ZWO OAG v2 thread Length?

[Xiga]

Hi guys

Recently i've had some situations where Phd2 has been losing the guide star on me due to low SNR, even though the weather has been completely clear. Visually, I also can't see that many guide stars on the screen (in both Phd2 or Sharpcap), although Phd2 still apparently manages to pick up close to 12 guide stars while doing multi-star guiding. I found this very weird, as I am using an Asi290mm, which is very sensitive, and i'm even binning it x2 as well. Note, i'm using it with an OVL OAG on my SW 80ed. 

Shortly after, I then realised that the aperture of the OAG stalk is actually really small. It's only a circle of about 5 or 5.5mm in diameter, which i think is causing a lot of vignetting of the Fov in Phd2, and i think this might have something to do with the issues i've been having with guide stars. One option i could try, is filing away the stalk aperture to make it more rectangular. The problem is, there is a teeny tiny grub screw holding the prism in place, and i can't see any way of removing it safely and getting it back on. I do have some very small allen keys, as small as 0.7mm, but none of them fit. I'd also have to find a way to then paint the inside of the stalk black again. Basically, that sounds like a lot of faff, especially for someone as averse to DIY as myself!

So i started looking at the ZWO OAG v2, as it's only 16.5mm wide. The OVL one i'm currently using is 16.3mm wide, but i am also using a 0.5mm M48 spacer on the scope side, so technically i can support up to 16.8mm wide for a replacement OAG (note, i've no option to find any more backspace, i'm currently at my max for the SW FF/FR). 

Now the OVL OAG has a really long M48 female thread, which allows it to take the full M48 Male thread of the SW FF/FR. I've tried to find out the length of the same M48 Female thread on the scope-side of the ZWO v2 OAG but i can't find it anywhere online. If someone has this OAG, i would be extremely grateful if you could precisely measure the length of this thread. Or, to be more exact, the distance from the outside of the OAG to the point where contact would be made with the prism stalk. If it turns out to be shorter than the OVL, then what would people recommend? One idea i had, was to try and shave down 1 or 2mm from the SW FF/FR thread. It's unnecessarily long imho, i don't need it to be 5mm, 3mm would suffice for my needs. How easy would this be to do though? Could it be done safely with a simple tool (which one?) without this tool (ie me) running the risk of ruining the threads on the FF/FR? 

[almcl]

Hi Ciarán

I have the ZWO V2 OAG and use it with an ASI 120 MM, but not sure I have understood which dimension you need (just a bit slow this morning).

If it's the one shown in the image below, then the thread depth is 5.19 mm.  If that isn't what you need, let me know (perhaps by noting on the image) which one?

[michael8554]

Hi Ciarán

 "although Phd2 still apparently manages to pick up close to 12 guide stars while doing multi-star guiding"

I take it you mean PHD2 is capable of choosing up to 12 stars.

"the aperture of the OAG stalk is actually really small. It's only a circle of about 5 or 5.5mm in diameter,"

Your ASI290mm has a sensor 5.6mm x 3.2mm, so a pretty good match for the stalk, I can't see any need for a different OAG.

Is the guidecam well focused, what HFD do you typically get ?

What exposure are you using ?

You could post a PHD2 GuideLog that includes the "Star Lost" situations.

Michael

 

 

 

[Xiga]

2 hours ago, almcl said:

Hi Ciarán

I have the ZWO V2 OAG and use it with an ASI 120 MM, but not sure I have understood which dimension you need (just a bit slow this morning).

If it's the one shown in the image below, then the thread depth is 5.19 mm.  If that isn't what you need, let me know (perhaps by noting on the image) which one?

Thanks so much for this mate! That's exactly what i needed, and it does look like the tread will just be long enough. 🙂 

[Xiga]

1 hour ago, michael8554 said:

Hi Ciarán

 "although Phd2 still apparently manages to pick up close to 12 guide stars while doing multi-star guiding"

I take it you mean PHD2 is capable of choosing up to 12 stars.

"the aperture of the OAG stalk is actually really small. It's only a circle of about 5 or 5.5mm in diameter,"

Your ASI290mm has a sensor 5.6mm x 3.2mm, so a pretty good match for the stalk, I can't see any need for a different OAG.

Is the guidecam well focused, what HFD do you typically get ?

What exposure are you using ?

You could post a PHD2 GuideLog that includes the "Star Lost" situations.

Michael

 

 

 

Hi Michael

Thanks for responding. From memory, i think Phd2 usually reports something like '9 / 12' (although the numbers can and do change) which i've always just taken to be the number of stars it is using for guiding out of the number it has picked up. The ASI290mm has a diagonal of ~6.45mm, which is about 1mm more than the diameter of the stalk, so i think i'm getting a good bit of vignetting in all 4 corners. I also suspect, that Phd2 is selecting some stars that are on the periphery of the central area, and that might be why it occasionally loses them. I should point out, it's not a problem that happens an awful lot, just every now and then, but i'd still like to get it sorted if i could. I think on my last session i was getting a HFD of around 3 using 2s exposuresso it should be pretty well focused. I should also point out, i use the main camera (Qhy268m) for doing Polar Alignment in Sharpcap, but if i try using the ASI290mm then Sharpcap can't solve the image. I find that weird, as its' such a sensitive camera, but maybe it's because the FOV is just too small? 

The reason for all of this, is that ever since i got my mount tuned i've been having real difficulties with data capture. I just can't seem to get a full night of subs with round stars on both sides of the meridian. After much hair-pulling, i deduced that the problem was differential flexure, so i ditched the Guide Scope in favour of an OAG. I'm still having problems though, but weirdly it seems to quite random in nature. I had the scope out 3 nights last week, and each time i had varying levels of success on both sides of the meridian. The worst night meant that not a single sub was useable after the meridian flip, even though the reverse was true only the night before. It was around this time i had the guide star being lost issue, so the next thing on my list is to just try and help Phd2 as much as possible, and i think that means an OAG with a bigger (ie rectangular) stalk aperture. 

I really should post up some Phd2 logs, just like you said. The mount is so sticky that finding optimal balance is next to impossible, so there's always going to be a good chance the guiding will be affected by that. Even so, the guiding was so bad on occasion there has to be something else major going on as well. 

[michael8554]

Hi Ciaran

PHD2 will guide well on 1 star when correctly set up.

It will guide even better with MultiStar and fast exposures that would normally be classed as "Chasing the Seeing".

And "9 to 12 stars" with an OAG is pretty good going.

I still think that changing the OAG may only be marginally better, and may well give you disappointingly similar guiding results.

Because I think your lack of round stars is down to mount and PHD2 settings, you admit to the mount being "sticky."

Concentrate on the real problem.

Michael

[Xiga]

Hi Michael

You're probably right. I'll likely continue with what I've got for the foreseeable future and will only change the OAG if I keep getting guide star lost messages for apparently no reason. 

It's just annoying really. After 5 years of routine data capture, most of which were 20 min subs without a single lost one, I now suddenly can't even do 2 min ones with any great confidence. I'm probably losing anything from 1/3 to 1/2 of a night at the moment. I can definitely improve things guiding wise, but I'm also pretty convinced now that the problem is mechanical in nature. 

Another suspect on the list is the stock focuser of the ed80. I'd tightened it down before, and did see some improvement, but not a massive amount. I've just tightened it again, so much so that the DC hand controller now can't move it! I can still move it manually though, so let's see how the next session goes. 

Are there any other obvious candidates, other than the focuser, that would lead to stars being bad post meridian flip, when they are fine pre-flip? 

[michael8554]

Diagnosing what happens post-flip will requuire the GuideLog.

Michael