umadog

Great suggestions. Thanks very much, I will look into these. 

Kriege's book is indeed a classic but, IIRC, it's also a little long in the tooth now. There are plenty of manufactures out there: take a look at their websites and collect design ideas from there. 

I have a car question, people. I would like to fit a large Dob and two passengers in a car to go camping. Trailer is not an option. Which car to go for? Background and details:

I recently(ish) moved back to the UK and brought my 18" Dob. I used to transport it in a station wagon with the wheelbarrow handles still attached but, as you can see below, then only one passenger is possible. The OTA is in the gray bundle between the posts of the wheelbarrow handles and would not on top of the mirror box anyway due to ceiling height. A roof rack would not help here, therefore.

Random things I have thought of:

0. My current ancient Fiat Punto will not do at all. 

1. Previously I would use ramps to push the mirror box up into the car and get it over the lip of the boot. I could now make bolt-on handles and have two people lift the scope into the boot. This would work if there no lip to the boot and sliding the whole thing in was an option. 

2. Might be helpful to have a car where removing or folding one rear passenger seat alone is possible. 

3. Taller car so stacking the UTA on top of the mirror box is possible. 

4. Rack for wheelbarrow handles and ramps. 

Any suggestions? I'm after second hand under 5k GBP ideally. ULEZ compliant. 

I reckon the cure is in two steps: 1) Buy the biggest telescope you can just about physically handle. You won't want to go bigger after that. 2) Go somewhere really, really, dark with whatever aperture telescope. After that you have dark-skies fever and you'll care less about the instrument. You can likely skip straight to step 2. 

Really?? In that case I'd return for refund since it's not fit for purpose. Ridiculous to have to return to factory for something that trivial.

Laser collimators are evil The only one I have ever seen that has been straight straight from the factory is the Hotech. May I point you to something that actually collimates your whole scope and not just the center five microns of it: The Catseye Collimation System. I got one and it is a heck of a difference. If I collimate with the Hotech and then try the catseye, I am looking at some major fiddling to get it right. You should try it! Just google it and you'll find it.

I too use the Catseye system too (Cheshire plus autocollimator), but it's misleading to say that the Catseye tools collimate "the whole scope" and the laser "just the center five microns." That's just not how it works.

A. The outward beam of the laser measures exactly the same thing--secondary tilt error--as a sight-tube (Teletube from Catseye), and is equally accurate as a sight-tube. Of course the laser has to be collimated but then again, so too do the cross-hairs have to be centred. Does anyone test that alignment of their sight-tubes? Either tool has the potential to be wrong. Further, I would suggest that it's easier to read a small error using a laser than using a sight-tube. In my opinion, the laser actually wins this comparison.

B. The barlowed laser (or return beam if you insist) measures exactly the same thing as a Cheshire (e.g. Blackcat) and, in this case, to pretty much the same accuracy.

The only difference between the two systems is that a sight-tube also allows you to round and centre the secondary (which is not an axial alignment anyway, and only effects field illumination), whereas a conventional laser does not. Otherwise, the two approaches are equivalent because they measure the same things in (virtually) the same way.

The difference in Catseye system is the option of the autocollimator. The auto-collimator has two advantages:

1. It reads to twice the accuracy of any of the above methods. That, I imagine, is why you need to "fiddle to get it right" after using the laser.

2. It measures a greater range of errors than other tools. For details: http://www.cloudynights.com/ubbthreads/showflat.php?Cat=0&Board=reflectors&Number=3532750

Outside of physically moving to a remote area, are there any techniques you employ to deal with light pollution?

I look at Saturn.

It's like anything in life: you get out of it what you put in and you won't get much out of it if you don't do your research. I know the photo/visual thing is a common misconception, but any introductory book on astronomy will tell you not to expect Hubble-like views and that everything will be colourless. Anyone who's serious about astronomy would have read a book or talked to someone knowledgeable before buying. I reckon most of these 49.99 department store scopes are sold to people who were never very serious in the first place. Frankly, the thing that's putting off most beginner isn't the cheap telescopes it's the light pollution.

