Autoguiding Explorer 130P-DS

[Stefan73]

Hi,

I have a Skywatcher Explorer 130P-DS on a unmodified EQ3 PRO Go-To and a Canon 550D for imaging.  I can manage 1 minute exposures that seem OK but want to improve on that.

I know the eq3 isn't ideal but first I wanted to get auto guiding working for that setup.  I was thinking about getting an OAG since it's not massively more expensive than a guide scope and will presumably future proof me.

Is the best OAG here just the thinnest one possible?  eg
https://www.firstlightoptics.com/off-axis-guiders-oag/ts-off-axis-guider-tsoag9-length-only-9-mm.html

Will this work ok with that scope, a DSLR and a coma corrector (https://www.firstlightoptics.com/coma-correctors/skywatcher-coma-corrector.html)?  I know some setups won't work at all.

Then for cameras would I get away with ZWO ASI 120MM-S, eg https://www.firstlightoptics.com/zwo-cameras/zwo-asi120mm-s-usb-3-mono-camera.html? I read somewhere you need a better camera for OAG than a guide scope.

Thanks for any help!

 

[vlaiv]

Although it seems like a good idea to go for OAG (and it is - I use it), it will be problematic for that particular setup.

OAG on the fast system is best placed as near the sensor as possible. Minimum distance to place it with DSLR is 55mm (or maybe slightly less if you get some sort of very low profile T2 adapter - usual one is 11mm optical path and there is 44mm of flange focal distance).

You are already at 55mm away from the sensor and at F/5 scope with 8mm pick off prism - you will start loosing light in the center at 40mm away from the sensor.

Then there is issue of coma corrector and focuser travel.

Most coma correctors are built so that they need 55mm of distance to sensor - they simply screw into T2 adapter. If you put OAG in between - you won't have proper working distance. You can use coma corrector with longer working distance - but then you risk running into issue with focuser. You need very low profile focuser to be able to rack it all the way in to accomodate for long working distance + OAG + T2 adapter + 44mm of DSLR flange distance.

For this reason, there are T2 adapters for DSLR with integrated OAG.

https://www.teleskop-express.de/shop/product_info.php/info/p2722_TS-Optics-Off-Axis-Guider-for-Canon-EOS-cameras---replaces-the-T-ring.html

You loose a bit of flexibility of OAG positioning as this one is non rotating (at least I think so). Also with regular OAG - you won't be able to freely rotate it as guide camera might hit body of DSLR.

However - you loose flexibility of OAG in this case as above unit is specially designed for DSLR.

Using guide scope is much simpler option for the setup you have in mind.

[Bibabutzemann]

I also have the EQ3 Pro and 130 PDS. If the conditions are not windy, guiding works quite good. Few days a ago i did 10x10min exposure and didnt have to throw away any data.

I agree with vlaiv. For this setup a guide scope is the better choice. With the guide scope you can also do pretty accurate polar alignment with Sharpcap.

You will reach the limit of the EQ3 with the 130 PDS though. For better balancing i would attach the Guide scope on the back of the Newton and in opposite direction in respect to  focuser+camera.

I have a 60mm guide scope, but 50mm is enough.

[alacant]

4 hours ago, Stefan73 said:

I can manage 1 minute exposures that seem OK but want to improve on that.

Hi

You can either modify the telescope [1] to make it acceptable for autoguiding using a separate telescope, Or you can use your OAG method. Both the items to which you liked will work fine although we'd recommend the 120mm, monochrome version of the camera for both guide telescope and OAG applications.

Both methods will work fine for the baby pds. We prefer the OAG and have never failed to find suitable guide stars, even in the barren depths of Leo.

To get really solid guiding, use both the OAG and do the modifications. 10 minute plus frames guaranteed. 

[1] Here's the list of stuff to do for the separate guide telescope. The main cost is in time. 

• Fit six 1.4mm wire springs. 3 replacements, plus three passive springs over the locking screws. Leave the latter loose.
• Seal the primary mirror to the cell using three generous blobs of neutral silicone sealant to coincide with skywatcher's cork. Leave to seat under gravity for 24h.
• Remove the mirror clips
• Set the tube rings at least 30cm apart on a Losmandy dovetail
• Tie the top of the rings with a rigid aluminium box section
• Remove the rubber o-ring spacers in the focuser
• Replace any metal or metal/plastic guide telescope retaining screws with all nylon types
• Mount the guide telescope directly to the top rail
• Drill and tap a third screw to the focuser collar

[Stefan73]

Thanks for your thoughts!  I guess you've both persuaded me against OAG...

My main worry with using a guide scope was weight on the EQ3 but if you've got it to work then I'm reassured and I'd be very happy with being able to take 10mins exposures.

What do you use as a guide scope and camera?  The obvious choice I guess is something like https://www.firstlightoptics.com/guide-scopes/zwo-mini-finder-guider-asi120mm-bundle.html as a combination but I've read to go for something more sturdy to attach to the OTA than the finder-shoe base.

 

[alacant]

Just now, Stefan73 said:

I guess you've both persuaded me against OAG...

