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Astrophotography and Sketching >> Beginning and Intermediate Imaging

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CounterWeight
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Re: Levenhuk carbon fiber 8" RC on the way! new [Re: Madratter]
      #6037187 - 08/20/13 07:31 PM

These days,

I'd only worry if I was undersampling by a margin large enough (as John mentions) to rob you of image quality - and you won't be doing that with the 8300M. There's plenty of reason not to worry about oversampling within reason - and if the supporting chipset allows binning pixels, there goes that worry. The 8300 chip's wild popularity among the manufacturers speaks for itself - as do the images made from them so I think you can buy with confidence.

I do think it's important to understand what the expression is taking about, detail or lack of it, and where in the grand scheme it matters. For us in general, small pixels and lots of real estate = good.

More important to me is the issue of using a 'OSC' version with it's Bayer matrix for color (and it's debayered resolution in each color plane), or the mono version where every pixel can provide resolution and detail. Also look at the aperture and filter discussions as relates to focal ratio. Also if the CCD is anti blooming or not, is microlensed, and then it's temp regulation specs.

There are reasons FLI, QSI, Apogee, SBIG, others adopted the 8300 chip, and there are differences in the exact models that were brought to market. So don't assume all the same and only different $. When I was going through it all when SBIG and possibly others initially announced I found it very helpful to spreadsheet all the offering and all the specs/options between them.

Re-reading your post (trying to be certain I'm not wandering OT) I think going 'mono' 8300 at your price point may not be possible outside a fantastic deal on the used market (and I wouldn't rule it out entirely). Adding in the cost of a filter wheel and filters puts you up and out of that bracket.

With the color or 'OSC' version I'd take a look at how they get the color and what that has to do with final resolution (arcsec/pixel and over/under sample) even after all the software bells and whistles.

When I jumped into imaging I bought an OSC and then sold it after saving some more $$ for a mono variant. I was in a hurry to get started and see what it was all about.


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rflinn68Moderator
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Re: Levenhuk carbon fiber 8" RC on the way! new [Re: CounterWeight]
      #6038132 - 08/21/13 11:10 AM

Thanks for chiming in about CCD's Jim! I understand I wont get filters and a wheel for that price. I was talking about the camera by itself. The SBIG 8300M is on sale at Astronomics for $1995 and QHY has their QHY9 mono (8300 sensor) with a filter wheel for $2099. The QHY9 looks pretty nice to me with a 2 stage TEC cooling and the SBIG is a single stage. I'm kinda leaning toward the QHY but they havent answered my email from several days ago regarding the filter wheel. It says its a 2" filter wheel and I was wondering if it will hold the 36mm filters. I also dont know how many filters it will hold. I suspect just 5 and I'd like to have an 8 position wheel. Then I could get the LRGB filters, Ha, OIII, SII and have 1 open for exposing my flats. Is this how you do it or do you expose flats with the Luminance filter?

I was able to get the RC collimated last night........sort of It looks near perfect on the intra-focus side but it doesnt look concentric outside of focus. What causes this? Also, stars in the bottom corners look pretty good but the top corners still look pretty bad.


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Madratter
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Re: Levenhuk carbon fiber 8" RC on the way! new [Re: rflinn68]
      #6038173 - 08/21/13 11:31 AM

Personally, I expose my flats using the luminance filter.

EDIT: Ideally you would take a set of flats for each filter actually used.

As for the collimation problems and it changing between intra and extra focal positions, I don't know how that can happen without slop in the focuser. If the star is staying centered in the exact same place (i.e. no slop), I'm at a loss.

I'm also wondering if you have some camera tilt.

Edited by Madratter (08/21/13 11:35 AM)


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jrcrillyAdministrator
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Re: Levenhuk carbon fiber 8" RC on the way! new [Re: rflinn68]
      #6038180 - 08/21/13 11:35 AM

Quote:

have 1 open for exposing my flats. Is this how you do it or do you expose flats with the Luminance filter?





