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Optimum guide scope


Robculm

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Thanks all for your comments / suggestions.

I will stick with the plan of upgrading to a 50cm Losmandy plate (so far I've only found the ADM in a 21" and that's £115 + out of stock at the moment) and add an aluminium bar to the top, where I will mount the current guide scope.

Mirror mod & 3rd tapped hole in the focuser is already done. Will add the extra springs (will drill and tap central to the others if I get around to it).

One thing that does beg asking though, maybe not for now, but longer term... OAG looks great & not especially expensive, so why doesn't everyone use this method? There must be a downside?

This one for example:

https://www.firstlightoptics.com/off-axis-guiders-oag/zwo-off-axis-guider-oag-v2.html

Only downsides I can see so far is that the extra space between camera / secondary means the focus draw tube will protrude further into the tube (I've already pushed the primary forward a little to keep it out), so will definitely need sawing off, but not a big deal. Also looks a little tricky to set up, especially with a DSLR (i.e. the OAG will interfere with the side of the main camera). but aside that it looks a great solution, so why do most people stick with a traditional guide scope?

Thanks again,

Rob

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31 minutes ago, Robculm said:

This one for example

The zwo won't work. Your combination gives only 11mm to play with. The one we linked is the only one which will work with an eos.

OAGs give you more good frames on cheap out-of-the-box Newtonians. I don't know why they get such a bad press. They spare you having to carry out many of the modifications we outlined and so save you time, energy and weight.

Best of both? Modifications and OAG. 

HTH

 

 

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I could see that being a problem in a small aperture refractor set up, but I guess in a Newtonian set up there's a huge amount more light to begin with.

Ignoring the central obstruction, the 200mm aperture has ~31,000mm2 of light collection compared with a 50mm guide scope at 2000mm2 or only 700mm2 for my current 30mm mini guide scope... So you don't need to pick off much of that to achieve an equivalent level!?

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In principle an OAG is the right choice for a reflector because mirror movement is always possible and its effects are corrected by an OAG but not by a guidescope. They have to fit, though, and since we're using a RASA 8 at the moment, they won't in our case.

Regarding expensive astro-bracketery, I agree with Alacant on this. Just make your own. If you are struggling to cut thicker alloy plate you can simply epoxy two or three layers of thinner sheet together and you'll get all the rigidity you need. Home made bits and bobs abound in our observatories. They're not trying to win a beauty contest (which is just as well! :D)

Olly

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9 hours ago, powerlord said:

difficult to get enough stars

Up to 1200mm, that's not what we find, so the OP at 900mm should be fine.

For which targets have you found problems?

8 hours ago, Robculm said:

 in a Newtonian set up there's a huge amount more light to begin with

I think that's it. But if you do put the OAG on a smaller telescope, what you lose in light gathering is made up by larger field of view from which to find guide stars.

On a -very similar to the OPs sw- 203mm f5 with a t7m  ( an asi120 clone) binned 2x2 in firmware (i.e. before it hits the computer) and with the prism aligned correctly along the long side of a dslr frame, we have never yet failed to find guide stars. Note the plural. The worst case was m106 where there were only two; one is the minimum requirement. Strangely, our preferred galaxy telescope, a 6" f8 finds just as many stars. Maybe because it doesn't need a cc.

Here is the setup with the t7m and the 150/1200mm. For completeness I've included the guide telescope on the f5 showing the mounting rail. Note the all nylon retaining screws under tension for the guide telescope. I'll add this to the modification list. With a properly modified Newtonian, I think that a separate guide telescope is fine for anything up to 5 minutes. For longer exposures use the OAG. Or just use the OAG throughout.

Cheers

6f8.thumb.jpg.54075cc85509014f021ed40e148d5217.jpghf2.thumb.jpg.e6ec70ff0e1d886ba2a7808fd3de61da.jpg 

IMG_20201126_171558.jpg.3eef83f67394639ffe6dbcc59a73ced2.jpg

Edited by alacant
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1 hour ago, ollypenrice said:

OAG is the right choice for a reflector because mirror movement is always possible

With the modifications we outlined here, this is all but eliminated. Having said that, we've never been able to get 100% success on frames of over 5 minutes duration without using an OAG. I'd be willing to concede however that this is due to the state of our mounts rather than the integrity of our tubes. 

Cheers

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3 hours ago, alacant said:

With the modifications we outlined here, this is all but eliminated. Having said that, we've never been able to get 100% success on frames of over 5 minutes duration without using an OAG. I'd be willing to concede however that this is due to the state of our mounts rather than the integrity of our tubes. 

Cheers

But the mounts will produce the same errors however they are guided, no?

Olly

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1 hour ago, ollypenrice said:

the mounts will produce the same errors however they are guided,

Good question. Does mount reaction to guide star motion on an OAG differ from that of the same camera on a guide telescope? I really don't know. I think that's why we setup guiding using stuff like size of pixels and focal length.

Why we get more usable frames when using an OAG on a Newtonian is I believe, almost all due to its ability to offset flexure; one is examining the same light cone as is arriving at the imaging camera. I'd guess there is less movement to have to pass to the mount to correct. Why then do we get more usable frames with a separate guide telescope but using a Newtonian with a non floating mirror cell and with a properly supported tube? I'd go for elimination of flexure once again.