We haven't! It's the best and easiest way to go.

Cheers

[Stefan73]

11 minutes ago, alacant said:

We haven't! It's the best and easiest way to go.

Yes sorry cross posted... oh no I was hoping for complete consensus! 😀

It looks like I can't get OAG to work with a coma corrector which I could live with but as a plus the weight would be more manageable and it's more future proof and accurate.

And on the guide scope side there's someone with a working setup with the same OTA and mount.

I'd certainly be interested if anyone out there is using a DSLR, OAG and the 130PDS successfully.

[Bibabutzemann]

25 minutes ago, Stefan73 said:

 

What do you use as a guide scope and camera?  The obvious choice I guess is something like https://www.firstlightoptics.com/guide-scopes/zwo-mini-finder-guider-asi120mm-bundle.html as a combination but I've read to go for something more sturdy to attach to the OTA than the finder-shoe base.

 

the ZWO 120mm mini is the one i would choose. Its future proof because its so popular, which means software issues will be sorted out faster.

I use the 60mm Artesky Ultraguide scope, but i think 50mm is really more than enough. 

I attached the guidescope like this

So i basically just screwed the dovetail of the bracket onto the top.

 

 

 

 

[alacant]

3 hours ago, Stefan73 said:

I can't get OAG to work with a coma corrector

The OAG works fine with the -any- coma corrector. We use it in preference to the separate guide telescope. On an eq3 it has the advantage of weighing much less.

Edited February 18, 2022 by alacant

[vlaiv]

3 minutes ago, alacant said:

The OAG works fine with the -any- coma corrector. We use it in preference to the separate guide telescope. On an eq3 it has the advantage of weighing much less.

How do you get it to work with coma corrector that needs 55mm distance and DSLR with standard T2 ring?

[alacant]

11 minutes ago, vlaiv said:

How do you get it to work

You don't use a t ring. The OAG has the EOS connection. The one you recommend to the OP!

How do you connect your oag to your 130pds?

 

 

Edited February 18, 2022 by alacant

[vlaiv]

2 minutes ago, alacant said:

You don't use a t ring. The OAG has the EOS connection. The one you linked to!

How do you connect your oag to your 130pds?

 

 

I don't use OAG with DSLR of 130pds. I use it with ASI1600 which has very short back focus requirement of only 6.5mm. I fit filter drawer between it and OAG.

OP wanted regular 9mm OAG from TS that he could reuse with other scopes and cameras. EOS OAG is not usable with other cameras.

Your initial post confused things a bit - since you were advocating OAG - without specifying which version. Yes, EOS OAG will work - but question remains if OP really wants that or something to be reused later with other gear.

[alacant]

11 minutes ago, vlaiv said:

OP really wants that or something to be reused later with other gear.

The 9mm oag can be used with a variety of connectors, eos included.

If you're only using EOS, obviously go for the dedicated EOS oag.

No confusion, unless you try including T2 rings of course!

 

 

Edited February 18, 2022 by alacant

[Clarkey]

4 hours ago, Stefan73 said:

but I've read to go for something more sturdy to attach to the OTA than the finder-shoe base.

Svbony do a cheap set of guide scope rings that are perfectly useable and slot into the guide scope mount.

If you have the original finder you could convert this to a guide scope with a converter:

https://www.firstlightoptics.com/adapters/astro-essentials-sky-watcher-9x50-finder-to-c-adapter.html

 

[Stefan73]

12 hours ago, alacant said:

The 9mm oag can be used with a variety of connectors, eos included.

If you're only using EOS, obviously go for the dedicated EOS oag.

No confusion, unless you try including T2 rings of course!

 

 

So would something like these two would work for connecting an EOS to an OAG while preserving other camera options and coma corrector possibilities?

https://www.firstlightoptics.com/adapters/ts-eos-connection-ring-for-ts-off-axis-guider-tsoag9.html

https://www.firstlightoptics.com/off-axis-guiders-oag/ts-off-axis-guider-tsoag9-length-only-9-mm.html

Certainly if it looks possible I'd like to go OAG since everyone seems to agree it's the best solution if it can be got to work.

What guide camera do you use?  Would  a ZWO 120mm be enough?

 

[wimvb]

3 minutes ago, Stefan73 said:

Would  a ZWO 120mm be enough?

A standard ASI120MM is a bit of a gamble. It is so wide that it may not fit between the imaging camera and the focuser. For some people it works, but for me it didn't. (With a dedicated astro camera on a Skywatcher 190MN). I invested in an ASI290 Mini. This camera is more light weight, more sensitive, and smaller.

[alacant]

6 hours ago, Stefan73 said:

Would  a ZWO 120mm

Hi

Yes, any 120mm will work with the ts9. We prefer the t7m however. Example setup attached. Note the use of a make-life-easier focuser;)

You can also see here that the old zwo120 will clear the imaging dslr. This is with a 130pds par-focal and ready to go with the 4 element GPU coma corrector.

Hands on experience tells us it's a good solution for what you aiming for.

Cheers.

