Jump to content

Narrowband

150PL - the good and the bad


RobertI

Recommended Posts

I had a great first light with my new 150PL last night - full observing report can be found here . In summary it fulfilled my hope as excellent double star splitter, clearly splitting a test pair of 1.5" separation and fleetingly splitting a pair of 1.0" separation in moments of clarity. The focuser was on the stiff side, making it hard to not wobble the scope at high mags - I may invest in £45 Skywatcher motorised focuser which many other folk seem to have done.

However, despite the excellent double splitting ability, the star test results were awful. Now I am no expert, but the extra focal image seemed sharp and more or less what you would expect, but the intra-focal image was just a fuzzy mush! I have done a very poor drawing below from memory (not sure if they'll confuse more than clarify) - extra-focal on left and intra-focal on right. If I understand the results correctly, it shows the mirror is overcorrected and possibly suffering from a turned down edge (depending on what you read). Assume the circles in the drawing are concentric and even!!

IMG_2162.thumb.jpg.3663ce17bcc9e7ae181a8c7df0442a5a.jpg

 

Also the in focus star didn't show the diffraction pattern I was expecting. Instead of one bright diffraction ring and possibly a fainter one, I could see several very fine and closely spaced rings of equal brightness - I don't think this was an illusion, but it was faint. Aplogies for the naff illustration again, what I was expecting is on the right, what I saw was on the left.

1622323401_Focussedstar2(2).png.adf1b6cd0ee017afcd8800ae0fdc2ce6.png

The airy disc seemd ok and well defined and the focus, whilst not 'snapping' seemd easy enough to find.

I have read that mirrors can become overcorrected as the temperature drops, but seems a bit unlikely as the scope was well cooled.

Can anyone explain what on earth is going on??

 

Link to comment
Share on other sites

9 minutes ago, johninderby said:

Have a read of this guide. Should help explain things.

http://www.astrophoto.fr/collim.html

Thanks John, had a quick skim of the article, and although it's similar to what I had seen elsewhere the diagrams do raise some questions about collimation. I'm pretty sure I was well collimated - firstly with a cheshire indoors and then with a defocused star on the night. I didn't notice any assymetry, but accept this can be complex.

What confuses me is how it could resolve close binaries after a poor star test?

Edited by RobertI
Link to comment
Share on other sites

I’ve found star tests are so dependant on good seeing. 

For collimating a newt I start out with setting up the secondary with a Concentre. Found a Cheshire gets you somewhere near but not spot on. Then use a laser collimator which unless it’s a good one usualy needs colimating itself.

https://www.teleskop-express.de/shop/product_info.php/info/p5507_TS-Optics-Concenter-1-25--Collimation-Eyepiece-for-Newtonian-Telescopes.html

Here’s a 150pl that I had but upgraded with OO 1/10pv mirrors. Wish I had kept it. 🙁

4AD9E9FA-DEF9-47AF-980D-6A928D9F1754.jpeg

Edited by johninderby
Link to comment
Share on other sites

7 hours ago, johninderby said:

I’ve found star tests are so dependant on good seeing. 

For collimating a newt I start out with setting up the secondary with a Concentre. Found a Cheshire gets you somewhere near but not spot on. Then use a laser collimator which unless it’s a good one usualy needs colimating itself.

https://www.teleskop-express.de/shop/product_info.php/info/p5507_TS-Optics-Concenter-1-25--Collimation-Eyepiece-for-Newtonian-Telescopes.html

Here’s a 150pl that I had but upgraded with OO 1/10pv mirrors. Wish I had kept it. 🙁

Thanks John, I’ve never seen a concentre before, it does look like an improvement over a Cheshire. A laser collimator is on my list. 
 

I had thought about upgraded mirrors if mine turned out to be dodgy - did you find them easy to fit?

Link to comment
Share on other sites

The Concentre is one of those bits of kit that once used you can’t go back to a Cheshire.for setting up a secondary.

Easy enough to fit the OO mirrors but used an OO 9 point mirror cell which fitted to the existing mounting ring on the scope.You would have to remove the old  secondary miiror from its mount but easier just ti buy a new seconday mount. I picked up the mirrors second hand on Astrobuysell.