The really serious folks do flats for each filter. That does have the advantage of handling dust on the filters. I think most of us do flats through the L filter. When I use flats (which isn't always) I use the L filter. I keep my filters clean, though.


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HunterofPhotons
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Re: Levenhuk carbon fiber 8" RC on the way! new [Re: Madratter]
      #6038292 - 08/21/13 12:43 PM

Quote:

I found the thread. See the statement by Kevin Nelson from QSI.

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Board/ccd/Number/57...




Thanks for the link.
I read it and the subsequent links, but I didn't see where there was a connection between 'low noise' and the number of hot pixels.
Temperature can affect the number of hot pixels, but that was the only direct connection that was cited in a subsequent link.
It's easy to fall into the trap of assuming a correlation is evidence of a cause-and-effect. I guess I'll remain skeptical. <g>

dan k.


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rflinn68Moderator
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Re: Levenhuk carbon fiber 8" RC on the way! new [Re: jrcrilly]
      #6038293 - 08/21/13 12:43 PM Attachment (2 downloads)

Ok, After looking at these on my desktop I can see I still have work to do but you can still clearly see the difference in how it looks inside of focus vs. outside of focus. This first one is outside of focus. I was on Arcturus and at ISO 1600 even 3.2sec over-exposed it for a test like this but when I cropped it I included the star to the left.

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rflinn68Moderator
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Re: Levenhuk carbon fiber 8" RC on the way! new [Re: rflinn68]
      #6038298 - 08/21/13 12:45 PM Attachment (3 downloads)

And here it is on the inside of focus....

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CounterWeight
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Re: Levenhuk carbon fiber 8" RC on the way! new [Re: rflinn68]
      #6038342 - 08/21/13 01:18 PM

Argh! just lost a post?! Must have forgotten to push the button after checking it over.

What I wrote was that I agree, flatting it all depends where the offending item is. If not on the filter directly using the Lum channel seems to work just fine on both the Baader and Astrodon filters I have used.

On another tack, the SBIG and flats requires special mention because of the mechanical shutter. I have the original 'ST' version and wheel and learned quickly that I had to increase my expose times for flats very dramatically to get rid of the shutter effect - more than 2.5 seconds or ~3 seconds seems adequate. This was what drove me to getting an illuminated panel for flats.

In all my astro purchases support figures in very heavily, and SBIG is fantastic. I think it's important to mention this. Clear nights to image are a premium in cloudyopolis, and may be when the problem shows is after a long wait to image again (it can and has been months between at times) and waiting days on end for support and 'all that' just does not float my boat. Neither does getting someone who has no actual expertise or concern and urgency when I finally do get through.

SBIG has exceeded my expectations which are high. I've sent mine in twice with the filter wheel and turn around was as fast as I could hope with clear communication always from them. If I remember correctly the first visit I was out only the shipping, and they went beyond the minimum in servicing the units.

Good luck with the final tuning of the optics. Takes some patience to learn each scope's 'personality' when it comes to tweaking in the optics. When these came out there were some pretty huge threads on dialing them in, i think in the 'Cat and Cass' forum?


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Madratter
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Re: Levenhuk carbon fiber 8" RC on the way! new [Re: CounterWeight]
      #6038424 - 08/21/13 02:05 PM

I can't tell much from Arcturus other than the collimation is not perfect.

The fainter star makes that completely clear. What is also clear is that there is some astigmatism. That is why the orientation of the overall oval shape changes.

I would not jump to conclusions on that. First off, the star isn't centered (I realize this is a crop so it is probably pretty close but still).

Second that astigmatism could be caused by things like camera tilt rather than the optics. Are you seeing the same behavior visually. Also, is there a diagonal involved in any way? It is very common for diagonals to be miscollimated.

Third, I am no expert on this stuff. I am a duffer at best. I would head over to the Cat/Cas forum and post there.

It is possible that it is normal for RCs to show this kind of intra/extra focal image when off axis.

At any rate, I would be very leary of making any pronouncements of optical quality until it is collimated properly.