 

Edited by alacant
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2 minutes ago, alacant said:

Good question. Does mount reaction to guide star motion on an OAG differ from that of the same camera on a guide telescope? I really don't know. I think that's why we setup guiding using stuff like size of pixels and focal length.

Why we get more usable frames when using an OAG on a Newtonian is I believe, almost all due to its ability to offset flexure; one is examining the same light cone as is arriving at the imaging camera. Why then do we get more usable frames with a separate guide telescope but using a Newtonian with a non floating mirror cell and with a properly supported tube? I'd go for elimination of flexure once again.

 

The test would be to look at the relationship between sub quality and guide trace. Flexure between guide scope and main scope can give a sound trace but a defective image.

Olly

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8 hours ago, alacant said:

Up to 1200mm, that's not what we find, so the OP at 900mm should be fine.

For which targets have you found problems?

I think that's it. But if you do put the OAG on a smaller telescope, what you lose in light gathering is made up by larger field of view from which to find guide stars.

On a -very similar to the OPs sw- 203mm f5 with a t7m  ( an asi120 clone) binned 2x2 in firmware (i.e. before it hits the computer) and with the prism aligned correctly along the long side of a dslr frame, we have never yet failed to find guide stars. Note the plural. The worst case was m106 where there were only two; one is the minimum requirement. Strangely, our preferred galaxy telescope, a 6" f8 finds just as many stars. Maybe because it doesn't need a cc.

Here is the setup with the t7m and the 150/1200mm. For completeness I've included the guide telescope on the f5 showing the mounting rail. Note the all nylon retaining screws under tension for the guide telescope. I'll add this to the modification list. With a properly modified Newtonian, I think that a separate guide telescope is fine for anything up to 5 minutes. For longer exposures use the OAG. Or just use the OAG throughout.

Cheers

6f8.thumb.jpg.54075cc85509014f021ed40e148d5217.jpghf2.thumb.jpg.e6ec70ff0e1d886ba2a7808fd3de61da.jpg 

IMG_20201126_171558.jpg.3eef83f67394639ffe6dbcc59a73ced2.jpg

that's good to know. I've been getting put off going down the OAG route because of posts that say they have had problems. I imagine I'd have less issues with my asi110 mini, but the other guide camera is an asi120mc, and is much less sensitive.  I'd hate to spent the 200 quid on OAG/focuser to find out it all didn't work. Sounds like it's maybe worth a try for me as well then.

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Thanks for your comments Alacant & Olly.

The OAG definitely looks an interesting solution. I'm glad I didn't rush out and buy another guide scope. Let's see for the modifications first, but it's definitely on my wish list now!

Cheers, Rob 

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20 hours ago, powerlord said:

issues with my asi110 mini, but the other guide camera is an asi120mc

We haven't tried either I'm afraid. Start a 'best guide camera' thread?

We use t7ms, bin 1 on guide telescopes,  bin 2 on OAGs. 

Edited by alacant
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  • 1 month later...

Just a quick update on my progress / lack of!

Found a nice 479mm Losmandy style plate from TS optics at a much more reasonable €75 😀.

Also discovered that there was quite a bit of slack in the DEC axis on my mount and followed the Astro Baby guide to adjust that.

Added extra springs on the locking nut screws. There was definitely some slack in the mirror, but feels pretty solid now.

IMG_20220224_172025.thumb.jpg.c0535905e41bfb29fd7b783e77938421.jpg

Have also bought an aluminium bar for the top, which I'm planning to mount the guide scope on. But haven't for now, mainly as it's quite a bit of extra weight & I'm worried it will impact the HEQ5 performance. Will give it a try in time, or wait until I upgrade to an HEQ6.

Should mention here that I'd also purchased a step down DC supply for the camera, so that I can finally be done with changing batteries & them running out... 

Got all set up for M81/M82. It was a couple of weeks back now so the frustration has faded! Terrible tracking! Usually I'm down at around 0.6RMS combined, but was up over 1. Spent a good few hours fiddling with the guiding, but no improvement. Wondered if for some reason it could be related to the camera supply (was running the mount, air pro & camera from the same power supply), so popped a camera in the battery & things definitely seemed to improve. 😁

Has anyone else found this? When maybe 'straining' the power source, the guiding is impacted? Hopefully that's the problem & I can resolve it with an extra power supply!

Then for the frustrating part... All looking good, set it running & went to bed (alarm set for 3am as I'm still not confident just to leave things going all night!). Lucky I did, as by 3am the bottom of the scope had hit the mount & stopped. Need to explore that, it's happened to me a few times recently. Perhaps I need to explore this meridian flip thingamibob? Anyway, to cut a long story short, popped the memory card in to my laptop the next morning only to discover the card was full! 🤣 In fact it had been full the whole night, so didn't record a single image!!! Oh well, we live & learn huh!

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13 hours ago, Robculm said:

extra springs on the locking nut screws

Like it. Nice job. You're almost there...

Top rail, seal mirror to cell, relocation of guide telescope, eq6 and a black shower cap are all that is missing.

Oh, and tighten those mirror cell screws to get more effect from the springs.

That's the tube done. You then have only the focuser and camera connection to go. 

13 hours ago, Robculm said:

I need to explore this meridian flip

Unless you plan to sit by the telescope throughout the session, automatic meridian flips are not really optional;)

Cheers

Edited by alacant
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