Finder Scopes

[George]

Any advice is welcome to help me get the best finder scope for my 8" SCT (Celestron, with GoTo).

I want to do some astrophotography with a Canon 30D, but my tracking is poor.

Any recommendations? Is the auto tracking thing-a-mabob a great way to go?

[turbo-1]

In which way(s) is your tracking poor? This information is critical. Do you have problems with image rotation, drift, periodic errors, or something else? The potential pitfalls of imaging are many, and there are no pat answers. By the way, you do not need a finder to fix this. You either need an ancillary guide scope or an off-axis guider. One of the things that can plague C8s is image shift due to settling of the primary mirror.

[George]

Image drift, I suppose, seems to be the symptom. In fairness, because I forgot to take my tripod with me on a long trip, I was able to make a steel mount that was not quite horizontal. I did not realize it till later. This certainly didn't help, no doubt. A following night, I carefully leveled the tripod mount but was not able to see the guide stars as I set up near the house to minize the wind. I thought I knew all the wrong things to do.

I knew the quarter moon would be easy, but even after setting for lunar tracking, I got drift. When I get frustrated enough, my solution is usually spending money, hence the desire for a finder scope. I have no finder now because the original broke.

By settling of the primary mirror, do you mean thermal adjustment? This might be a factor when I shot the moon. It was only about an hour after sunset when I took images.

For long exposures, I just assumed I would have to have something to keep on target.

[redshifter]

For a finder, I'd recommend a Telrad hands down. Dunno how that applies to astrophotography though, sounds like two different questions (finder vs. tracking)?

[Kaptain K]

George,
From what I've read, you are looking for a guide scope, not a finder scope.

Is your SCT alt-az or equatorial?

[George]

From what I've read, you are looking for a guide scope, not a finder scope.
Yes, that is likely the case, unless some sort of automated alternatitive suggests otherwise. There seems to be some choices out there, which is why I was curious of other's opinion.


Is your SCT alt-az or equatorial?
Alt-az.

[Kaptain K]

From what I've read, you are looking for a guide scope, not a finder scope.

Yes, that is likely the case, unless some sort of automated alternatitive suggests otherwise. There seems to be some choices out there, which is why I was curious of other's opinion.

Basically, your choices boil down to:
1) Bolt a guide scope to your SCT and guide manually.
2) Same as above except use a CCD guider hooked up to your scope's drive.
3) Use an off axis CCD guider and a "pickoff mirror" between the scope and camera.
For more information on electronic guiders, I would suggest checking out the SBIG (Santa Barbera Instrument Group) website.

Is your SCT alt-az or equatorial?

Alt-az.

Well, that's going to limit you to short exposures. Otherwise, you will get streaked stars, due to field rotation.

[George]

Well, that's going to limit you to short exposures. Otherwise, you will get streaked stars, due to field rotation.
Yes, I don't recall an auto camera rotater option. :) A wedge would help but SBIG suggests (http://www.sbig.com/FAQ/General/Do_I_need_a_wedge.htm) an equitorial instead.

The pricing for much of SBIG's cool gear is too high for me. A finder scope, at least, is a must for me since I have none. [I've been placing a green laser pointer on the tube frame but it is off about 3 or 4 degrees.] My hope was to spend a little more and get something to improve tracking for photography. This latter idea now makes less sense due to the mount. I've enjoyed portability with the C8 but I could consider stepping up to something that would be kept at my dark sky observing spot (near Charlotte).

[Kaptain K]

A wedge is what you need to convert your alt-az to EQ. Celestron has a couple:
For CPC Series, Nexstar GPS or Ultima 2000 scopes
http://www.celestron.com/c2/product.php?CatID=52&ProdID=382
For NexStar 5i or 8i
http://www.celestron.com/c2/product.php?ProdID=381

A finder is not as necessary with a Goto scope, once you've aligned the scope (pick a bright star and center it in the scope, repeat for two more stars). But a guide scope will be necessary to keep the scope exactly aligned during exposures.

[George]

Thanks for the help. I may go with a nominal finder scope for now and save money for something fancy later.

[turbo-1]

By settling of the primary mirror, do you mean thermal adjustment? This might be a factor when I shot the moon. It was only about an hour after sunset when I took images.A friend of mine had a classic C8 (full fork mount with wedge and tripod) and that scope had quite a bit of image shift when focusing. The implications of this are not good for astrophotography, because if the focusing mechanism causes a slight bit of tilt in the primary mirror (as evidenced by the image shift), then it is entirely possible for that primary to "relax" into a less-stressed position during your exposures. If you are using an off-axis guider that uses the main OTA's light, you may be able to compensate for this. If you are using an ancillary guide scope, you will never see the shift occuring, and your astrophotographs will be no good if this happens.

I have no further experiences with Celestron's SCTs, so perhaps their current focusing mechanism is far superior to what they were producing ~20 years ago. Perhaps there is a way to reliably compensate for image shift - for example: go out of focus in one direction and back into focus in the other. I just don't know. I know that he and I started in astrophotography at the same time, and I was getting some great shots through my 6" f:8 APO refractor while he was struggling to even get shots that I would have rejected as poor-quality.

