f4 Newt - Collimation, Tilt or Spacing?

[AbsolutelyN]

I'm trying to get nice stars on my Quattro 250 f/4 Newtonian and just wondering if anyone used to collimating an f/4 Newtonian can make any suggestions on what to tweak?
Star test looked ok last night but atmosphere was not not really steady enough to be absolutely certain. Camera is 2600mc so aps-c. Its using the f4 Aplanatic Coma Corrector and spacing with ZWO adapters - 16.5 + 21 + 17.5 of the camera. 

Any pointers appreciated, it would be a lot easier to sort if it didn't get dark quite so late 🙂

 

Edited June 2, 2022 by AbsolutelyN

[Astroscot2]

Slight elongation on all the stars, could be flexure?. How are you guiding, OAG or guidescope 

[ONIKKINEN]

Looks to me like like spacing needs addressing as stars have this circular elongation thingy going on around the edges. I thought this meant there was too much spacing between the corrector and camera and not the other way around? Cant find a source for that now though so hopefully someone can confirm that, but could also be the other way around.

[AbsolutelyN]

27 minutes ago, Astroscot2 said:

Slight elongation on all the stars, could be flexure?. How are you guiding, OAG or guidescope 

Thanks, not thought of that. It's just got a little 50mm guide scope mounted on a dovetail spanning the tube rings. Never tried OAG but the current setup with guide scope has always worked fine with my 250PDS which was longer focal length.   

[AbsolutelyN]

24 minutes ago, ONIKKINEN said:

Looks to me like like spacing needs addressing as stars have this circular elongation thingy going on around the edges. I thought this meant there was too much spacing between the corrector and camera and not the other way around? Cant find a source for that now though so hopefully someone can confirm that, but could also be the other way around.

Brill thanks .... I think you are right. Found this on google which would indeed suggest too far away. Not sure how that's possible though, will go take the camera off and double check the exact spacing. 

[newbie alert]

Certainly looks like you're too far away rather than too near.. did you measure the spacing distance as a 10mm spacing isn't always 10mm

APS-C sized sensors need far more critical spacing than smaller sensors

[AbsolutelyN]

So the spacing seems to be bang on 55mm. No filters anywhere in the optical train.

Perhaps get some M42 extension tubes and switch 21mm for 20mm? I can then possibly tweak with 0.3mm / 0.5mm spacers. 

https://www.firstlightoptics.com/adapters/astro-essentials-t2-m42-extension-tube-set-4mm-5mm-6mm-7mm-8mm-9mm.html

https://www.firstlightoptics.com/adapters/astro-essentials-t2-m42-extension-tube-set-5mm-10mm-15mm-20mm.html

Edited June 2, 2022 by AbsolutelyN

[Clarkey]

5 hours ago, AbsolutelyN said:

Perhaps get some M42 extension tubes and switch 21mm for 20mm? I can then possibly tweak with 0.3mm / 0.5mm spacers. 

That would be my thought. The 55mm spacing should not be taken as gospel - there is always some deviation. I have a collection of small spacers that get regular outings!

[AbsolutelyN]

13 minutes ago, Clarkey said:

That would be my thought. The 55mm spacing should not be taken as gospel - there is always some deviation. I have a collection of small spacers that get regular outings!

Thanks - I'll order some and give that a go. I've never had any coma corrector spacing issues before but that was at f/5 so probably a lot more tolerant than f/4.  

[Dan_Paris]

The backfocus requirement of the GPU corrector depends on the focal length (in order to correct field curvature on top of coma). The exact values are as follows:

• F=600mm,  backfocus = 51,66mm
• F=800mm, backfocus = 53,66mm
• F=1000mm, backfocus = 55,0mm
• F=1200mm, backfocus = 54,66mm
• gtom 1500mm, backfocus 54,60mm

So in your case 55mm should be OK.

Did you check the thickness of the ZWO extension tubes with a vernier caliper? I found that the GPU corrector is extremely sensitive to backfocus requirement, it should be dialed with 0,1mm accuracy. I concur with the idea of buying a set of good quality spacers and spend some time to tune the backfocus.

Do you have the stock focuser?  Which tools do you use for collimation?

 

Dan

 

Edited June 3, 2022 by Dan_Paris

[AbsolutelyN]

2 hours ago, Dan_Paris said:

The backfocus requirement of the GPU corrector depends on the focal length (in order to correct field curvature on top of coma). The exact values are as follows:

• F=600mm,  backfocus = 51,66mm
• F=800mm, backfocus = 53,66mm
• F=1000mm, backfocus = 55,0mm
• F=1200mm, backfocus = 54,66mm
• gtom 1500mm, backfocus 54,60mm

So in your case 55mm should be OK.

