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Astrophotography and Sketching >> Beginning and Intermediate Imaging

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Madratter
Post Laureate


Reged: 01/14/13

Re: About to give up on the 8" RC [Re: bseltzer]
      #6194800 - 11/14/13 03:22 PM

Well, I overstated it. I looked up the STF-8300m (my camera) and it is 1.8 pounds. I also the FW8-8300. I can't find a weight for it, but it is probably less (though probably not a lot less) than the camera once loaded with filters. So 3.6 pounds or a little less is probably closer to the mark.

Edited by Madratter (11/14/13 03:28 PM)


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Madratter
Post Laureate


Reged: 01/14/13

Re: About to give up on the 8" RC new [Re: mpgxsvcd]
      #6194804 - 11/14/13 03:26 PM

Quote:

Wow that is heavy. I am dealing with 1.2 pounds for my heavy camera and .85 pounds for my lighter camera. Maybe an RC would work for me?




I won't say it will be a non-issue. Any weight will flex this design some. But I would anticipate it being a minimal issue with that weight.

The design IS clearly non-optimum. There is a reason they can sell these for so much less than other RCs. I personally find mine quite usable at 1190mm. I don't know how I would feel about it at 1625mm as I haven't tried yet.

I will also say I have seen Phil's images at 1625mm. Obviously I haven't been able to do FWHM on his images, but I thought his images were very acceptable despite this issue.

I knew about this issue before I bought the scope.

Edited by Madratter (11/14/13 03:29 PM)


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Phil Hosey
Carpal Tunnel
*****

Reged: 10/29/08

Loc: LaGrange, GA
Re: About to give up on the 8" RC new [Re: Madratter]
      #6194843 - 11/14/13 03:49 PM

Hey guys sorry I'm late to respond back. I've read all the responses so far and I want to address a few questions. First of all, Madratter, the images that I have posted so far at 1625mm seem to be a lucky shot, I haven't been able to repeat them in 3 consecutive tries. My guiding error is less than .5 arcsec RMS but yet I'm getting 'pear' shaped stars. With seeing around 3" FWHM my stars should be round.

My camera, which is the QSI 683wsg-8, weighs over 3 pounds, the moonlite 2.5" focuser weighs about as much. The Moonlite focuser actually makes the situation worse because it adds even more weight to the back of the tube. The problem is that the rear thread that everything attaches to is connected to the primary mirror cell. The focuser itself weighs almost as much as the camera. I probably have about 7 pounds of stuff hanging off the back when you include the guide camera, focuser extensions, etc. By the way, my camera connection to the focuser is all threaded so there should be no flex there.

When I get home this evening I'll post some images of the issue I'm having now for everyone to see, but I'm pretty certain that it is being caused by this flexure. If the weather is good tonight I may give it one last go, I have a couple of things to try, but at the very least my aim is to confirm my suspicions. Oh, one more thing about the Moonlite.. last night it was about 25F (-4C) outside when I was doing this and the fine focus knob would not move the camera inward. I'm not sure if it is the weight of the camera or something to do with the cold or both.


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Madratter
Post Laureate


Reged: 01/14/13

Re: About to give up on the 8" RC new [Re: Phil Hosey]
      #6194874 - 11/14/13 04:00 PM

One thought Phil, and you have probably already been there done that. But this the collimation is a push-pull. Is everything real tight?

I haven't actually changed the collimation of my back cell because it was pretty good to start with. I have no idea how tight they come.


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Phil Hosey
Carpal Tunnel
*****

Reged: 10/29/08

Loc: LaGrange, GA
Re: About to give up on the 8" RC new [Re: Madratter]
      #6194934 - 11/14/13 04:30 PM Attachment (2 downloads)

Quote:

One thought Phil, and you have probably already been there done that. But this the collimation is a push-pull. Is everything real tight?

I haven't actually changed the collimation of my back cell because it was pretty good to start with. I have no idea how tight they come.




