Skywatcher 150 PDS issues

[AbsolutelyN]

I've just picked up a Skywatcher 150PDS in the hope I can set it up (and take it down) much quicker on less favourable nights rather than hauling out the huge 250PDS which requires a 100% good night for it to be worth the effort. On inspecting the scope the drawtube protrudes a long way into the tube and after a few googles this seems to be an issue with the 130pds/150pds which I did not spot before buying. I cant seem to figure out which coma correctors solve or exasperate the issue.  I'm using the skywatcher 0.9x coma corrector. Does anyone know if I'll need to hacksaw the draw tube for good stars?  Current conditions don't allow for even taking the scope outside.

The other issue I immediately found was balancing. Being so much smaller the imaging end with camera/filter-wheel/focus motor seems considerably heavier than the mirror end. I noted ankle weights were good for the 130pds but  suspect the 150 is too big for those - does anyone have any recommendations for balancing the 150pds?  

Any thoughts much appreciated.

Edited October 27, 2021 by AbsolutelyN

[vlaiv]

I've read on several occasions that GPU CC pushes focal point further out so you need to rack out tube more - and it solves issues with protruding focuser tube.

You'll also want to mask off mirror clips for perfect stars.

https://www.teleskop-express.de/shop/product_info.php/info/p6706_TS-Optics-NEWTON-Coma-Corrector-1-0x-TSGPU-Superflat---4-element---2--connection.html

[AbsolutelyN]

Thank @vlaiv much appreciated. I'll be 3d printing a mask for the mirror clips but I just had not anticipated the focus tube issue. Being on a limited budget the coma corrector you mention is great but is out of budget (already have the skywatcher) and that kind of defeats the point of buying the scope as it basically costs the same as the scope itself. Perhaps I'd have been better saving for the TS 150 f/4 and that corrector ... but as is perhaps the hacksaw may the the best option. Interesting though, I dint know those properties of the TS corrector so thank you. 

Edited October 27, 2021 by AbsolutelyN

[vlaiv]

I've heard that some people dealt with the issue with hacksaw - so that is an option too.

 

[Clarkey]

1 hour ago, AbsolutelyN said:

Perhaps I'd have been better saving for the TS 150 f/4 and that corrector

Be ready to change the focuser too. I have just got the TS f4 Photon and then had to spend another £500 on a new focuser and Aplanatic CC as the Baader MPCC was not good enough. (Claimed to be OK at F4, but I could not get it to work).

[AbsolutelyN]

Thanks - yes I'd heard the TS f4 focuser was not great. I'd have gone with it but as you've found out it adds a lot of cost needing to upgrade the coma corrector. 

All the mods now done so ready for first light. 18mm chopped off the focus tube, flocked inside, aperture mask around the mirror, bobs knobs added and dovetail switched to Losmandy. Just need a clear sky to test it ... hope I've not copped too much off the drawtube 🙂 

 

[AbsolutelyN]

Not yet complete but I've compiled a list of mods that I've made to this scope so far here - https://www.absolutelynothing.co.uk/blog/skywatcher-150pds-modifications/

First light image:

[Rallemikken]

Get some 5mmx50 bolts with recessed heads, and move the mirror upwards in the tube. Did that with my 200PDS, works like a charm. Made 3 spacers 15mm thick, this will give room to move mirror 15-20mm. No problem with eyepieces. The drawtube is now flush with the tube when my Canon is in focus.

This mod can be reversed without the need for a hacksaw. Make a drawstring bag in some dark cloth and tie it around the bottom of the tube. Theese tubes leak light unmodified, and the rising of the mirror makes it even worse. I've not done anything regarding the three clips, but I've painted the sides and bottom of the mirror matt black.

[david_taurus83]

On 01/11/2021 at 12:32, AbsolutelyN said:

Not yet complete but I've compiled a list of mods that I've made to this scope so far here - https://www.absolutelynothing.co.uk/blog/skywatcher-150pds-modifications/

First light image:

That is some first light!

[Padraic M]

On 07/11/2021 at 21:11, Rallemikken said:

Theese tubes leak light unmodified

Interesting - just found that out yesterday while trying to take flats/darkflats in daylight. Had to wait until after dark and cover with a bin liner. In your view does this also impact lights?

[AbsolutelyN]

1 hour ago, Padraic M said:

Interesting - just found that out yesterday while trying to take flats/darkflats in daylight. Had to wait until after dark and cover with a bin liner. In your view does this also impact lights?

I would think if there is a light leak it will affect the flats too.  

I'm having a lot of problems with what I think (but not at all certain) is caused by a light leak. Narrowband seems absolutely fine but if I attach my 2600MC the results are awful. Currently I only use this scope for narrowband. I've painted the back of the mirror black, I have a fan attached which covers and blacks out a lot of the back of the tube ... next step is going to be to complete seal off the primary mirror end of the tube to ensure not light getting in at all. 

