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To OAG or to ST80 for guiding with a 130pds?


Russe

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Hi guys,

thinking about spending some more money. I've been guiding with a 9x50 finder guider/QHY5 but am limited to 10min subs (usually go for 300s), because of egg shaped stars. I'm suspecting flex, which I'm trying to eradicate. One thought was to change to guider setup.

What would you do? ST80 guider on top of my 130 pds or an OAG like this one: http://astronomia.co.uk/index.php/accessories/photographic-accessories/guiding/sky-20280.html

Cheers

Florian

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Whilst you have an HEQ5 which will be up to the job of handling the additional weight of the ST80, the OAG does eliminate a lot of the issues of flexure etc as you are guiding on a target through the scope you are also imaging with.  I guess the downside is that you are placing something in the optical path of the scope you are imaging with...which may reduce contrast or have some other affect. I am speaking theoretical here as I've never used an OAG, but would be interested in your results if you opt for that method of guiding.

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Neither OAG or ST80, just keep using your finderguider mounted on top of your tube rings (not in the finder shoe), saves weight, saves money and its easy to set up. It guided my 130pds for 8 hours straight last night with no dropped subs and perfectly round stars in 15 and 20min subs.

If you suspect flex, put your finderguider in mini guidescope rings and tighten them up real tight, also velcro up all your cables to there is no strain on the cuide camera.

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The standard finder bracket has one spring loaded support and two screw adjusters. The spring loaded one is a recipe for flexure if you have it in use. I know the factory finder guiders have three screws but the modded standard one may not. First step is to get it bolted down hard.

You're sure you don't have mirror movement?

Usually if you can do 5 minutes you can do twenty.

Cables all sorted? 

The egg shapes are right across the chip? Distortions due to PA show up in the corners on long exposures.

Olly

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I have both an OAG and  the ST 80 and I would advise against using either , the 50mm is capable of guiding up to 1000mm of FL if your set up is right. Getting the OAG parfocal with the mainscope is a right royal pain and depending on your set up at times impossible . ST 80 is bulky and slow compared to a 50mm @ F3.2 and does not bring any new  money to the table. The biggest problem with the 50mm is the ridgidity of the adjusting bolts and the mounting shoe. For my Apos I have installed tube rings and the finder is screwed to the dovetail bar on top of the rings, super rigid, for the newt it is WIP at the moment although I use a 60mm guider nowadays.

Regards,

A.G

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Neither OAG or ST80, just keep using your finderguider mounted on top of your tube rings (not in the finder shoe), saves weight, saves money and its easy to set up. It guided my 130pds for 8 hours straight last night with no dropped subs and perfectly round stars in 15 and 20min subs.

If you suspect flex, put your finderguider in mini guidescope rings and tighten them up real tight, also velcro up all your cables to there is no strain on the cuide camera.

Which mini-guidescope rings could you think of? Still have it in the shoe (* ashamed)...

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The standard finder bracket has one spring loaded support and two screw adjusters. The spring loaded one is a recipe for flexure if you have it in use. I know the factory finder guiders have three screws but the modded standard one may not. First step is to get it bolted down hard.

You're sure you don't have mirror movement?

Usually if you can do 5 minutes you can do twenty.

Cables all sorted?

The egg shapes are right across the chip? Distortions due to PA show up in the corners on long exposures.

Olly

I don't know what to replace the spring screw with...

Maybe if I could sort that, I could get a better result? I'm waiting for better weather for a test...

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Ah, you have the springy thingy? That's your problem, very possibly. You do need to lose the spring. You could drill and tap a hole for a proper bolt to go through and hold the finder firmly or find some other way to jam it solidly in its bracket. Or lose the standard bracket and make something to hold the finderscope itself very firmly.

Lensman, regarding F ratio we need to have a care. Aperture matters in a guide scope because stars are, to some extent, point sources and don't follow the rules of F ratio. (They would be point sources in perfect optics and minus the atmosphere.) However this complex equation plays out, I suspect you would get more useful guide stars in an ED80. I don't know because I've never used a finder guider but I have endless hours of positive experience of the ED80. I have never, ever, in thousands of hours, needed to move to find a guidestar.

Olly

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The ST80 is a good choice BUT differential flexure is a potential problem with this otherwise good guide 'scope. The advantage of a 'finder-guider' is that it is close-coupled to the imaging 'scope which reduces the risk of flexure. However as Olly has pointed out, that neat spring-loaded 'third' mounting bolt can introduce flexure and swapping it out for a fixed bolt would be a very good idea.