Edited February 19, 2022 by alacant
Foto

[vlaiv]

@Stefan73

As far as camera goes - I guide with color camera - ASI185mc (less sensitive) at 1.6m of focal length F/8 system and have no issues, so I don't think camera sensitivity will be any issue with OAG - just take care that body of the camera won't hit either focuser or DSLR body.

There are slim options of camera available.

Do also pay attention to the overall optical length of the setup.

OAG9 is 9mm of optical path - if you use 2" connection, but if you use M48 - it is 11.4mm long

(above is from TS website).

What sort of connection your coma corrector has? Which model is it?

If it has T2 connection - you also need to account for M48/T2 adapter on telescope side.

Something like this:

https://www.teleskop-express.de/shop/product_info.php/info/p9307_TS-Optics-adapter-from-M48---2--filter-thread-to-T2-male.html

This will cause problem for coma corrector as you will have:

44mm + 11.4mm + 2.2mm = 57.6mm

That is more than prescribed 55mm and you might get less than optimal correction in corners.

Depending on type of CC you have - you might make better or worse combinations, like these:

Skywatcher x0.95 CC - has M48 thread so you don't need T2 adapter - working distance is 55m - total optical path will be 44mm + 11.4mm = 55.4 - very small mismatch, correction will probably be as good as with 55mm

Baader MPCC - it has both T2 and M48, with T2 it requires 55mm but with M48 (when T2 adapter is removed) it requires 58mm of optical path.

44mm + 11.4 = 55.4 - you'll require 2.5mm M48 extension / distancing ring to get to 58mm correct distance (or 57.9 to be precise - but that is the same as 58).

[Stefan73]

6 hours ago, vlaiv said:

@Stefan73

What sort of connection your coma corrector has? Which model is it?

I don't have a coma corrector at all at the moment.  The skywatcher one https://www.firstlightoptics.com/coma-correctors/skywatcher-coma-corrector.html  was one I was looking at and it sounds like alacant is using https://www.firstlightoptics.com/coma-correctors/ts-newtonian-coma-corrector-10x-gpu-superflat-4-element-2-nosepiece.html successfully.

I might leave that for now and concentrate on the auto guiding first.  Then see how much the coma ends up annoying me in practice once I've got better guiding.   The main thing is I could add a suitable coma corrector to the OAG if I wanted to later, which it sounds like I could as long as I chose well.  It's all an expensive business so it's really good to have an upgrade path which doesn't involve having to get new everything!

Anyway I really appreciate all the guidance here from everyone.

[alacant]

15 minutes ago, Stefan73 said:

might leave that for now and concentrate on the auto guiding first

Very wise:)

The gpu (the second one to which you linked) leaves the focus position unchanged, and is by far the better of the two you cite. It simply screws to the m48 thread of either oag  available to you. No calculations or spacers necessary. In fact, it works better at 650 focal length at just under 55mm. Perfect.

 

 

 

 

 

 

Edited February 19, 2022 by alacant

[Bibabutzemann]

On 19/02/2022 at 11:31, alacant said:

Hi

Yes, any 120mm will work with the ts9.

Hands on experience tells us it's a good solution for what you aiming for.

Cheers.

now you also got me interested in OAG.

I just wonder is polar allignment through sharpcap still possible? The fow seems to small with OAG.

Also i wonder if you always find multiple stars with the OAG.

Because multi star guiding improves the EQ3 guiding significantly.

[alacant]

26 minutes ago, Bibabutzemann said:

polar allignment

There are many methods for polar alignment you can use. As you'll already be using it, rather than add yet another app, we'd recommend using PHD2

26 minutes ago, Bibabutzemann said:

multiple stars with the OAG

We have always found multiple stars. The worst case we ever witnessed was at 1200mm whilst imaging m106 with a 6" f8. Then there were only two stars available for guiding. Drop that to 650mm with a proper cc and you'll have plenty of stars. The minimum requirement for guiding is one. Bear in mind also that we use indi to control our hardware which includes an eq6 mount.

If you have any doubts, perhaps the best advice we can give is to go along to an astro club and try it.

 

Edited February 21, 2022 by alacant

[Bibabutzemann]

On 21/02/2022 at 11:14, alacant said:

There are many methods for polar alignment you can use. As you'll already be using it, rather than add yet another app, we'd recommend using PHD2

We have always found multiple stars. The worst case we ever witnessed was at 1200mm whilst imaging m106 with a 6" f8. Then there were only two stars available for guiding. Drop that to 650mm with a proper cc and you'll have plenty of stars. The minimum requirement for guiding is one. Bear in mind also that we use indi to control our hardware which includes an eq6 mount.

 

 

Ok well i dont really like using PHD for polar allign, because its way slower.  But NINAs polar allignment feature should do the trick.

One thing im still scepitcal about is using the oag while having the L extreme filter on the front of my CC.

[alacant]

9 hours ago, Bibabutzemann said:

L extreme filter on the front of my CC

Fit the filter on the camera side. 

[Bibabutzemann]

On 23/02/2022 at 10:24, alacant said:

Fit the filter on the camera side. 

Is there a known solution for 48mm Filters like the L extreme? Whith Clip Filters this would be no problem.