Yes, in fact that was a concern of mine earlier, so I made double sure it was all locked down tight. You can see what I am dealing with now by looking at the image here.

This has basically been the same issue for the last 3 outings, ever since I got the heavier Moonlite focuser. And just to make it more confusing, here is the CCDI plot:


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gdd
scholastic sledgehammer


Reged: 11/23/05

Loc: N Seattle suburb, WA
Re: About to give up on the 8" RC new [Re: Phil Hosey]
      #6194955 - 11/14/13 04:41 PM

I saw pictures a while back where someone used an extra long dovetail (or attached an extension to it) to provide support for the camera. If I can find it again I will provide a link.

Gale


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gdd
scholastic sledgehammer


Reged: 11/23/05

Loc: N Seattle suburb, WA
Re: About to give up on the 8" RC new [Re: gdd]
      #6194985 - 11/14/13 04:53 PM

Found the link to another CN thread showing a picture of supplemental camera support for the AT8RC:

http://www.cloudynights.com/ubbthreads/showflat.php?Cat=&Board=cat&Nu...

Gale


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Madratter
Post Laureate


Reged: 01/14/13

Re: About to give up on the 8" RC new [Re: Phil Hosey]
      #6194993 - 11/14/13 04:56 PM

Well, I see the problem. I also see that it doesn't line up with the tilt shown in CCD inspector.

And it doesn't line up with my recollection of what the RA axis or declination axis would be here for this object.

I know you use are using the OAG port so it is highly unlikely it is flex.

On the other hand, because of what I am seeing, I have my doubts it is Collimation either although that would seem otherwise along with tilt to be the likely suspect.

Humor me with this, and you certainly don't need to try it because I am probably way off base here, but one thing I would try is removing the field flattener and see what happens to the stars in the center of the image.

Also, to take any possibility of it being tracking problems, take very short photos (maybe 5 seconds of a rich Milky Way field) and see what the stars look like then.

I'm just trying to narrow the problem down.

I'll grant that the problem probably involves the moonlight focuser since that is what has changed. But I'm not convinced collimation is the problem here. Actually I would have guessed Tilt if it weren't for the CCD Inspector plot. And it is reporting some. Just not where I would expect it.


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bseltzer
scholastic sledgehammer


Reged: 10/28/07

Loc: East S.F.Bay, CA
Re: About to give up on the 8" RC new [Re: gdd]
      #6195014 - 11/14/13 05:08 PM

Phil,

That is confusing because that CCDInspector data doesn't look that bad. Matter of fact, I've managed to get reasonably round stars with worse collimation than that.

If it's any help at all, I think the posts Gale is referring to were authored by 'blueman' AKA Floyd.


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Phil Hosey
Carpal Tunnel
*****

Reged: 10/29/08

Loc: LaGrange, GA
Re: About to give up on the 8" RC new [Re: Madratter]
      #6195135 - 11/14/13 06:12 PM

Madratter,
There is in fact more to the story, I just haven't had a chance to get to it yet. Last night after taking the image above, like you I began to suspect something with the field flattener so I removed it and took some shorter sample exposures. I didn't keep these as I should have, but the stars in the center were round but the field curvature is quite severe without a flattener, CCDI measured over 50%. As I was removing adapters and such to put the original focuser back on, my 68mm adapter that screws into the focuser drawtube because stuck. I couldn't get a grip on it because the only thing protruding were the T-threads on the camera nosepiece which sits inside of the adapter. In my frustration I tried to remove it with a filter wrench and didn't realize I was scratching the *BLEEP* out of it in the process. Anyway, the adapter is still stuck in the drawtube. I spoke with Ron at Moonlite this morning and I'm going to have to send the focuser back to him so he can get it out. I seem to have all kinds of bad luck when it comes to gear, some of it is my own doing because I get frustrated and impatient. Having said all that, if I were using the stock focuser and a DSLR, there would be no issues that good collimation couldn't take care of. As it is though, I can't use the scope but at f/8 because I can't get the AP reducer close enough get good stars in the corners and even if I could, I don't think I can off-axis guide on seagulls. Maybe I should evaluate how much detail I can get on galaxies with my refractor before thinking I need a longer focal length. 910mm and 1.22"/pixel should be good enough for my seeing which is rarely better than 3" FWHM. But can't seem to leave well enough alone. I don't blame the scope exactly, I knew of this possible issue before buying it, I don't know why but sometimes I think these things don't apply to me then they happen and I think, well damn, I should have listened.


