Ricochet

I would start by downloading some planetarium software. On a phone or tablet I suggest Sky Safari (I believe the most basic version is now free) and on a computer you should download Stellarium. These should help you to start learning where things are in the sky. As you need to locate things manually, you should spend a good 10 minutes at the start and end of each session just trying to spot and learn the constellations without using your telescope. Orion is a constellation most people can spot so start with that and then from there you can look for Taurus etc. The book "Turn Left at Orion" will also be useful for finding targets to look at with your telescope. With regards to the equipment you have, I think it is best to get some experience with your telescope and 25/10mm eyepieces and then work out what you feel needs improvement. A 32mm Plossl will maximise the field of view with your telescope, which will help with finding objects (1.84° vs 1.44° with your 25mm) and also give you a good exit pupil if you want to buy a UHC filter to get a better view of the Orion nebula.

I think the steel tripod would be good if you have space to store it near your observing location, but if you're carrying all your equipment a distance a lightweight, good quality photo tripod would be a better choice. I've got a manfrotto tripod and it is clearly significantly better than the cheap clone tripods that are all over Amazon and eBay.

What do you mean by parallax with respect to eyepieces?

I think it is too big for that. Even if it was, that shadow would be offset in roughly the direction it is offset. (Which suggests I might be wrong about the relation to the light source direction). See the picture below of the view through a collimation cap in my 8" dob. Notice how much smaller the secondary shadow is. (Not the best picture as it is too dark to see the edge of the secondary, but you get the idea.)

The only thing of importance is the brightest circle, which is pretty well centred in the overlaid red circles. The darker shadow within the brighter circle had nothing to do with the collimation. There must be a piece of card/paper in the tube blocking the reflection of the primary and the Newtonian is in a location with a light source slightly off centre to the axis of the OTA, so the spider is throwing a shadow onto the card, but this just signifies the angle of the tube compared to the light source, not the collimation of the telescope. There is no need to move anything.

It's fine as far as being central is concerned. What you really need to do is to look through a Cheshire and see if the cross hairs line up with the doughnut on the primary, as that is the important bit. Once you have that done it is a case of adjusting the primary to get the shadow of the eye hole of your Cheshire or collimation cap inside the doughnut.

If the 8" dob is too big then an 8" reflector on an eq mount will also be too big. Eq mounts are big and heavy, to the extent that I would categorise the dob version as the smaller overall set up.

You're right about that. I had one with a really cheap spotting scope many years ago. Ok with a spotter when you're holding the handle and panning around but not really suitable for a scope. Knowing how good the included diagonal really is would also be nice as it isn't standard and won't be easy to replace. It made me think that maybe a small Mak would be a nice grab and go planetary/lunar scope, but perhaps a slightly larger one would be better.

Your telescope takes 1.25" filters, so any filters you buy at this stage should be that size. My favourite type of moon filter is a neodymium filter. You can buy the premium Baader version, but the cheap Chinese version available on eBay is also good. For light pollution the best type of filter is to observe from a darker location. You can buy a CLS type filter, but these are aimed at the old sodium lighting, and for LED lights they are not as effective and you would probably be better spending your money elsewhere. If you wish to view nebulae then good (Astonomik/Televue!) UHC and OIII filters are worth getting, but you will probably need a 32mm/40mm plossl in order to generate a large enough exit pupil to use them with your scope. For stars you need high magnification rather than a filter to dim the background sky whilst maintaining star brightness. A 32mm plossl and/or 24mm 68° eyepiece would give you the maximum field of view available with your scope. You probably don't need to go much shorter than a 10mm eyepiece and your telescope is quite kind on eyepieces so, aside from picking something to fill in the gap between your eyepieces, your main reason to upgrade will probably be to choose eyepieces with a wider apparent field of view.

If you can see the obstruction then you have defocused way too much and are not doing a star test. You need to have a high power eyepiece, equal or shorter than the telescope's focal ratio, and to have the star perfectly centred on the field of view. At this magnification at least the first diffraction ring should be visible when the star is in focus, you then want to slightly defocus to make the rings more visible. You should see a bright disk with a series of concentric rings around it. The concentricity of the rings shows the collimation, but beware that any star not quite centred will not show concentric rings and so using an off centre star to collimate could make your collimation worse.

Ah, I should have guessed it was either end of a zoom you were talking about. With regards to cooling, I keep my 8" newt in a shed, have a primary cooling fan and I will still get it out to cool early if I think it will be clear and suitable for high power views.

There is quite a difference between a 24 and 8mm eyepiece, do you have anything between the two? Using an 8mm in an 8se will require very good atmospheric conditions, as well as properly cooling or insulating the telescope.