Differential flexure (DF) is always a risk with any guide 'scope unless you take all precautions to obviate it. An OAG does generally stop DF BUT getting the spacing correct to ensure that the guide camera and imaging camera are parfocal AND the spacing is correct for any coma corrector (Newtonian) or field flattener (Refractor) can be a real pain - even then, I have seen flexure in the 'turret' of both my OAGs although shimming them with kitchen foil and adding a further retaining screw has resolved this.

For now, with your current focal length, I'd fettle your existing finder-guider to make it more solid - drilling and tapping the existing sprung mounting hole and insertinf a bolt with a nylon cap would make for a good solution.

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I normally guide with the QHY5 / ST80 combo, but as I happen to have a spare Phillips SC900 laying around, and the adapter to fix it to the finder on my 200P I thought I would set it up and have a go.  OK I'm limited with the web cam to targets near bright stars, but so far in tonight's trial it's guiding more than its correcting, and the subs look OK.  I'll do some further analysing of the subs later to see how the stars look, but so far there is little difference in the graphs showing +/- 0.5 arc sec peak to peak.  - Might look at getting another adapter for the QHY5 to fit the finder (sold one when I got the ST80) and a set of rings at some stage if it out performs the ST80 :)

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For now, with your current focal length, I'd fettle your existing finder-guider to make it more solid - drilling and tapping the existing sprung mounting hole and insertinf a bolt with a nylon cap would make for a good solution.

Where would I get bolts with nylon caps from?

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Where would I get bolts with nylon caps from?

For this sort of work, I just use standard bolts (usually M6 or 1/4"-20 UNC) and then put protective ferrules over the end to protect the tube of the guider. If it helps, I have some ferrules suitable for either 5mm or 6mm bolts and I also have some 6mm nylon bolts that would also work well without a ferrule. If these are of any use to you, PM me your address and you can have 'em! The two nylon bolts on my 9 x 50 are in fact 6mm as a guide. I haven't tried removing the sprung unit on mine but if you are really lucky, it may screw into a 6mm tapped hole already!

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Which mini-guidescope rings could you think of? Still have it in the shoe (* ashamed)...

This is the one I use:

http://www.teleskop-express.de/shop/product_info.php/info/p735_Quick-Release-3---Finder-Bracket-for-50mm-Finders---D-62mm.html

Its all hex bolted together, so you can remove the bit that attaches to the shoe, just leaving the rings and a mini dovetail. Bolt that mini dovetail to a regular sized dovetail bar, then mount that on your tube rings. If you have two of those countersunk photo screws you always get with SW scopes (very handy), you can mount it on those and it will save your paintwork or potentially denting your scope (ordinary hex head bolts wont work,the head is too large). 

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Does the finder guider ever need to be moved relative to the imaging scope in order to find a star? My ST80s never do. If the answer is no, why spend £85 on an adjustable system? I don't fuss too much about what my rigs look like so I just take simple solutions. The springy thing could be locked solid using epoxy resin, for instance. Or a hard little block could be bonded in place next to the spring and the two opposed screws used to lock the scope tight.

Mounting hardware is always brutally expensive. The hardware for our dual rig, bought second hand, came to 700 euros. Gulp. I economise where I can and your problem seems ripe for a quick fix!

Olly

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What Olly says about locking the sprung unit does indeed make sense as it is not critical that the guide 'scope should be pointable - just pointing in the general direction of the imaging telescope is absolutely fine and I can't imagine you having any problem finding suitable guide stars with such a wide FOV.

In the interests of curiosity, I dismantled my own Sky-Watcher 9 x 50 finderscope clamp this morning and removed the spring loaded unit. Fortunately, this is bolted in but unfortunately, it has an unusually fine pitch. The specification of the tapped hole is M8 x 0.8 pitch and my usual suppliers don't stock matching bolts!

My temptation would be to use one of the nylon bolts that I am sending you and suggest that you epoxy it in place as a fixed post - unless you can find a source of suitable M8 0.8 pitch bolts.

An alternative, if you can't get this size bolt and you still want adjustment, would be to drill a 5mm hole as close to the sprung unit as possible, tap it to 6mm and use one of the 6mm bolts I am sending you to give you the third adjustment bolt.

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