£150 is nowhere near enough to get started in astrophotography, unless you want to limit yourself to photographing the moon.
For anything else, even getting the very basics right is going to skin you. You could probably get a half-decent SkyWatcher brand telescope, with a reasonable aperture (100mm+) for £150. But then you're going to need a very good, sturdy, equatorial mount, which is going to cost more than the telescope itself. Something like
this is probably the very least you could get away with, so that's another £250
at least. Then you'll be looking at some kind of motor to drive the mount so it tracks the apparent movement of objects across the night sky; without this you can forget about long exposures. At the very least, you're looking at about £60-100 for a basic motor which will move your mount at the appropriate rate. You're also going to need to align your scope with the pole star, otherwise your motor is useless. For this you'll need to spend about £30-40 on a polar scope (and you'll also need to invest a lot of time and patience learning how to use it correctly). Your dSLR adaptor will be about a tenner. If you want to do planetary imaging on a budget (don't expect incredible, detailed images), you'll probably need to get a 3X Barlow lens, or an eyepiece projection adaptor, so that's another £40-100.
And that's all just an outright beginner's kit, which will get you started and help you learn the principles but it won't have you producing the kind of "WOW" deep sky or planetary images that we think of when we think of astrophotography.
Then you have to learn a whole litany of techniques that are
not easy: polar alignment (this is an honest-to-god nightmare, and if you get it wrong, your exposures will not be sharp, and may be utterly unusable), star-hopping, locating objects that you can barely see, focusing the image on your sensor through a telescope, stacking, etc. Factor in the Great British weather, the long hours in the cold and pitch dark necessary to practice your technique, finding good dark sky sites (forget it if you live in a town or city), the fact that astronomy season only lasts about six months of the year, then actually learning all of these skills to a satisfactory level becomes an outrageous frustration.
I'm a keen amateur astronomer, and I've dabbled in astrophotography, producing some images that have pleased me in a "that's quite cool" sort of way, but are far, far, far below the standard I would want before showing anyone else. I don't have the time, or money, needed to get more serious about it.
So, before investing any money in astrophotography, ask yourself how much you're really going to put into it, and how much you expect to get out of it.