Well, here's a review: http://www.cloudynights.com/item.php?item_id=2318

$150. Nah. Look here: http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/5280448/page/0/view/collapsed/sb/5/o/all/fpart/1 You may need to be a member of CN for a particular length of time to see that, not sure, but it's a 10" Dob for $250. That's within your price range and will show you far, far, more than that Meade. It'll pull in almost 5 times as much light and only 60% more money. It doesn't have auto-star. But do you really need that? In NE Texas you're probably very near to some very dark skies. You want aperture to take advantage of it! You'll learn the sky: that bit is easy and fun. You just need a star atlas and a little patience. There'll be other similar bargains out there. I reckon you can do better than the Meade.

The difference at the eyepiece between 8" and 10" is noticeable but not huge. The other difference between the scopes is focal ratio. In order to make the 10" of similar focal length to the 8", manufacturers must obviously make the 10" mirrors of faster focal ratio. There is, therefore, noticeably more coma in a 10" f.4.7 than in a 8" f/5.9 (which is often how the numbers end up). It's unlikely you'll be forking out a few hundred for a coma corrector so you will end up with some blurring in your low power fields of view. Cheaper eyepieces (or just badly designed eyepieces) will show astigmatism below f/5 but at a lot less around f/6 and slower. So going for the 10" will affect your eyepiece choices. I'm not saying any of this to put off: like others on this thread I have a faster 10". But you do need to know that there are more differences than just aperture and if you want to get sharper views more cheaply than an 8" is a better way of going about things.

There seems to be a bit of confusion here. Or maybe I'm the one who is confused. Anyway, this is my take--

Yes, there is always confusion because, as you say, everyone calls the combination tool a "Cheshire"

I would rather have these separate, because the sight tube is used once in a blue moon to align your secondary mirror, while the Cheshire eyepiece is used to collimate your scope maybe every time you take it out.

The sight-tube portion has two uses.

1. Rounding and centring the secondary in the focuser. Hopefully that is done rarely. For this you frame the secondary in the round opening of the tube. The OP could use the long-tube quite happily, I would think.

2. Secondary tilt adjustment. This is an axial alignment (just like primary tilt) and is distinct from the above rounding of the secondary. It's achieved by centring the primary centre spot under the sight-tube cross hairs. This requires tweaking probably every time you use the scope (at least that's been my experience). The secondary tilt and primary tilt adjustments need to both be correct in order for the telescope to be "collimated." For this reason you need both a cheshire and a sight-tube to be aligned and this is why the combo tools are popular. If you're a perfectionist, I'd recommend dedicated cheshire and an auto-collimator. Much, more accurate and easy to read than the sight-tube and can be used to align both secondary and primary tilts.

Yeah, that's about right, Tom. It'll be the focal ratio that you use to select the sight-tube: http://www.catseyecollimation.com/sxl-out-640.jpg

That said I've used the longer combo-tool tube at f/5 and found it to work ok.

Maybe im wrong but cant see if it will make any difference tbh, im probably going to get corrected though by someone. What you waisting money on them for when you could make one out of a film cannister?

You will It's not a waste of money because it will do a better job than the film cannister: http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/5052617

Briefly, if it's silvered on the inside then the film cannister will serve as a Cheshire and can be used to adjust the primary tilt. It can't be accurately used to adjust the secondary tilt. For that you need a sight-tube or a laser. The combination tool the OP is referring to contains a sight-tube and a Cheshire. The length of the sight-tube serves to create a "frame" with which you can more accurate round and centre the secondary in the focuser. I don't know exactly what length is suitable. The one I have is adjustable. Maybe you can get more information on what's right from this page: http://www.catseyecollimation.com/ So long the length is vaguely correct you'll be ok.

If you can't stomach finding your latitude and longitude and typing it in then a GPS unit is for you. Otherwise, I can't see the point unless the goto has no other way of entering one's location (which seems unlikely).

Paul, you can tap the scope or you can simply sweep the scope left/right or up/down across the field of view. The idea is simply to add motion (the size of the object won't change). Probably the reason this works is that you're adding extra information by moving the image. This information comes in the form of correlations. Your brain identifies objects in the visual scene by looking at correlations in neural activity (so brain cells firing simultaneously, basically). With a static image you only have contrast to go on. If you add motion then you can also look for correlated activity in this domain.