This is a quick stack of 19 mins with a dynamic background extraction to really emphasis the issue - no flats applied so there are some marks caused by dust. But the main large circular area surrounded by slightly lighter halo I cannot get rid of.  As mentioned above next step will be total blackout of back end of the scope to see if that makes any difference. So far brilliant scope for HA but starting to wish I bought a different scope now, never had any issues even slightly like this with my 250PDS. 

[Rallemikken]

2 hours ago, Padraic M said:

In your view does this also impact lights?

No, unless you have stray light from windows, torches or cars around you. But I don't like it. I have made two mods to remedy this: First I made a ring out of plastic with the inner diameter like the outer diameter of the mirror, and the outer diameter like the inner of the tube. I have flocked it on both sides, and gently pressed it down around the mirror (after taking it out first). Slots for the clips, and the clips themselves is hidden with small patches of flocking material. The ring sits a few mm below the top of the mirror. The sides of the mirror is also flocked. Almost like a mirror mask, without restricting aperature. And tight fit against the tube, no light leaks any way. I've also made a sort of string-bag to tie around the bottom of the tube. Inside it I've hidden a small dew-heater band. Actuall keeps both mirrors from icing up. Must keep an eye on focus through the night. Tmp often drops below -10 C. This mod gave me better frames, less mess around the bright stars.

[AbsolutelyN]

16 minutes ago, Rallemikken said:

No, unless you have stray light from windows, torches or cars around you. But I don't like it. I have made two mods to remedy this: First I made a ring out of plastic with the inner diameter like the outer diameter of the mirror, and the outer diameter like the inner of the tube. I have flocked it on both sides, and gently pressed it down around the mirror (after taking it out first). Slots for the clips, and the clips themselves is hidden with small patches of flocking material. The ring sits a few mm below the top of the mirror. The sides of the mirror is also flocked. Almost like a mirror mask, without restricting aperature. And tight fit against the tube, no light leaks any way. I've also made a sort of string-bag to tie around the bottom of the tube. Inside it I've hidden a small dew-heater band. Actuall keeps both mirrors from icing up. Must keep an eye on focus through the night. Tmp often drops below -10 C. This mod gave me better frames, less mess around the bright stars.

Sounds really interesting thanks - would you be-able to post a photo of your mods? 

[Padraic M]

@AbsolutelyN I'm enjoying the scope generally. Using it for HA RGB. Gathered some luminance for the first time over the weekend and that's when I noticed - such a bad effect it would have been impossible not to notice. I didn't keep any of the subs unfortunately, but there was a strong curved line across the middle (probably a reflection from a glass edge) and an embossed pattern in the middle - more detailed than a standard vignette. Initially I was worried that the Lum filter was cracked but when I looked closely I could see it on the colour filters too. Most of it was removed when I covered the mirror end with a black sack but not all. 

Took fresh flats last night and they looked and worked fine.

28 minutes ago, Rallemikken said:

But I don't like it.

I agree - if it lets a lot of light in during the day, it's bound to let a little in at night so might have a contrast/definition impact. I'll look into flocking and a mirror mask to cover the clips and block the back-lighting. I also like your 'shower cap' idea!

When I get more time, I'll experiment to see if there is light leaking from anywhere else. 

[Rallemikken]

6 hours ago, AbsolutelyN said:

Sounds really interesting thanks - would you be-able to post a photo of your mods? 

I've been trying to get a good shot of Alnitak, Flame nebula and the horsehead since I got my rig last year. So this will be my testbed for so long. It is a difficult target with the composition I have choosen, and my location at 64 degrees north. Low. Pictures taken with same camera, but modded on the two last. Don't be fooled, at first glance the darkest seems the best. Pay attention to the foggy mist around the star. The halo's on the two last is caused by overexposure, or so I belive. This is details from RAW files, no adjustments, from the same Canon 600D.  I have learned the hard way that this target don't cope with long exposures, due  to the spillover of light around the bright star. Next session (whenever that will be) will be spent with 30 sec @ ISO 800.

I have two aperture rings, one upper and one lower. The first sits sturdy on the spider, and shields the spider nuts from light. I also think it shields the edges of the primary mirror of some extend. The cutout is for the seem on the tube. The lower one looks almost exactly the same, but has additional cutouts for the mirror clips (which themselves are camouflaged with small pieces of flocking material). Primary mirror is flocked on sides and bottom. Had to remove the three filt pieces in the mirror cell to get it in place. As for the long bolts on the cell; on occasion I raise the mirror in order to get rid of the protruding focus tube, depending on which camera I use. I use 15mm aluminium spacers between the upper part of the cell and the springs.

Here's To You, all brave men (and women) that choose reflectors! When the clouds roll in we can spend time tinkering! I know theese telescopes can take incredible pictures, and I'm learning for each mod.

Edited January 31, 2022 by Rallemikken