Jump to content


EP / Scope question

Recommended Posts

Im just starting out in the Hobby and so far I am really enjoying it. For my first scope I purchased an Orion XT6 Dobsonian and I am very pleased with it, really enjoying the time Im spending in my yard learning how to star hop.

I had my first very clear night opportunity to try and find a Nebula this past Saturday and I was able to find both M57 and M27. Firstly, it was an absolute thrill for me to be able to find them at alll and then to take in the idea of what it was I was looking at and just how amazing it was. I was using a 40mm Orion Highlight Plossl with a Orion Narrowband filter to first locate the objects, then I took in closer views with a 25mm Plossl. I was surprised at just how much a difference the filter made, especially with the Dumbbell Nebula, which was faint almost to the popint of obscurity without the filter. I was also surprised to see that unlike a cluster, it seemed I could hold a good view with higher resolutions (10mm) although they were darker because of the higher magnification EP.

I have a few questions though. I understand with Plossl EP's the higher the magnification the darker the viewable image is. Can somebody explain how / why that happens?

Secondly, as I was observing the objects I started wondering how different they would appear in a larger scope. I opted for the 6" Dob because although I really wanted to jump in with a larger scope I thought it would be best for me to take my time and learn to find what Im looking for. There are times though (Saturday night included) I wish I had a gone with the larger scope. The 6" has optical diameter of 150mm and a focal length of 1200mm. The 10" numbers were 254/1200. What I don’t understand is that would objects appear any larger from a 6" to a 10" dob or just more visable because of the additional light intake?

Link to comment
Share on other sites

hi there and welcome to SGL!

your scope is a decent starter scope and you have done well to be finding objects straight away - it's always very encouraging when this happens.

with any increased magnification you get a darkening of the image and eventually the 'seeing conditions' (effectively atmospheric of local air disruption) or the maximum resolution of the scope make the image blurry too; in other words there's a maximum magnification you can generally use for each size and type of scope (generally but not always larger scopes allow more resolution) for the seeing conditions (which affects all scopes about the same). I tend to work on 1.5x the aperture in millimetres as the maximum magnification - so in you case around 225x. sometimes you can get a bit more (especially on the moon and double stars) before the image really breaks down but it's often less too.

the reason for this is that all objects (in space) have a fixed amount of light and as you look at the thing more and more stretched out in the view as you increase magnification, it reduces the brightness. they are at their brightest when seen as pinpoints with the eye.

re larger scopes, objects appear brighter not necessarily bigger. with faint objects in the sky you will see increasingly more detail (or be able to see things at all) with more aperture. there's a limit though and most DSOs will always be relatively faint fuzzy whiffs of cloud in space. it's all about area a 6" scope has an area of 3" x 3" x 3.142 = 28" square. an 8" = 50" square, a 10" = 79" square and a 12" = 113" square. so if for example you could magically increase your scope to a 12" scope with the same focal length you'd see M27 around 4x brighter.

magnification is based on focal length of the scope so if you scope is 150mm of aperture and a focal length of 1200mm then it's a f8 scope (1200/150) and with a 10mm eyepiece would give 120x (1200/10) magnification.

hope this helps a bit.

Edited by Moonshane
Link to comment
Share on other sites

Thanks for the detailed reply, I appreciate it. I don’t regret the purchase of this scope in the least, but I would be lying if I said I can’t wait to get a larger one someday – haha.

That same session I also viewed a really brilliant blue / orange double in the Vulpecula constellation. The constellation itself was very faint and I was aiming towards what I thought to be the star Anser. After viewing the charts more closely and looking online when I got in the house I’m not sure I was looking where I thought I was though. If anyone is familiar with such a double in the Anser neighborhood can confirm that would be great. It was the first time I could clearly distinguish a color difference in what I was looking at.

It was an all around fantastic night, the best I have had in the eight weeks I have been out searching around with the scope. I’m hooked!

Link to comment
Share on other sites

ps you can never have a scope that's big enough so just enjoy what you have now and make good use of it. in time you can always get a larger scope when you can justify the invest. broadly it's always worth it. I am currently putting together a 16" which will be as large as I want to go (for now :)).

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Recently Browsing   0 members

    No registered users viewing this page.

  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.