8" Dobsonian Focuser question

[AndyM001]

Hi, hopefully someone will be able to answer a question that I've struggled to find on searches. I'm currently looking to purchase a Dobsonian soon, something like a Sky-Watcher Classic 200P Dobsonian or the Ursa Major 8" Dobsonian. After a fair bit of research, I've noticed that some people carry out various modifications, like a mask to hide the primary mirror clips to get rid of diffraction errors, and some people have stated that anything in the light path causes diffraction. After looking at pictures of the 2 scopes, the focuser appears to protrude in to the tube a bit on the Sky-Watcher, but not apparently on the Ursa Major. How much of a difference does this actually make to the image in real world viewing? I'm going to concentrate on the Moon and planets initially, but will move on to DSO's later. Thanks.

[M40]

I moved my 200P dob on about 3 years ago now and I used it for visual and with a dslr but from memory I must say that I never noticed anything protuding into the image or through the eyepiece although I will never class myself as a serious imager. Hopefully if someone has noticed it they will let you know. All the best.

[John]

I found the same with the Skywatcher focuser, all though not as extreme as in the above photo. I now use a shorter eyepiece extension tube - 35mm whereas the original 1.25 and 2 inch adapters are around 50mm. This change means that the focuser is racked out a little further to get eyepieces to focus so the end of it does not intrude into the light path, or at least not as much. I can't say that I've noticed a difference in the views though, to be honest. 

It is also worth bearing in mind that not all eyepieces come to focus at the same point. There can be as much as 15mm difference depending on the brand / model of eyepiece. So that can affect the degree to which the focuser extends into the light path as well.

 

[AndyM001]

Thanks for the replies, forgot to mention I live in Bortle 5, not sure if that makes a difference.

I've seen a couple of second hand Dobs for sale at reasonable prices, so if I purchased one of them I could potentially upgrade the focuser if needed.

[The60mmKid]

Worry not; it won't make a difference! I've owned a couple of dobs with focuser tubes that intrude on the light path, and I've also used accessories (e.g., Paracorr, binoviewer GPC) that do so. Even when using high magnifications, there isn't a visible impact. Like you, I worried about this at first and consulted with a couple of respected telescope makers, who shared the perspective that it's a non-issue.

[Spile]

There is nothing wrong with the Skywatcher focuser. It can be upgraded to dual speed and that is worth doing in my opinion. The focus tube extends into the tube a few cm  if an extender is used when non is required. If you are buying new I would recommend the StellaLyra Dobsonian as it is better equipped than the Ursa Major or Skywatcher. 

[John]

33 minutes ago, AndyM001 said:

Thanks for the replies, forgot to mention I live in Bortle 5, not sure if that makes a difference.

I've seen a couple of second hand Dobs for sale at reasonable prices, so if I purchased one of them I could potentially upgrade the focuser if needed.

My skies are Bortle 5 as well. The Skyliner 200P that I have recently acquired performs very well now that I've made a couple of minor tweaks to it. Optically it's very good indeed. I owned one about 10 years ago and have forgotten how good they are.

 

 

[AndyM001]

Thanks for the replies, definitely put my mind to rest.

I did look at the StellaLyra, and it does look very tempting, but I'm on a budget at the moment. If I can resist buying for a few more months and it's in stock, I may well go for it. As I said, I've seen a couple of second hand ones that look okay, so may look in to getting one of those and possibly upgrading.

[Zermelo]

With a Dob (Newtonian), you already have the secondary mirror and spider vanes obstructing the light, but you don't see them (if the image is in focus). What you do get is some diffraction at the edges of the obstruction, which reduces image contrast. A focuser extending into the tube will have the same effect.

I have a similar situation with my Newt. What I was concerned about was the draw tube being a very shiny silver on the outside, which I thought might give some unwanted reflections.