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Ionizar

Wedgeitis

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What a weird title for my introductory post on here.

So first off hi I'm new to these forums and it looks like a nice place to get info by like minded folks into the same hobby as me.

I have an 8" SCT, specifically the Celestron Nexstar 8 GPS, and was wondering if it is worth it to get a wedge for astrophotography through it. So far I've only managed a 30s exposure of M31 and clearly, it needs more like 30 minutes or more before it's going to show any detail I suppose. So I'm getting the cable release for my 350D which I plan on using in prime focus through the SCT.

Question is should I get the wedge which can cost from 400-700 depending on brand and build or if I'm going to spend that much just get a GEM instead? How long of an exposure can I expect to be able to take with Celestron's wedge?

Also would it be preferable to take many short exposure pics or few long exposure ones to show the most detail? For example would 300 30s exposures show more or less detail than say 30 5 minute exposures which both amount to 150 minutes of exposure if I did my math right.

Thanks for helpin out a newbie!

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Hi .....?, and firstly, welcome to SGL.

Having a Fork Mounted, its pretty much essential to mount it on an equatorial wedge, if you intend to do long exposure astro imaging. Without this, your images will from Field Rotation problems.

The is no reason, if you have reasonable DIY skill, that you can't build your own EQ wedge. I once made on out a wood, and it work quite well.

As you suggest, the other option is to take the OTA off of the 'forks' and mount it on a 'driven' German EQ mount. However, I reckon the cost of a reasonable mount, will exceed the cost of buying a 'wedge'.

Hope this helps.

Dave

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Hello,

I have a NexStar 4, although its a single fork mounted scope and the base model in the NexStar range, it does have a built in wedge, which is one of the reasons I bought it. I have found that the wedge is essential for astrophotography and the celestron hand-book way of polar aligning is easy once you know how to do it.

I have seen others with say Skywatcher telescopes on an Equatorial mount taking ages to set their scopes up, and I am glad to say that mine takes just a few minutes.

As for your exposure length question, I can only say that yes 300x30second exposures will give you the same as 30x5 minutes, although you have to realise that if you have an ISO of say 800, the noise will be greater on the 300x30 as you have taken 300 images at 800ISO rather than just 30, so your images will be less grainy if you do 30 rather than 300.

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Would you site your scope permanently if you place it on a wedge? I think it has a bearing on whether it would be worth spending a bunch of money, if the scope is not going to be permanently sited. Because if you do site it, you could go to great lengths to accurately align the polar axis, and that is going to be a big plus when you come to do long exposures. The guiding, if manual, will be a lot easier for you. If you have a mount that allows autoguiding, I reckon that would be more accurate too. Field rotation will not be an issue. I know wedges are pretty expensive, and Dave's suggestion about making your own is a very sensible one, if you have DIY skills. You could take the route of taking shorter exposures and stacking the images, but that is an area for the people with that experience to advise you on. I am not sure about stacking DSLR images.I think someone has already explained that to you.

Anyway, good luck whatever you decide.

Ron. ;)

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Hi Ionizar and welcome to the forum.

For ease of use I have a C8 GPS like you. Great for sit down visual observing, and good for imaging the planets and the moon - as you don't need a wedge. You can also grab images of the bright globular clusters ( M13 , M15).

For Deep Sky Objects you need a wedge for the C8. You can buy Celestron's own wedge and adjustor from the two suppliers (see logos at top). With the SGL 10% discount you can get one for about £300. Not a brilliant wedge by all accounts but it should give you over 2 mins unguided. With stacking this can give you some pretty good DSO images. With guiding you should be able to manage 5 mins.

You can get decent images of M31 with the Canon 350D EOS using 15 to 20 images (subs) of 2 mins each @ 800 ISO.

Having some decent software to process your subs can do wonders to your images as well. Stacking shorter but more subs can often give you lower noise levels than fewer longer subs. One or two bad subs in a stack of 20 is not a disaster, whereas one or two out of three or four longer subs is a disaster. Remember we all kick the mount from time to time when we're imaging!

At the moment I image with the HEQ5 equatorial and the TAL with the Canon 350D EOS at prime focus. For wider field imaging I use the Canon (with tele lens) piggy backed on the TAL.

As I'm moving soon I intend to site the C8 permanently on a wedge. The ability of the C8 GPS to hibernate will be very useful then, since once you've polar aligned the scope you can go to hibernation mode and power off the scope. When you power back on the GPS will not need re-aligning.

MD

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Thanks everyone for the warm welcome and informative replies. I'm learning lots here, great site!

Unfortunately I won't be able to mount the scope permanently so I take it adding a wedge and EQ alignment will increase my scope set up times dramatically each time I go out observing? Well I suppose 2-5 min. exposures would be worth that, as long as it's not too difficult. I was just reading the Celestron manual on EQ alignment and it mentions having to find meridian. I haven't a clue how to do that accurately. I mean I realize its an imaginary line between north and south but how do I point the scope to an imaginary line and get it right on? What if I'm off by some amount? They don't go into much detail. I guess maybe it's not critical to a proper alignment then.

Well thanks again for the replies. It looks like I probably will go for the celestron wedge at some point and give it a go. 2 minute subs being 4 times the exposure of the current 30 sec. subs I can do in alt az should give me some better images.

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Hello again,

The easiest way to find your meridian is to use a compass. Place it on the ground where you will be using your scope and see where the pointer shows North, then take either some chalk or masking tape and draw a line in the North South direction. This method works in two ways as you can then place your scopes tripod over the line with the hinge for the wegde nearest north!

The Celestron handbook is a bit backwards in some respects, in order to set up your telescope for both polar and wedge alignment you need to do the following:

1) Tilt your scope using the wedge to your latitude, using the engraved points on your wedge drop down 'stick'. Remember that you must have the hinge of your wedge North (or south) of the rest of your tripod. Then perform a EQ North (or South) align using the hand control, it will ask you to index the scope, just level the two index markers. Then you have to find the meridian, using your N/S line on the ground align the scope so it follows this line (it doesn't have to be pefect as the following aligning of the stars will tell the scope of your position). Next you have to choose two stars for the scope to automatically slew to. Allow the scope to fully goto (in most cases, near) the star then using your finderscope properly align the scope with that star, you may find that the scope is a little away from the intended star, so align it properly, repeat for the second star.

2) Now you need to wedge align, this is in the 'Utilities' feature of the NexRemote, the scope will automatically slew to where it thinks Polaris is, then all you have to do is move the scope MANUALLY (NOT USING THE NexRemote!!) to align your scope with Polaris.

3) Perform another EQ North / South align then you are ready to go!!

Hope that this helps, it works for me!

  • Thanks 1

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