Unable to achieve focus after reflector modifications - Forums

Topic starter

7 days ago

So last week I received my new spider, primary mask and rear plate(?) off AliExpress (link for reference - https://www.aliexpre....51d31802lnldIB) for my Sky Watcher Quattro 250P F4

My steps so far as follows:

I installed the spider and mounted the secondary mirror - algood no problems
I removed the primary mirror, cleaned and installed the mask - algood no problems
I installed the red plate on the base of the scope behind the primary mirror cell - again algood no problems
I collimated and waited for the next clear night

Now in-between collimating the scope and actually taking it out I had removed the ASI2600MM and attached it to a standard camera lens for other purposes but then reattached it to the reflector as it was before and never had a trouble with it setup as follows:

Coma Corrector ---> OAG-L (threaded onto corrector) ---> ZWO 7x36mm EFW (physically screwed to OAG) ---> ASI2600mm with tilt plate removed (physically screwed to EFW)

I had always gained focus with this setup so left the EAF settings as they were prior

Took the scope out about an hour before this post and I could not gain focus whatsoever!! The EAF position when I done the upgrades to the scope was at 2680 approx from memory and that was perfect focus!! When I done the upgrades and went to focus again, I had to move the EAF position down to 1 to almost get focus but I could never achieve it!! I tried various positions between 1 and 4000 and the closest was always down close to 1

It seems like the sensor need to get closer to gain focus which surely is not right

I apologise if this is hard to follow along with

Any help to sort my focus issue is much appreciated

Photo’s to come

Scope - Sky Watcher 250P F4 with Sky Watcher Coma Corrector
Camera - ZWO ASI2600mm with ZWO EAF+EFW

Tony Gondola

Frequent contributor

7 days ago

Obviously the distance from the primary to the secondary has changed. Forget about the EAF, disconnect it and find focus manually. Reconnect the EAF and Bob's your Uncle. Or in my case, my cousin…

Sam Badcock

Topic starter

7 days ago

Tony Gondola:
Obviously the distance from the primary to the secondary has changed. Forget about the EAF, disconnect it and find focus manually. Reconnect the EAF and Bob's your Uncle. Or in my case, my cousin...

Is Fanny your aunt too?? 🤣

Jokes aside, I’ll disconnect to EAF and try a manual focus though I don’t think that’s going to do much (could be wrong) as it seems I have too much back focus in a sense!! Seems like the sensor needs to be closer!!

Spitballing here, if I adjust the height?? of the secondary mirror via the centre adjustment screw within the spider, which effectively will change the distance between it and the primary, would that be another thing to try??

andrea tasselli

Frequent contributor

7 days ago

Are you sure you mounted the primary in the same exact place it was before? I bet it isn't. As for the secondary, unless you move the focuser up or down on the tube there is ONLY one position that is optimal. All the others result in uneven illumination and pointing offsets, whilst be more difficult to collimate too.

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Sam Badcock

Topic starter

7 days ago

andrea tasselli:
Are you sure you mounted the primary in the same exact place it was before? I bet it isn't. As for the secondary, unless you move the focuser up or down on the tube there is ONLY one position that is optimal. All the others result in uneven illumination and pointing offsets, whilst be more difficult to collimate too.

I’m pretty sure it’s in the same position 🤔 I will remove it and see if there’s a way to tell!!

Tony Gondola

Frequent contributor

7 days ago

If the sensor needs to be closer to the tube then you have less back focus than before. not more. How much of a difference are you seeing?

Sam Badcock

Topic starter

7 days ago

If the sensor needed to be closer wouldn’t that mean I have too much back focus?? By closer I mean closer to the secondary, atleast that’s my thinking!!

As I was moving the camera closer to the secondary via the EAF, the stars were getting sharper until the point the EAF had no travel left!!

Tony Gondola

Frequent contributor

7 days ago

That means you need to increase the back focus. You have to move the point of focus away from the secondary. There are only two things that could have caused the point of focus to move inward. The primary is lower in the mount than it was before or the secondary has moved towards the top (sky end) of the tube. As was mentioned, there is only one right position for the secondary. Looking through the focuser the circular outline of the secondary should be located in the center of the opening. If it’s not then you need to adjust it until it is. If that looks right then the issue is with the position of the primary. If the difference is small, you might be able to fix the issue by just using the collimation bolts to shift the primary forward. If you think a moment about the geometry or draw it out on paper, all of this will be clear.

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Sam Badcock

Topic starter

7 days ago

Been a long time since I’ve done anything scientific 🤣 I’m following along with ya now though!! I’ll fire it all up tonight and while doing live view, adjust the secondary vertically see if that does anything but only after I check the primary!!

I do wonder if it’s the secondary being in the wrong position after what Andrea mentioned about full illumination as one side of the image was significantly darker than the other side!!

Regardless I’ll post images later on just incase

Rick Krejci

7 days ago · Edited: 7 days ago

Did you re-collimate and if so, how? If you did with any kind of sighting tube/cheshire, I would have to assume you would have noticed the secondary not being centered in the field of view and/or the primary was cut off. If it was just a simple laser, then you may not have noticed. Should be obvious just standing several feet away from the focuser and centering your eye through the focuser and seeing that the secondary centers the primary.

Like Tony said, assuming the secondary is OK, then you need to move your primary closer to the secondary to push the focus point out further

Sam Badcock

Topic starter

7 days ago

Yep I did re-collimate the same way I usually collimate!! I have a home made cap with a small hole in it to look through, on the inside is some tin foil with a + and x combined to indicate the centre!! I generally move the secondary around to centre the little circle of the primary in the centre of the cap

Maybe it is a case of the secondary being in the wrong position and I haven’t noticed

Tony Gondola

Frequent contributor

7 days ago

That’s step 2 of collimation which is usually all you have to do when the secondary is already adjusted. The problem is, you can still do that even if the secondary is in the wrong longitudinal position. Since you’ve had both elements out of the scope, you need to go back and make sure that the outline of the secondary is centered under the focuser and perfectly round. Only when you have that can you go on to step two.

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Sam Badcock

Topic starter

7 days ago

I think that’s my problem there Tony, I’ve attempted to collimate with an out of position secondary

It’s not looking good weather wise tonight but I’ll pull the primary out to make sure it’s seated correctly and also need to check something with the clips!! While that’s out I’ll adjust the secondary longitudinal position

CraigT82

7 days ago

I bet your primary collimation screws are screwed down tight (fully compressing the springs). Loosen them off about half way to close the distance between primary and secondary and recollimate, that will push out the focal plane a bit

Helpful Concise

Sam Badcock

Topic starter

7 days ago

Okay so I adjusted the secondary mirror along the longitudinal path and managed to almost get focus at a point above step position 1

The primary mirror collimation screws are loose and I think that’s the reason I’m not properly collimated though this is a big step in the right direction I think!!

From here I believe I just need to collimate properly, and I should be good to go!! I may drop that secondary mirror a tiny bit just so the potential focus step position is higher up the ladder!!

I ran an AF routine and the above step position is what it gave despite not being well collimated