[George]

Thanks, Turbo-1. Even more concern comes in the play within the drive system. The heavy weight of my large colorimeter piggybacked to track the sun may be the cause of some unwanted slack in the gearing. Once tracking is engaged, it is not sloppy, fortunately.

Sounds like I need a plan for upgrading to a Newtonian drive. I'll probably just sell the 8" and get something nicer, when funds allow.

[I came here for help on getting a finder scope, and now I'm lead to a much nicer scope.....thanks, I couldn't have planned it better. :)]

[a 6" APO, huh. Nice. ]

[turbo-1]

Thanks, Turbo-1. Even more concern comes in the play within the drive system. The heavy weight of my large colorimeter piggybacked to track the sun may be the cause of some unwanted slack in the gearing. Once tracking is engaged, it is not sloppy, fortunately.Usually, fairly massive accessories can be piggybacked (within reason), but you have to be quite cognizant of when the drive is heavily loaded raising such a load and when it is lowering while resisting the weight of such a load. Your drive may not act predictably in both circumstances, and it cannot be expected to behave AT ALL during the transition between raising and resisting.

[turbo-1]

Thanks, Turbo-1. Even more concern comes in the play within the drive system. The heavy weight of my large colorimeter piggybacked to track the sun may be the cause of some unwanted slack in the gearing. Once tracking is engaged, it is not sloppy, fortunately.

Sounds like I need a plan for upgrading to a Newtonian drive. I'll probably just sell the 8" and get something nicer, when funds allow.

[I came here for help on getting a finder scope, and now I'm lead to a much nicer scope.....thanks, I couldn't have planned it better. :)]

[a 6" APO, huh. Nice. ]Don't jump into the deep end (big APO refractor) until you have tried out a few. Maybe a few star partiies are in order. Roland Christen's APO refractors have a well-deserved reputation for quality, and although it is commonly accepted that 50X/inch of aperture is max, lots of well-respected amateurs are claiming that given appropriate seeing conditions, they get good results with 100x/inch. There are other high-quality scopes out there, now, and Roland has gone into some catadioptic designs, as well. I chose the APO because of the high contrast, lack of central obstruction, etc, and I have never regretted the decision. If I can afford to put in a roll-off roof observatory in the next year or two, I'm going to give serious consideration to making it big enough to also house a 14-20" Dobsonian - perhaps a Starmaster with a Zambuto primary. (Drool)

[George]

Usually, fairly massive accessories can be piggybacked (within reason), but you have to be quite cognizant of when the drive is heavily loaded raising such a load and when it is lowering while resisting the weight of such a load. Your drive may not act predictably in both circumstances, and it cannot be expected to behave AT ALL during the transition between raising and resisting.
Nicely stated and all correct. The risk assesment in my mind said go for it; anything for heliochromolical pioneering. :)

I tend to favor aperature, so an RC reflector with an equatorial mount would be dandy. It might even happen this year. It depends on several circumstances, however. Time will tell.

[turbo-1]

Nicely stated and all correct. The risk assesment in my mind said go for it; anything for heliochromolical pioneering. :)

I tend to favor aperature, so an RC reflector with an equatorial mount would be dandy. It might even happen this year. It depends on several circumstances, however. Time will tell.Might I venture "Holy Cow!", since a well -appointed RC is just a distant dream of mine? I hope you can pull this off and would love to share in your enjoyment of such a system. Good luck!!

For those new to this thread, George started out by asking about finder scopes and tracking issues, and has decided that a new RC scope might settle everything. I cannot offer a word of discouragement, because, if I could afford such a scope and mount, I'd order one today, with my main question hinging on "how much aperture can I afford?". Can I come to the "first light" party?

[George]

Might I venture "Holy Cow!", since a well -appointed RC is just a distant dream of mine? I am being optimistic, of course. My father has a small ranch with dark skies about 1.5 hours away. If he elects to pitch in, I might just bite the bullet and do something nice regarding a new scope. Unfortunately, this has been a very tough year financially but things are looking up. We shall see.

Can I come to the "first light" party? Hmmm, that would be fun.

[turbo-1]

I am being optimistic, of course. My father has a small ranch with dark skies about 1.5 hours away. If he elects to pitch in, I might just bite the bullet and do something nice regarding a new scope.I have spent only one night in the hill country north of San Antonio, but it was a gorgeous sky. If you can get out 1.5 hours from San Antonio (and not toward another light source), that should be pretty nice. The skies here in north central Maine are better than I saw in the hill country, and the skies an hour north of here are incredible. I am about 20 miles from the first town with 7000+ people in it and the skies are great, but up north where I fly-fish, I always have to stop at the sandpit on the drive back out of the woods to look at the Milky Way and see how much I can see naked-eye. M31 is naked-eye off my back deck every clear night (you can't help but see it), but in the sand-pit in Parlin Pond Plantation, it is a beacon.

[George]

Sounds great. It is my understanding that an Atlantic laminar air flow is a real asset to seeing conidtions. Some surprisingly great images have come out of Florida for that reason. Perhaps Maine has this condition, also.

What strikes me the most "back at the ranch" is how the Milky Way becomes 3d after dark adaptation of the eyes. [I was going to give you lattitude by not mentioning it, so I won't. ;)]

[George]

I bought a 9x50 finder scope and its mount (about $90). It has a right angle prism that eliminates any flipping of the view.