Did you check the thickness of the ZWO extension tubes with a vernier caliper? I found that the GPU corrector is extremely sensitive to backfocus requirement, it should be dialed with 0,1mm accuracy. I concur with the idea of buying a set of good quality spacers and spend some time to tune the backfocus.

Do you have the stock focuser?  Which tools do you use for collimation?

 

Dan

 

Thanks Dan that is really useful info. Previously I used the coma corrector on my 250PDS and was very happy with the stars. Image below taken with same corrector, exactly the same 55mm spacing but at 1200mm so 200mm longer focal length. 

I've measured the spacers and they come out at 37.49mm (both together - 16.5 + 21) so no issue there. From your table I'm guessing it needs to be about 0.66mm less spacing assuming the corrector was spot on at 1200mm. 

Focuser is a Baader SteelTrack and I've collimated with a new Farpoint laser and cheshire I got specific for the job. I'm confident the collimation is pretty good, star tests look good. I think it will just be a matter of getting a 20mm spacer and then very fine adjustment spacers - and a bit of time consuming trial and error testing.    

Cheers

Edited June 3, 2022 by AbsolutelyN

[Dan_Paris]

31 minutes ago, AbsolutelyN said:

star tests look good

I am not saying that this is the issue that you have, but a star test cannot detect whether the focal plane is tilted w.r.t. the sensor plane. It only asserts that the optical axis and the mechanical axis of the focuser intersect at the focus position. It does not tell whether they are coincident.  For that you need a well-collimated laser, or a Catseye autocollimator.

 

The Steeltrack focuser is a reliable model, but how is the corrector attached to it ? Compression ring, clicklock or threaded connection ?

 

Dan

 

 

[AbsolutelyN]

6 minutes ago, Dan_Paris said:

I am not saying that this is the issue that you have, but a star test cannot detect whether the focal plane is tilted w.r.t. the sensor plane. It only asserts that the optical axis and the mechanical axis of the focuser intersect at the focus position. It does not tell whether they are coincident.  For that you need a well-collimated laser, or a Catseye autocollimator.

 

The Steeltrack focuser is a reliable model, but how is the corrector attached to it ? Compression ring, clicklock or threaded connection ?

 

Dan

 

 

Good points, understood. The corrector is just attached with compression ring. It's much better than than skywatcher compression ring, and I've done my best to put even pressure on with the three compression screws but I guess tilt could always come in via that connection. There is no thread on the coma corrector so I can't see any way of making it a threaded connection. I've never understood why coma correctors for reflectors are not threaded. I was planning on getting the farpoint autocollimator. 

[Dan_Paris]

1 hour ago, AbsolutelyN said:

There is no thread on the coma corrector so I can't see any way of making it a threaded connection. I've never understood why coma correctors for reflectors are not threaded.

The point is that refractors have loads of focus travel, so correctors can be screwed after the focuser, while on Newtonians they should sit inside the focuser, which requires a different type of attachment.

On the Teleskop Service version of the GPU corrector, on can unscrew the stop ring which  releases few millimeters of extra M48 thread. I don't know about yours.  If it is the case you would need a step ring with 55mm outside (to screw in the focuser) and 48mm inside.

 

1 hour ago, AbsolutelyN said:

I've done my best to put even pressure on with the three compression screws but I guess tilt could always come in via that connection.

The best advice I could give is to simply remove one of the three screws from the focuser. If you apply equal pressure on the three screws, the corrector "floats" inside the drawtube an nothing guarantees that it is parallel to the optical axis. The crucial point is that parallelism is more critical than centering. A proper procedure is as follows:

1. While the screws are loose, push the corrector stop ring against the focuser edge (a bit firmly but not too much) . Do not release the pressure till the end.
2. Tighten fully the first screw to push the corrector against the wall of the focuser drawtube, which guarantees that the optical axis and the mechanical axis of the focuser are parallel to each other.
3. Tighten then the second screw, but not too much, in order to remove the last degree of freedom.

 

Edited June 3, 2022 by Dan_Paris

[AbsolutelyN]

That's great thanks, I shall have have a go as you describe. 

The stop ring doesn't seem to be detachable at all, seems to be part of the same mould. If I'd known that I'd have gone with the GPU. I did a quick search to see if it was removable and by chance came across this thread: https://www.cloudynights.com/topic/744140-coma-corrector-for-the-skywatcher-quattro/

That has cast slight doubt as to if it is the Skywatcher f/4 Aplanatic Super Coma Corrector or if they have slightly changed it. That might explain why 55mm is not spot on for f/4. Unlikely hopefully. Mine is exactly as pictured here - comma corrector used on reflectors - f4/5

Edited June 3, 2022 by AbsolutelyN