Fully focused high power lens (250x) offers a smudged image

[velis]

I started my (daughter's) astronomy journey with a Celestron Powerseeker 127eq. It came with a 20mm EP, 4mm EP and a 3x barlow.

I have great success with 20mm EP: I can see clear images, focus is easy, etc.

Less so with the Barlow: I can't seem to focus it properly - the image is softened regardless of what I do. Note that this is 150x magnification, which should be perfectly in order for this scope, I understand. So far I have not invested significant effort into making it work.

Finally, my greatest issue and the subject of this post is the 4mm lens:

I'd like to use it to observe Jupiter and Saturn (goal is to see Cassini's division) as they are nicely visible in the early night sky right now. I realise that the 250x magnification this EP offers is on the optimistic side of the spectrum, and I concur: the planets are so low on horizon that I can't really expect a sharp image, especially with all the light pollution where I'm observing from.

But, even that is not the issue. The issue I have with the eyepiece is, that with the best focus I can get with the EP, the observed object is smudged: it looks about like a comet. Half of it is in focus, but the other half looks like another planet just hit it. With debris and all

Anyway, I tried rotating the EP in the mount and the "comet" thingy doesn't rotate with it.

So I'm assuming this is about primary mirror / 45° mirror alignment. Sorry about my terms, I'm really green here.

I don't really care if my analysis is correct, so: is there a way I could fix this issue?

Edited October 30, 2021 by velis

[Stu]

1 hour ago, velis said:

I started my (daughter's) astronomy journey with a Celestron Powerseeker 127eq. It came with a 20mm EP, 4mm EP and a 3x barlow.

I have great success with 20mm EP: I can see clear images, focus is easy, etc.

Less so with the Barlow: I can't seem to focus it properly - the image is softened regardless of what I do. Note that this is 150x magnification, which should be perfectly in order for this scope, I understand. So far I have not invested significant effort into making it work.

Finally, my greatest issue and the subject of this post is the 4mm lens:

I'd like to use it to observe Jupiter and Saturn (goal is to see Cassini's division) as they are nicely visible in the early night sky right now. I realise that the 250x magnification this EP offers is on the optimistic side of the spectrum, and I concur: the planets are so low on horizon that I can't really expect a sharp image, especially with all the light pollution where I'm observing from.

But, even that is not the issue. The issue I have with the eyepiece is, that with the best focus I can get with the EP, the observed object is smudged: it looks about like a comet. Half of it is in focus, but the other half looks like another planet just hit it. With debris and all

Anyway, I tried rotating the EP in the mount and the "comet" thingy doesn't rotate with it.

So I'm assuming this is about primary mirror / 45° mirror alignment. Sorry about my terms, I'm really green here.

I don't really care if my analysis is correct, so: is there a way I could fix this issue?

It sounds very much like the scope needs collimating (mirror alignment) as you suspected.

A star test will show it clearly, look at a star like Polaris or similar and defocus it slowly. Everything should remain concentric, if it flares off to one side then your collimation is out. Should be fairly straightforward to correct once you get the hang of it.

[Captain Scarlet]

What Stu means by “defo is” is “defocus” … his spellcheck needs collimating 😄

[Stu]

13 minutes ago, Captain Magenta said:

What Stu means by “defo is” is “defocus” … his spellcheck needs collimating 😄

Indeed! Thank you Magnus 🤣. I will correct it now to avoid more confusion.

[Carbon Brush]

Hello and welcome to SGL.

Yes 'comets' sounds like a collimation issue.

You didn't say where the scope came from, or if it is new or used.
A benefit of buying from an astro retailer is that you can usually pop into the shop and get the scope collimated - with a free lesson on how to do it for yourself.
Failing that, pop to a local astro club and there should be many eager hands. I did this with my first newtonian scope.

Having said that, it shouldn't be difficult to improve the situation. There are various web guides and threads on SGL.
If I have identified the scope correctly, it is almost F8, so should be relatively easy to collimate. Or putting it another way, is tolerant of mis-collimation.
As collimation is very much a daylight job you can do indoors, you don't need good weather.

Really a 4mm EP (250x mag) needs a good stable sky, and a well set up scope. So don't expect good views every time out.

Enjoy the journey.

David.

 

[Ricochet]

Unfortunately, previous reports about this scope suggest that you are unlikely to ever be able to use such magnifications, with an exit pupil of 1mm (127X magnification) being about the limit. It does sound as if your scope is probably miscollimated, so checking the collimation could improve things. The scope is a Jones-Bird style starter scope and so doesn't quite follow the standard Newtonian procedure. I believe the instruction manual has guidance on collimating the telescope with a collimation cap so I suggest that you follow that advice. I wouldn't bother removing the corrector lens from the focuser for collimation as seen on some youtube videos, as previous feedback has been that it doesn't make any difference to the views at the end of the day. 

The eyepieces and barlow supplied are also of low quality so you may also see some improvement in the views by upgrading those. The 10 and 23mm Vite Aspherics are good for very little cost, but have plastic lenses and so are more susceptible to damage when cleaning than more expensive glass lensed options.

[Carbon Brush]

Thanks @Ricochet
I hadn't realised it was a Bird Jones, otherwise my post would have been rather different.
I should have looked at the scope more closely.🥴

To @velis
Don't forget, any eyepeices you may buy are of course transferable to a different scope, so don't always look at lowest cost.

[doublevodka]

Collimation is almost certainly the main contributor here, along with seeing conditions no doubt.

I'd suggest having a read of this lovely thread by @Alan64  which has info on collimation but also a full overhaul on this much maligned scope, hopefully some of it helps, if not it's a great read anyway

 

[AstroMuni]

@velis dont be too disheartened by the bird-jones comments. I have the 130eq which is a slightly bigger version of what you have. It has a slightly better reputation I guess but not too far off I got mine collimated with help from a friend and its worked well for me. Do NOT expect to view with high magnifications but it should do a lovely job for viewing DSO and star clusters. As others have said the mount is its weakest point so I used to attach sandbags/milk bottles filled with water etc. to stablise it.

I decided to take the plunge into astrophotography after I upgraded my mount and was pleasantly surprised with the results (see link in my signature).

Good luck.

Edited November 9, 2021 by AstroMuni