EDIT:

I think that was probably a bit cryptic, so let me explain this another way. Imagine you're a single photoreceptor and are detecting light from a tiny area out in the world. You see your little patch of world become brighter than darker again. There are two things that could have happened:

1. Something that was already there became brighter and darker again.

2. A brighter thing passed transiently through your little bit of the world.

So the first is scenario where something flashed on and off. The second is a scenario where there was motion. You can't disambiguate these two things just by looking at the activity from a single photoreceptor. This is because, as you've probably guessed by now, single photoreceptors can only provide information about luminence not motion. To extract motion information you have look over many photorecptors and compare what they're all telling you. This allows you to diambiguate brightening from motion.

So the brain is wired up to pool activity from very many photoreceptors and what you see (particularly at night) is the average activity of vast numbers of photorecptors. You take advantage of this averaging whether there's motion or not, but motion activates extra pathways in the brain. It's rather interesting why this is the case. Some pathways coming out of the eye are, loosely speaking, specialised to detect shape and others specialsed to detect motion. A single photoreceptor (rod or cone can feed its signals into both pathways. This is a form of parallel processing, allowing the brain to process information rapidly by splitting it up. So if you're looking at a static image, you're activating the shape pathway only but if you move the image you are activating both. So possibly the improvement you see when you're moving the telescope is because you're providing the brain with extra information by activating a neural pathway that otherwise wasn't being used.

Damo has already answered your question You can use the Cheshire/sight-tube tool to adjust everything that needs adjusting. Some people do find the AB guide confusing, but that's partly because it covers everything in quite some detail. You may not need all of the information in it so the extra stuff may confuse. It's worth knowing about, though, as it is pretty complete. Take a look at these links: Rob Campbell's Home Page Pay particular attention to the second link, which is simple.

You current image shows that you're very far off. Take it slowly and one step at a time. Adjust the secondary tilt first until the primary mark is under the crosshairs. Then repeat and adjust the primary until the Cheshire spot and primary donut meet and are centred. Iterate back and forth between the primary and secondary until you're done. Use the third link on that page to correct secondary position in the focuser if needed.

The sight-tube portion of the combo tool is designed for centring and rounding the secondary. You can use the collimation cap for this purpose if you wish, but it's not designed for the job.

The difference between a 16" and a 20" is only about as great as that between a 10" and a 12". As you say, best off keeping your eyepieces and the 16"!

No, don't even do that. Just quietly substitute one scope for another and nobody will notice

The thing with aperture fever is that, to an extent, it is self-limiting. I think it strikes hardest with smaller apertures: if you have a 4" then an 8" seems tempting because it's so manageable. Then a 12" seems tempting from an 8": a 12" is still quite a managable scope if it's a truss. Even as a solid tube it's not *that* big. If you put a 12" next to a 16" you'll realise that. However, after the 12" the next significant jump is >16" and a lot people just aren't willing to go that big. I keep hearing about people who bought a scope in the 16" to 20" range then sold it and went back to a 12". After a 16", you really are looking at a ~25" and that entails a trailer, a mortgage, and a divorce. See? Cures itself!

The other side to all this is that bigger scopes aren't better at everything. The field of view is more narrow and so a lot of larger objects look at least as nice in a smaller scope. The other thing that gets pushed aside sometimes are the skies: dark skies matter a lot!. Yes, even in light polluted areas a bigger scope shows you more; but a smaller scope out in dark skies will still trounce the large, light-polluted, scope on many if not most objects.

For me, 18" is my absolute maximum. I've observed though as large as 32" (albeit briefly) and whilst that's nice, it's not something I want to dabble in myself. What I would like, however, is a 12" f/3.... Will resist!

I do see what your point, it may help to have a small scope there too as a sanity check. But the question is what the smaller scope is a reference for.

If the views in the big scope don't match the small one after stopping down then what that tells you is that there's something "wrong" with the bigger scope. It doesn't speak to the effect of aperture on seeing, which is what we're trying to test. Only an improvement in the views through the bigger scope will do that. The improvement doesn't have to match the smaller scope. This is why it doesn't make a good reference for this test.