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Phil Hosey
Carpal Tunnel
*****

Reged: 10/29/08

Loc: LaGrange, GA
Re: About to give up on the 8" RC new [Re: Phil Hosey]
      #6195169 - 11/14/13 06:29 PM

Oh, one other thing on the subject of collimation, in case I missed something in doing this. I'm going to describe how I collimated my scope and the issues that I've seen and what I did to 'correct' them.

When I first got the scope, with the stock focuser in place, I checked the collimation according to the manual, which basically has you use a cheshire. I got everything looking textbook so I was surprised when I was focusing on my first star that the secondary shadow was quite visually off-center. At this point I proceeded to refine the collimation based on the DSI method, which first calls for removing on-axis astigmatism, basically geting the secondary shadow centered. From there I did the next part which involves checking off-axis astigmatism and making sure the the stars in each corner are balanced about the center. Keep in mind I did all of this with my DSLR because it is much easier to do with Live View. Once this was done I put the CCD on with the AT flattener. The image of NGC7331 on my Astrobin page was done with this configuration. So, I get the moonlite focuser and noticed that collimation was way off so I tried to re-collimate it using the cheshire. I never could get the primary and secondary collimation to coincide with the mechanical axis so I thought it was the focuser. I put the original focuser back on and bam, had it collimated in minutes. Put the Moonlite back and.. out again. Ok, so then I collimated the Moonlite to make everything fall into place, it all looked good in the cheshire. I get it outside on a star.. on-axis star's secondary shadow was off again! Arggg. So I used the DSI method again with the Moonlite in place and did the final tweak with CCDI. The image that resulted from that was the one of Mel15 I posted in an earlier thread that everyone that looked good for 20 minute subs at 1625mm. But, I went and looked closer at that image earlier today and realized it has the same star shapes, just not as bad as the vdb142 image above. So....either there is something very screwy with my collimation of the moonlite et al, or there is some kind of flexure. Either way, at this point it is more than I want to deal with. I've lost too many clear sky hours jacking around with it.


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Madratter
Post Laureate


Reged: 01/14/13

Re: About to give up on the 8" RC new [Re: Phil Hosey]
      #6195172 - 11/14/13 06:32 PM

Oh, man. Phil, I feel for you. And I'm afraid wanting to do galaxies is something of a curse. It is very much an interest of mine, and it is just so demanding on gear.

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Phil Hosey
Carpal Tunnel
*****

Reged: 10/29/08

Loc: LaGrange, GA
Re: About to give up on the 8" RC new [Re: Madratter]
      #6195245 - 11/14/13 07:03 PM

You know, the thing is I bet this scope would do quite well with a DSLR. In fact, I've been thinking I might do some DSLR imaging with it. I just don't happen to own a modded DSLR anymore, but I do have a stock T2i. By the way, I just checked the collimation with a cheshire after all the tweaking from last night and it is dead-nuts on. But that's with the stock focuser.

