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Martin,
I think it probably depends on how large the image is when you upload it. The ‘expand’ option seems to display the image at the original size. For example my Ptolemaeus image is 1200×1755 pixels and the expanded image displays as that while on the pre-expand screen it is scaled down to fit. Consequently one appears larger than the other.
Your last image of Mars seems to be 500×550 and displays as that when expanded but does not need to be scaled down on the previous screen and therefore appears the same size in both.
Cheers,
David.
Martin,
Is this what you mean when you hit the expand button? https://britastro.org/node/15748
David.
Dominic,
Thanks for the prompt response. I had no idea that things were tied down that tightly nowadays. Truly remarkable.
Does anybody know the claimed accuracy of the USNO or Handbook values? I’ve had a quick look and could not see anything.
My point is this, is the two minute difference actually significant? If the USNO figure is accurate to 10 seconds then yes, if accurate to 10 minutes then no. Same with the Handbook value.
To assess the actual difference I have done a rough calculation which may well be wrong – I am not a mathematical astronomer by any means so feel free to fault my approach. What I did was as follows:
- From the Handbook calculate the rate of change in declination by taking values either side of the equinox.
- Pro-rata this to arrive a shift in the two minute difference of roughly 0.2 arcsec.
- Using the polar radius of the Earth, 0.2 arcsec translates into 61 metres.
- So in the two minute difference the position on the Earth’s surface where the Sun is directly overhead moves northward by just 61 metres.
Are the Earth’s orbit, the planet’s shape and axial tilt sufficiently well defined to give this level of accuracy?
David.
Hi,
I had a dim memory in the dusty recesses of my mind that many years ago S&T ran an article on detecting Sirius B and the benefits of using a hexagonal mask.
I have now dug it out and it may be of some interest and indeed utility. The issue in question is June 1975 p407.
Basically Dennis di Cicco created a carboard mask that fitted over the aperture of a C14. Cut out from the cardboard was a hexagonal hole scribed on a 13.25 inch circle.
The idea is that this creates six strong diffraction spikes around the primary which suck a lot of the glare out of Sirius A. Rotate the mask so that the secondary is positioned between the spikes and Sirius B is more readily seen.
The reported results look good, it was indeed more easily detected with the mask. Also included in the article are photographs with and without the mask and the difference is clear.
While this approach is fine for refractors and SCTs, something like a Newtonian with a spider mounted secondary which already creates diffraction spikes might be more problematic.
Anyway, it could be worth a try.
David.
You could try this method based on one I’ve used in the past to find the balance point of a telescope tube.
- Fit the telescope to the wedge and stand it on the floor.
- Slide pencils in from opposite sides, level with each other, effectively making a see-saw.
- Slide the wedge back and forwards on the pencils until it is in balance.
- The vertical plane through the pencils now contains the COG.
The only issue you might have is if the COG lies outside the wedge and it tips over when placed on the floor. In this case you could attach an outrigger to prevent tipping before putting the mount on the pencils. As long as it is light in weight compared to the scope it will not materially shift the COG. After all it will shift in use if you switch from say a light eyepiece to a heavy camera.
Just a thought.
David
JR,
Click on ‘My Account’ top right. This takes you to your Members Page. If you wish you can add a bit about yourself here.
At the top left of that page is an ‘Upload Image’ tab. From here you can add your photo along with any details of the exposure and/or description.
People can then comment on it if they wish and also ‘Like’ it.
David.
- Click on “My Account” on top right of screen.
- Click “Edit Account Details” below your name and towards the right.
- Scroll down to “Full Name”, change to your preferred name and save.
David.
Hi Dominic,
I’ve used a Vixen Polarie for some years and am quite happy with it. The reason I went with the Polarie is that I was something of an early adopter and at the time the market was more limited than it is now.
STRENGTHS.
- Solid and well built.
- Tracks well.
- There are only a couple of thumb screws to tighten and these are not really fiddly
POTENTIAL ISSUES.
- For longer exposures or longer focal lengths you will need to purchase the polar alignment scope which is an optional extra.
- The polar alignment scope does not have an illuminated reticle so you may have to obliquely shine a torch down it. Having said that, there does appear to be a new version out which works differently with an app.
- Unlike some newer offerings it does not have polar alignment adjustment screws so you have to adjust the tripod head which can be a bit awkward as it is likely out of balance. However with practice it does become relatively simple.
At some point I will likely make a permanent mount in the garden for it. Probably just a post with a suitably aligned top piece. Then all that will be necessary is to clamp the Polarie to the post when I want to use it and it won’t need any further alignment. This should resolve most of the issues.
Lastly, there is a further thread here with additional discussion and an expanded version of this post.
Regards,
David.
Hi Tor,
Click on ‘My Account’ at the top right of the page. This takes you to your member’s page and above your image are the buttons you need. The ones on this page work on the thread you are in, not the member’s page.