The boundary layer can be seen through the eyepiece if you defocus whilst looking a bright star. There's a pretty obvious difference between bad seeing and a boundary layer. More subtle tube currents may be harder to detect, however.

Really nailing this one with convincing experiments would probably be quite hard, I must say. More effort than it's worth. At the end of the day, we all know which scope in our arsenal gives the best views for particular objects and we just choose appropriately. We may not know exactly why one thing works well and another does not.

People may not say "smaller aperture alone", but this is implied. It's implied because the argument being made is that the purported reduction in image quality is due to seeing affecting large scope more than small scopes. Clearly everything else must be equal for one to be able to make this judgement. As you say, quite possibly this is not the case.

Observing a better image in a small compared to large scope doesn't tell you why the difference is there. This is why this is an unsuitable test. Stopping down is a more controlled experiment since everything else is held constant. Sure, the large scope may, for various reasons, produce a worse image than a small scope overall. This isn't relevant, however. What matters is whether stopping down the large instrument provides a relative improvement in the views. This would be evidence for the seeing hypothesis. It doesn't really matter if the views are now the same as an equivalently sized smaller scope. That's a different question.

There are caveats to stopping down. Firstly, it assumes that the mirror doesn't have a turned edge or any other local defects which the mask occludes. Secondly, it assumes that seeing is the limiting factor. If the scope isn't cooled down or has horrible optics then these may mask a "seeing benefit" should one actually be occurring. The test scope would therefore have to possess excellent optics, be in excellent collimation, and well cooled down. In practice, knowing these things to be the case is possible so the experiment is feasible. The test can even be done objectively: simply image the PSF of a star using a webcam. The movie sequences can be analysed and compared.

Indeed, this is the point. The reason that beginners should be steered toward f/6 Newts is because this is a cheap way of getting good images. The mirrors are easier to make well and the eyepieces need not be expensive. Coma correctors aren't necessary. Large (and fast) can be better, but quality is a lottery if you're buying Synta or GSO, although those manufactures can produce some nice stuff. If you go home-grown you have a much better chance of a good mirror but you're paying a lot more for it. In the end a lot of this comes down to economics not optics. Finally, there's the hassle factor. To get the best out of a refractor you don't have to do anything very special. To get a good view out of a Newt (particularly a big one) you have to plan ahead with cooling and collimation.

seeing predominantly shifts the entire image of a stellar object, whereas in larger scopes blur, but less motion is the result (because multiple seeing "cells" contribute to each star image along the line of sight).

Hmmm... I see plenty of motion shifts in Jupiter's moons through an 18". Even double images of the moons for fractions of a second. I have not experienced a situation where a smaller aperture gives better results. A well cooled and well collimated larger scope has always done better. I suspect this larger scope and seeing thing is a myth. S&T even ran a piece on it a while ago: Four Infamous Telescope Myths - Visual Observing - SkyandTelescope.com It's true that masking out a bad edge can help, though.

Larger and faster scopes have their problems, but these problems are collimation, cooling, and quality control during manufacturing. All of which are more critical when you push the envelope. Secondary obstruction size, and perhaps seeing and focal ratio, are often blamed for poor performance when in fact they aren't the root cause anything.

We too live under the jet stream and have bad seeing. Very frustrating! I've looked through a variety of scopes around here and so far a well made 18" f/4 has given the best Jupiter views. I too found this rather surprising to be honest, but it's true. Also, it's not better by a little bit, it's better by a lot and it's better consistently. Even first-timers can see the difference. It's true the view is brighter, but I find this helps one to see colours, such as the purple streaks along Jupiter's equator.

I've also had some very disappointing views through larger scopes where everything looks mushy and unpleasant. So I know what you're talking about. Making a large scope well is very hard and some of these instruments may have defects. Also, having a fast scope correctly dialled in takes a little more effort. Cooling is very important with a large mirror. When these things aren't done right the instrument gives bad views and this is blamed on the seeing, the image brightness, or any number of other things.

It is true, of course, that to make a faster scope work well you may want a Paracorr and you do need nicer eyepieces. So that costs more. However, the quality of Chinese mirrors is pretty good now. I had some rather good views through a 12" f/5 Orion scope.