bayer CFA. Enough at least to question something as simple as a statement that full frame is always better than smaller sensors ;-) Now, if we are starting to stray into the personal animosity stage of forum life, it's time to retire from the fray.
  16. I don't why would I should have heard of this Atik 383. Talk to me about CP950, E10, D100, 14n, SD9, SD14, DP1, DP2 Merrill, 350D, 450D, 5D classic, Nex 3, NEX5n, G1, G3, GH2, G6, K5, X10, XE-1, XA-1, S5Pro, E-410, GF1 (all cameras I own, owned or used) and we can swap stories. Atik is not a company I'm familar with. I'll take your word it's a staple of the field. I have been doing astro photography for 1 day and photography for 40 years so of course I have my preferred terminology. I have also looked through my telescope in anger twice to date, so I hope I can be forgiven for not exactly being up on the units of the field. Arc-seconds and the like is not familar territory yet. Give me time ;-) ps One thing that is being missed in all this is the type of sensor. A mono sensor and a sensor fitted with a Bayer pattern and a low pass anti-aliasing filter are very different things, Likewise, the Foveon sensor with its 3 layer co-incident photosites and absence of AA filter. The Bayer colour filter array + AA filter problem reduces the resolution 'per pixel; by 50%.
  17. You have some quibbles with some of the things I said and vice versa. However, I think in essence we actually both believe the same thing whilst arguing with one another. The basic rule is that to resolve something well with a digital system, you need to get as many pixels as you can on the subject. If the subject is very small in the optical image, the pixels have to be significantly smaller to position a lot on it. Generally, that is best done with a small pixel dense sensor. It can also be done with a larger sensor with the same size pixels and a higher pixel count (even though most of that sensor's pixels will go to waste). Much of the debate seems to have strayed from the point somehow; I'm going to do a quick review of the whole thread and remind myself of the real debate. Edit: Ok, the crux of this seems to have been the question "Should I get a full frame camera to replace my APS-C" and a response that said something like "Yes, it will have more pixels and therefore you will have more resolution which allows better cropping capability". You then replied you didn't see what sensor size had to do with it, as pixel size is what matters. And I said that sensor size does matter in the case of sensors with the same pixel count but in the opposite direction ie smaller is better as pixel density is higher. .....Which equals smaller pixels (exactly as you said). So we were in effect agreeing. Where there is a difference in the conversation is an earlier poster recommending full frame on the assumtion that full frame sensors always have higher pixel counts than smaller sensors - which isn't always true. Mostly this thread revolves around semantics and assumptions, the usual reasons for arguments!
  18. If the 383 (whatever that is) has smaller pixels and is a larger sensor then it must by definition have a higher pixel count than the 314l. That is why is also has a higher pixel density. Everything I have discussed assumes sensors of different sizes that have the same total pixel count eg my APS-C sized 16MP sensor vs m4/3 size 16MP sensor vs 16MP full frame 36x24mm sensor. If the pixel count is the same and the sensor sizes are different then the pixel densities of the smaller sensors are higher and the resolution per sq mm is higher. If Saturn appears as 5mm x 5mm in my telescope, the ideal 16MP sensor to image it is a 5mm x 5mm P&S sensor, not a full frame because it puts all its 16MP on Saturn while the full frame wastes almost all it pixels imaging space. You don't really need much technical physics for this to be perfectly obvious!
  19. I haven't read that article in full (I will) or worked through the subtleties of context involved but that quote makes no sense to me. Let me put the argument another way: If you photographed a resolution chart with a 1MP, 2MP, 10MP, 50MP camera, which would achieve the highest line pairs per inch score? The answer is the one with the highest pixel count, irrespective of sensor size. In terms of resolution, a full frame camera with the venerable 6MP Phillips full frame CCD as fitted in the short lived Contax N full frame DSLR will be utterly outresolved by a modern throwaway £50 16MP P&S. Until the point that noise becomes too intrusive, more pixels always improves resolution. It is perfectly obvious that if you have a full frame sensor that puts 20 pixels on saturn and a small sensor that can put 100px on Saturn, you are going to get more resolution from the small sensor. It may be true that noise and weak dynamic range and the effects of excessive noise reduction algorithms may make small sensor images look ugly, but more pixels always = more resolution, all else being equal. Take a look at the image quality difference between two cameras I happen to own, a 2003 Nikon D100 6MP DSLR and a modern 24MP Nikon. The newer sensors blow away the older camera for image quality. It's quite trendy in some circles to pretend that fewer pixels is better (low noise) but outside super high ISO use, it isn't so. Now, I'll and read that Wiki and see what they really meant ;-) Edit: I've read it now. It's a peculiar paragraph, just kind of thrown into the article almost out of context. You have to work out what it means because of this and the wording could be better. What I think it is saying is that a large high resolution sensor with fat pixels can have a lower pixel density than a lower resolution but much smaller sensor. This is pretty much what I've been saying. In photographic terms it's not really meaningful or helpful because you would normally fit each camera with a focal length lens appropriate to the sensor size such that you would compare identically framed images and highest total pixel count would yield the highest resolution. It is applicable in astronomy because the fixed focal length of the telescope produces a fixed size image of the subject. Therefore getting the most pixels on the subject (highest pixel density) wins.