60523450-4D20-466F-92B9-9DF9E24C5434.jpeg

664806B7-1C31-4548-BAA7-5ABD58ED4E5B.jpeg

 

93EB5923-5CBF-4095-BDE1-90B1C751A7C6.jpeg

Edited by johninderby
  • Thanks 1
Link to comment
Share on other sites

The stock scope was very good actually but with the OO mirrors it was exceptional. Really noticed the difference. 

Wish I had never sold it. A mistake I still regret. 🙁

I have seriously thought about picking up a cheap 150PL and fitting OO 1/10pv mirrors.

Edited by johninderby
  • Like 1
Link to comment
Share on other sites

If your scope is splitting 1 arc second doubles, even if that "comes and goes" it's doing very well. I would re-check the star test a few times under varying conditions before jumping to any conclusions about the mirror quality.

 

 

  • Like 2
Link to comment
Share on other sites

21 minutes ago, John said:

If your scope is splitting 1 arc second doubles, even if that "comes and goes" it's doing very well. I would re-check the star test a few times under varying conditions before jumping to any conclusions about the mirror quality.

 

 

Thanks John, I will definitely redo the star testing each time I observe, I’m sure the things will become clearer. I suspect there are lots of factors at play here, as always it will be an education! 

Link to comment
Share on other sites

24 minutes ago, mdstuart said:

My star test on my big dob is similar to yours. After observing for a few hours the fuzzyness reduces and in and out become similar. So be patient.

Mark

Ah that’s very re-assuring, thanks Mark. 

Link to comment
Share on other sites

On 30/01/2020 at 10:05, Captain Magenta said:

+1 for the Concenter. I got infected with the "Concenter is absolutely essential" virus by @johninderby, and have myself infected two more people.

I have been using a 2" version for a while, damn good. I also have been infected by the bug.

Nadeem.

  • Like 2
  • Thanks 1
Link to comment
Share on other sites

  • 4 weeks later...

Just fitted a home-made cooling fan to the back of the scope using instructions from a fellow SGL member. I didn't do a fantastic job (missing some vital tools), but it does blow a nice breeze up the tube, has a variable speed and is vibration free. My hope is that it speeds up cool down time, but more importantly improves high powered views of lunar, planets and possibly doubles, by removing the boundary layer of warm air that sits over the mirror. It's clear outside but I have a streaming cold so will wait for another time to test it.

 

fan.jpg

Edited by RobertI
Link to comment
Share on other sites

  • 4 weeks later...
On 28/02/2020 at 03:59, RobertI said:

Just fitted a home-made cooling fan to the back of the scope using instructions from a fellow SGL member. I didn't do a fantastic job (missing some vital tools), but it does blow a nice breeze up the tube, has a variable speed and is vibration free. My hope is that it speeds up cool down time, but more importantly improves high powered views of lunar, planets and possibly doubles, by removing the boundary layer of warm air that sits over the mirror. It's clear outside but I have a streaming cold so will wait for another time to test it.

 

fan.jpg

Just one question; are cooling fans required for a 6 inch Newtonian?

I've always read that 6" scopes cool down in around 20-30 minutes, while bigger aperture (8" and up) take longer to cool down, and so a cooling fan is necessary for them.

Or am I missing something here?

Link to comment
Share on other sites

5 hours ago, DanyalAG said:

Just one question; are cooling fans required for a 6 inch Newtonian?

I've always read that 6" scopes cool down in around 20-30 minutes, while bigger aperture (8" and up) take longer to cool down, and so a cooling fan is necessary for them.

Or am I missing something here?

A fair question. I bought the scope specifically for double star and planetary observing and from my research it seems that a fan can improve a newtonian's  ability to resolve close doubles and fine planetary detail by removing the 'boundary layer' of warm air above the mirror. Apparently this boundary layer is always present to a greater or lesser extent, even if the mirror has cooled, and even on smaller scopes. The fan at low speed creates a gentle current of air up the tube which keeps the air moving over the mirror and removes the boundary layer problem. I've read it works well, but I have not had a chance to test it the difference with the fan on and off under different conditions.

  • Like 1
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.