I think it would be best if you take a photo of what you've got and attach it to a post here so we can better understand what it is that you've got and are having trouble with.

Have you tried Celestia?

Yes, but if you are taking the time and effort to set up a tripod you might as well set up a telescope. A small telescope with a lightweight mount that fits on a photo tripod probably won't be much more difficult to carry than a large pair of binoculars that need tripod mounting. If you do want to stick with binoculars then in my experience I think you want to be looking at 8x40 or 10x50 for a hand held pair, or 15x70 with a trigger grip and monopod. Any magnification over this narrows the view to the point that I think you will also need a red dot finder, so you're looking at a tripod to keep things steady as you switch from finder to binocular and my previous comment about getting a telescope applies.

If you want a pair of binoculars then the 8x42 Opticron Adventurer T is the pair I would be looking at. I believe that @BinocularSky uses these when he does observing events for children. However, I disagree with the idea that binoculars are as good as a telescope. Some people prefer the low magnification, wide field views that a binocular gives, but I think that this is quite a small minority of people. For more detailed observations of DSOs or planets a telescope is really what you want. The cheapest range of decent telescopes is the Skywatcher Heritage range. However, most of these are manual telescopes and your child would need to learn to "nudge" the telescope to keep the object inside the field of view. The heritage virtuoso models have motors that keep the object in the field of view, but you will still need to find the object first. If you want a telescope that has goto to both find the object quickly, and keep it in view once found, then I would suggest looking for a model from the Skywatcher AZgte/gti range.

£500 is not throw away money. I think I would rather put that towards the cost of a 100ED rather than a big, heavy achromat that I suspect would end up being sold on for a significant loss.

That doesn't sound right. You should easily be able to add or remove an eyepiece as small as a BST without needing counterweights. Have you got the friction set right on the altitude axis?

How good are the mirrors? If the mirrors are top quality then the addition of a nice focuser would make it a really nice instrument. Of course that is assuming that the poor focuser is a major reason in the lack of use. If a short 6" newt isn't your preferred type of scope and a new focuser won't result in you using it more you might as well let it find a new home where it will be used.

The 25mm BST would give a field of about 2.3°. The 5° of a 9x50 RACI is quite an improvement on this.

What is the aperture of your telescope? This will partly determine which eyepieces are usable with your telescope. If you use a 5mm eyepiece with a telescope with 1000mm focal length telescope you will have a magnification of 200x. At this magnification you should see Jupiter as a small disk with up to four moons around it, and Saturn as a small disk with rings. If you do not see this, the first thing I would do is to check the alignment of your finder scope with your telescope as the difference between a planet and a star should be obvious.

I am a big fan of 9x50 RACI finders, but you are right that it might cause balance issues as it is a significant size compared to your telescope. I don't think there is really any way to know how it affects the scope except to try it. You can get a smaller 6x30 right angled finder, although I can't recall if RACI is an option, but obviously this has both less magnification and light gathering ability. If you do get some sort of right angled finder, my preference would be to relocate the red dot. The Telrad in particular is a very large finder and could even obscure the optical finder on an scope of that size. Also, which BST have you ordered?

On my 9x50 I used self amalgamating tape to replace a missing O-ring. It was supposed to be a temporary fix but maybe 5 years later the tape is still there and the finder is very stable.

I agree with the following sentiment: However, I think the photo already posted in this thread is evidence of my opinion that a collimation cap simply isn't an accurate enough tool to achieve this because the visual gap between the focuser edge and the secondary edge makes it too hard to judge when the secondary is centred. With a long tube from a cheshire/sight tube you can adjust the focuser so that the edge of the tube is almost the same size as the secondary mirror appears and then it is so much easier to accurately get your secondary under the focuser. With the crosshairs at the bottom of the tube you can see exactly where the primary doughnut should be and you can then tweak the secondary so that they are aligned (this is the important step of secondary collimation). With a collimation cap there are no crosshairs and so it is very difficult to know is the doughnut is precisely in the centre. For primary collimation you want to get the reflection of the eye hole inside the doughnut. Here the crosshairs get in the way so it is time to remove the cheshire and use the collimation cap. Alternatively, I prefer to use a short cheshire I bought cheaply on ebay, and simply unscrewed the crosshairs as this can be angled to give a brighter reflection than a collimation cap. Also, it is not obvious to me why the linked article suggests to collimate the primary, then the secondary and then tweak the primary. The first step doesn't seem to do anything, you only have to collimate the secondary and then the primary.

The secondary itself should appear central through the focuser but the shadow of the secondary in the reflection of the primary will be slightly offset. If you take a photo of your collimation and post it here people will be able to advise on adjustments.