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bseltzer
scholastic sledgehammer


Reged: 10/28/07

Loc: East S.F.Bay, CA
Re: About to give up on the 8" RC new [Re: Phil Hosey]
      #6195487 - 11/14/13 08:59 PM

Phil,

I'm assuming you got the collimatable base on the Moonlite focuser. If so, I've found collimation gets a whole lot easier if it's used to align the mechanical axis of the focuser with the optical axis of the optics first. If not, you'll need some way to make focuser alignment and primary alignment independent. I've got the AT focuser alignment collar under my FT focuser, and the first step I take in collimating the scope is using the HG laser to align the focuser. I do this by gently adjusting the push-pull screws on the collar until the beam is reflected exactly back into the laser. Then I user a cheshire to adjust the secondary to get everything concentric. This is an iterative process, and you'll have to back and forth between the laser and the cheshire until they are both dead nuts on. It may take 3 or even 4 iterations, but they will converge. Then I screw the concentric circle hologram attachment onto the HG laster and use it to tweak the primary to get all the rings concentric and equally bright all the way around. Also as you rack the focuser out, one of the outer rings will disappear. It needs to dim out equally all the way around. Of course this is iterative as well, so if the primary was out by much, you will need to back as redo the focuser/secondary adjustments. If you're gentle, making the smallest possible adjustments you can at any point in the process, everything will converge with 2 or three complete cycles for sure. My last check is a start test in the center of the field with a 6mm eyepiece (~277X) It usually take only the slightest tweak on the primary to get perfectly concentric intra/extra focal doughnuts. If I do that final check and adjust with my camera, the resulting CCDI analyses show the collimation at 0.5 - < 2.

Now that said, I know for a fact that good visual collimation disappears as soon as I hang a camera off the focuser. Depending on where I've got the camera on board, it usually takes anything from barely cracking to at most 1/32nd of a turn on the lowest primary adjustment screw to get those perfect doughnuts back. Now I realize the minute I point the scope somewhere else, it'll be a wee bit out again, but it appears this is just the nature of the beast.

In any case, I only mention all this because I agree with MR that what I see in your CCDI display and your photo makes me think something other or more than collimation is at work here. BTw, I weighed the combination of my 2" FT fouser and the AT focuser collimation collar, and they came out to almost 3 lbs. together. That would lead me to believe that the weight of the Moonlite isn't too big a factor.

Edit... One final note is that Ive found that the scope can be well out of alignment even if the view through the cheshire looks good. Which is to say, the cheshire view needs to be as near perfect as possible along with all te other checks to be in good alignment, but the cheshire by itself is a poor indicator of good collimation.

Edited by bseltzer (11/14/13 09:04 PM)


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orlyandico
Post Laureate
*****

Reged: 08/10/09

Loc: Singapore
Re: About to give up on the 8" RC new [Re: bseltzer]
      #6195853 - 11/15/13 01:27 AM

Travis, I'm suffering something similar with my AT8IN.

My new trick is an aluminum backing plate -
http://orlygoingthirty.blogspot.sg/2013/11/astro-tech-8-imaging-newtonian-par...

I am still on the stock focuser, however. If the above fails, I'll need to get a Feathertouch. If the tube still sags, I will need to get a rolled-steel backing plate that covers the entire interior of the tube (360 degrees) and bolt it to the tube at several locations.

The current (larger) backing plate is already bolted to the tube at 4 locations (aside from being held by the existing focuser mounting bolts). I am not sure if it is enough, I haven't tested extensively. I am using the 0.73X Keller reducer.


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mpgxsvcd
Carpal Tunnel


Reged: 12/21/11

Loc: Raleigh, North Carolina
Re: About to give up on the 8" RC new [Re: orlyandico]
      #6196199 - 11/15/13 09:53 AM

Quote:

Travis, I'm suffering something similar with my AT8IN.

My new trick is an aluminum backing plate -
http://orlygoingthirty.blogspot.sg/2013/11/astro-tech-8-imaging-newtonian-par...

I am still on the stock focuser, however. If the above fails, I'll need to get a Feathertouch. If the tube still sags, I will need to get a rolled-steel backing plate that covers the entire interior of the tube (360 degrees) and bolt it to the tube at several locations.

The current (larger) backing plate is already bolted to the tube at 4 locations (aside from being held by the existing focuser mounting bolts). I am not sure if it is enough, I haven't tested extensively. I am using the 0.73X Keller reducer.




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