Regards,
David.
Hi Tony,
My observatory is a roughly 6’x4′ run off shed with the entire building rolling off so I guess the overall weight is not wildly different to your run off roof. Also because I have built on a sloping site the rails are supported on short posts not resting on the ground so the rails are taking the weight not the ground. So again there is a similarity to a roll off roof.
For the rails I have used 70mmx45mm pressure treated timber from, I think, B&Q. The wheels run on the 70mm face and the weight is therefore supported by 45mm of timber. There is an important point here however. The shed runs on eight wheels, four on each side, and each rail is supported on eight posts. These posts are arranged such that whenever the shed is fully open or fully closed the wheels sit directly over a post so the load is transferred straight to the ground and there is no opportunity for the rail to sag under the weight of the shed. I would strongly urge you do something similar.
The guides are quite simple. Most DIY superstores sell lengths of 10mm threaded rod and matching nuts and washers. I cut four roughly 25cm lengths of the rod and mounted one at each corner of the shed frame extending down beside the track. From simplybearings.co.uk I obtained four roller bearings. These slide onto the end of each rod, are held in place by nuts and washers and roll against the side of the track.
For what it’s worth the wheels I used for the shed were these.
The observatory has been in use for a couple of years now and so far there have not been any problems.
Regards,
David.
James,
From the programme of the Mars Section:-
“Other atmospheric phenomena White clouds, frequently seen at the limb and terminator, can occur in conjunction with specific topographic features, such as the great volcanoes like Olympus Mons, those in the Tharsis region, or Elysium Mons. Such topographic or orographic clouds are carried round with the planet’s rotation, and may be enhanced with the recommended blue and green filters. The clouds over the Tharsis volcanoes can sometimes coalesce on the evening side and form a ‘W’-shaped cloud (or ‘M’ for ‘Mars’ if viewed with north up). Some basin areas act as cold-traps for volatiles: thus Argyre and Hellas can show frost patches at the appropriate season. Another interesting phenomenon is a band of equatorial white cloud, the so-called Equatorial Cloud Band, which is best seen in blue or violet light and most apparent between about Ls = 50° to 145°, and whose brighter extremities at the E and W limbs give rise to the evening and morning clouds viewed in white light.”
The last part of this probably applies to the image you have displayed as Ls at the time was 78.
I guess the reason they appear brighter at the limb and terminator is that you are viewing obliquely, looking through a greater depth of cloud than when they are on the meridian and you look straight through them.
David.
Hi James,
The article here may be of interest. It includes links to a number of other resources including the original professional paper.
David.
Hi David,
I have a Vixen Polarie and am extremely satisfied with it. You can see a couple of results on my Member’s Page.
Clearly polar alignment is key. The polar sight has a field diameter of 8.9deg, I’ve attached a copy of a table from the manual showing the permitted exposure times assuming you can align on the pole to within 2deg.
If you intend to use longer focal lengths or much longer exposures I can recommend the optional polar alignment scope which is a very solid piece of kit.
Lastly, with longer focal lengths and exposure times you need a really solid tripod to mount it on. That much is obvious, less obvious to me was the need for a really tall one otherwise you end up kneeling on the cold and wet ground to align the device.
Hope this helps, if you want to ask anything else feel free to drop me an e-mail.
David
Jack,
Is this what you mean?
If so, then it is enabled already. The thing is and this threw me to start with it does not appear on your own postings but is visible to other people.
David.
A few points in response.
- My post perhaps could have been worded better. Where I said “observations that are new” what I meant was those that are newly posted not necessarily the most current so while there was an apparent contradiction it was not real or intended. My bad!
- The downside remains for anybody who like me wants to see fresh posts is that they would need to trawl through to find them.
- The point about it being for current observations not an archive is interesting. I do not recall seeing anything about that elsewhere though I certainly would agree with it. Perhaps the fact needs highlighting on the submission page?
David.
I would quantify my support as half-hearted at best. I fully understand your point and see its logic but I would not want it to be the default. My rationale is simple, when I go into the Members Pages I want to see those observations that are new and not have to search for them. Displaying in date submitted order gives me this. You are right of course, in a lot of cases this will be the same as date of observation but some members are posting old observations, some from several years ago and depending on their posting mix these could be well down the list with your suggested scheme and thus difficult to find.
Ideally it would useful to have a choice of sort order so everybody could have what they want and maybe the default held against each members individual profile. However I suspect this is a step too far and likely beyond the capabilities of the tool used to create the site.
David
Hi Dominic,
Gallery page, click on “Recent Images” top right centre of screen takes you to https://www.britastro.org/gallery_recent
Click on any of the thumbnails and you get a message eg for the first thumbnail The requested page “/gallery_image/8438” could not be found.
Hope this helps.
David.
