Jump to content

sgl_imaging_challenge_banner_31.thumb.jpg.b7a41d6a0fa4e315f57ea3e240acf140.jpg

Recommended Posts

I posted on this forum a little while ago about trouble I was having focusing in on distant objects - planets. Still having problems focusing on distant objects even after purchasing a 7.5mm Ep and using a Barlow x2, the problem only ever occurs with the Barlow. Had some advice from a follow startgazer with more experience then I and they definitely said I was focusing in on the current planets.

I have double checked the collimation and it is spot one.

So I have a daft question regarding the Crayford Focuser. When you focus in and the focuser moves down towards the tube, the actual barrel of the focuser goes inside the tube across the line of sight of the primary mirror. Would this affect the actual image projected back up to the Secondary Mirror?

Link to post
Share on other sites

It will have an effect, but it'll not cause the issues your seeing.

If the problems only occur when you use a barlow, maybe the barlow is faulty or poor quality. Have you taken it apart?

The effect of adding a barlow is to generally move the "in focus" position further out - so the issue of the focuser draw tube becoming visible in the light path actually reduces.

What effect are you actually seeing when you use the Barlow?

Ant

Link to post
Share on other sites

David I can state that the focuser does move into the OTA and its reflection can be seen in the primary.

With regard to sharp focus you may find that you might need to purchase an extension tube to gain focus with certain EPs - link to give you an example - http://www.astronomica.co.uk/accessories/adapters/2-inch-extension-tube

To check before you buy - move the focuser to its limit and then, by hand, move the EP/barlow away from the focuser to see if you gain focus.

Tell us how you get on and remember you can always do this task on a distant telegraph pole rather than wait for a clear sky.

Mark

Edited by MarkC
Link to post
Share on other sites

Is it outward or inward focuser travel you are running out of when you use the barlow ?

With focus problems that have been reported in the past, the solution has been much easier to arrive at when a photo or two of the way the focuser has been setup for observing has been posted on here so we can see exactly how things are set up. Could you do that ?.

Apologies if you have already posted such a photo.

Link to post
Share on other sites

If you are saying that you can't get to a focus, ie the image reaching its smallest size before getting larger again as you continue to focus inwards then there is a spacing issue. If you mean that nowhere between these limits you do not get a clear focus then either the Barlow is not a good one or the magnification with the 7.5mm eyepiece plus Barlow is too great for the seeing conditions, or both.

Link to post
Share on other sites

If you are indeed reaching focus, you might find that 7.5mm + a 2x barlow (about 200x magnification) is perhaps a little too much for the scope/barlow optics or the seeing to get a clear image. Also, you need to ensure the scope has had plenty of time to cool down.

Link to post
Share on other sites

If you are using the 7.5mm and the barlow then probably too much magnification. I tend to use the idea that an eyepiece equal to the scope f number should be OK. If smaller then the f number then some good luck may be needed.

Your scope will be f/5 so dropping below 5mm is asking for a bit of luck. Also in general below 5mm the eyepiece has to be good. The angles of refraction involved are fairly extreme so the eyepiece has to be engineered accordingly.

If the barlow isn't good then the high magnification and a poor barlow will give a poor image.

What is the 7.5mm eyepiece?

Which barlow?

Link to post
Share on other sites

You are using to much magnification, with the Barlow in your reducing the 7.5 down to 3.75mm

Most scopes work best with the maximum power of between 8mm-10mm EP

So best not to use the barlow with such a small EP.

Link to post
Share on other sites

agree with my 10" most of the time 200x is pushing it, its only at rare times does the atmosphere relent and then 400x is possible. Most of the rest i use 150x for planets small and sharp is better than a big blob.

Link to post
Share on other sites

Hi,

Many thanks for all the replies.

We are experiencing a Welsh Summer at the moment - cloud and rain, so have not been able to check.:)

I do however think it is the Barlow which came with the scope so I'll speak to FLO and see if they will exchange it and try again.

I'll report back.

Link to post
Share on other sites

If it is the one that came with the scope they all seem poor. Chances are the swapped one will be the same, maybe worse.:hello2:

Afraid that the simple option is to buy a better one. The Tal always gets good comments, trouble is actually getting hold of one. Retailers get them, astronomers then buy them as fast as they arrive. The TV offerings are good but thay are also damn expensive by comparison.

Think the Celestron Delux ED or something is good, but check with others. I do not use barlows so cannot suggest one from experience. I just have lots of eyepieces:evil6::):evil6: From 3.2mm to 40mm.

Link to post
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.