  20. Pixel density is the significant thing. If a planet takes up a tiny fraction of the field of view, I want to put as many pixels on it as I can. If I have a full frame and a 4/3 sensor with the same pixel count, the the higher pixel density of the m4/3 is going to put more pixels on the planet. The full frame will waste its pixels recording the black sky. That will make a difference to recorded detail (assuming the optical image from the telescope has sufficient resolution, seeing etc). Edit: This quote "(If the pixel count is the same on two chips of different sizes then the bigger chip has bigger pixels and, therefore, lower resolution." perhaps shows the danger of loose terminology. The "resolution" of a sensor is defined by the number of pixels on the sensor: the more pixels, the higher resolution. A full frame 16MP sensor has exactly the same resolution as a 16MP m4/3 sensor (or a 16MP medium format back or 16MP 2/3" sensor). The thing that changes between sensors of the same pixel count but different format is the pixel size and therefore pixel density. Smaller sensors have higher pixel densities and put more pixels on every square mm of the optical image. As planets will be very small in the frame, that is an advantage of small sensors.
  21. I tried it out on a chimney pot today. The Panasonic Ext Tele mode is really interesting. Usually with large sensor video, the camera downsamples or lines skips to get from the native 16MP to 2MP needed for each HD frame. Panny offer this special mode whereby instead of line skipping or downsizing, they just use a 2MP slice out the middle of the sensor. Effectively compared to standard HD video, it is gives you a 2.5x teleconverter built in with no loss of light or image quality. Handy when you need more reach from your lenses! Astrostakkert seems to be able to use the .mp4 files without conversion.
  22. Focus with my camera didn't seem to be a problem even though it is easily disturbed. 10x mag in the viewfinder + focus peaking seems to do the trick. I used the electronic shutter to avoid vibrations from a clacking shutter and fast burst mode but no cable release. Should have used one. I might try HD video with the special 2x mode that panny use (this uses a 2MP slice of the sensor rather than digital zoom and supposedly avoids line skipping/downsizing problems).
  23. Actually the sensor size does have an impact on resolution. Let's pretend we have a scope with infinite resolution i.e. every square mm of its optical circle could support any arbitrary level of magnification. Then what defines the resolution of the final image is the number of pixels per sq mm. If you have for example a choice between a full frame and a 4/3 sensor of the same pixel count, the full frame would cover a 4x wider area of view but the imager resolution would be half of the 4/3 because the pixels are spread over a larger area. The upside is improved noise and dynamic range characteristics. Another idea jumps out at me, too. Even if the coverage of a telescope is relatively small, small format cameras could be used with a shift adaptor to allow small mosaics to simulate the effect of a full frame. For example, I have a shift adaptor for my m4/3 camera. This is intended to be used with full frame lenses which have a larger covering circle than native m4/3 lenses. The idea is that you shift left take a pic, centre take a pic, shift right take a pic. Then the you can flat stitch the 3 frames to produce a mosaic image with 3x the area and pixel count. I keep seeing comments to the effect that photography is somehow different from astronomy but the optical principles are the same. Maybe some of the terminology and rules of thumb can confuse but there is no actual difference.
  24. Hi Rik Thanks again for all your help. One point I'd like to make: of course you are correct in saying that the crop factor doesn't change the physical focal length. However, it does make a difference when comparing sensors of different sizes but the same pixel count: the pixel density is higher. For example, if I compare my m4/3 16MP to a full frame 16MP, my m4/3 camera is concentrating all its pixels in an area 1/4 of that of the full frame. Which means I can blow up the frame 2x bigger compared to the full frame without loss of resolution (assuming the scope has sufficient resolution to support the magnification). The downside is smaller pixels, more noise and less dynamic range. I had a quick go at the moon with my camera lens (300mm f4 + 2x teleconverter + m4/3 panny g6 = 1200mm equivalent compared to full frame with same lens). I downloaded and installed some free stacking software (PIPP and Autostakkart). Don't really know what I'm doing with it but here's my first attempt: I shot about 60 frames and selected the best 15 (100% - 80% quality). The pic makes a decent 7x7 inch print as long as you don't try and look too closely ;-) I'm not sure what is the biggest quality constraint: the mount, the lens, shutter speed, camera resolution, seeing, magnification, number of frames, processing but sharpness is a long way from great, even with loads of processing...
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