And again, I could be all wet. The people in Cat/Cas will be much more helpful than I could ever be, simply because some of them actually know what they are talking about.


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rflinn68Moderator
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Re: Levenhuk carbon fiber 8" RC on the way! new [Re: Madratter]
      #6038604 - 08/21/13 03:44 PM Attachment (2 downloads)

Thanks guys....Madratter, yes it is cropped and no I do not use a diagonal when imaging. I would be VERY interested in this astigmatism issue!! They have a 45 day return policy and I might want to take them up on that. Its not the most expensive scope in the world but it was a lot of money for me and I want a nice scope for that kind of money. Here is the image uncropped to give you an idea of where the other star was in the frame. I know Arcturus was overexposed and thats why I cropped it and included the other star.

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Madratter
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Re: Levenhuk carbon fiber 8" RC on the way! new [Re: rflinn68]
      #6038635 - 08/21/13 03:58 PM

That other star is so far off axis, I wouldn't make any conclusions based on it.

AND it turns out that RC telescopes have this particular abberation (astigmatism) off-axis.

http://starizona.com/acb/basics/equip_optics101_astigmatism.aspx

Edited by Madratter (08/21/13 03:59 PM)


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rflinn68Moderator
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Re: Levenhuk carbon fiber 8" RC on the way! new [Re: Madratter]
      #6038655 - 08/21/13 04:10 PM

Thanks!! This is good to know. You scared me for a minute there! I'll try to tweak on it a little more over the next night or two and hopefully get it lined out by the start of our dark window. Its way too pretty of a scope to send back.

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rflinn68Moderator
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Re: Levenhuk carbon fiber 8" RC on the way! new [Re: rflinn68]
      #6038693 - 08/21/13 04:28 PM

But the article says they employ a "correcting lens" to eliminate any residual astigmatism. So they are calling the secondary mirror a corrector lens? Please forgive my ignorance.

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Madratter
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Re: Levenhuk carbon fiber 8" RC on the way! new [Re: rflinn68]
      #6038776 - 08/21/13 05:22 PM

No. Apparently the expensive RC scopes, have lens elements (just like a normal scope might have a field flattener or coma corrector) to get rid of it.

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jrcrillyAdministrator
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Re: Levenhuk carbon fiber 8" RC on the way! new [Re: Madratter]
      #6039030 - 08/21/13 07:39 PM

Quote:

No. Apparently the expensive RC scopes, have lens elements (just like a normal scope might have a field flattener or coma corrector) to get rid of it.




I can't think of a single example of an RC with an internal astigmatism corrector, let alone "most". I think it's just the unfamiliarity of the writer with his topic. For the most accurate information on a telescope design it's probably best to go almost anywhere other than to a vendor who sells only competing designs.

Folks who buy RC scopes tend to do so because they prefer pure reflectors. Folks who don't mind a requirement for add-on internal optics can usually save a bunch of money by selecting CDK or EdgeHD models (or Starizona's Hyperion). A couple of the top end RC vendors do offer optional flatteners, though.


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rflinn68Moderator
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Re: Levenhuk carbon fiber 8" RC on the way! new [Re: jrcrilly]
      #6039306 - 08/21/13 10:40 PM

I feel much better now after a little more tweaking with the collimation tonight. I'll post some new pics tomorrow but it looks a lot better on both sides of focus now. Now I'm ready for some dark skies!

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CounterWeight
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Re: Levenhuk carbon fiber 8" RC on the way! new [Re: rflinn68]
      #6039441 - 08/22/13 01:00 AM

John C,

Take a look at RCOS and DSI - the top names as far as I know in the scope design true R/C that is - both offer the corrector.


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jrcrillyAdministrator
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Re: Levenhuk carbon fiber 8" RC on the way! new [Re: CounterWeight]
      #6039481 - 08/22/13 01:45 AM

Quote:

John C,

Take a look at RCOS and DSI - the top names as far as I know in the scope design true R/C that is - both offer the corrector.




Hi, Jim.

Those are both flatteners, which I did mention.


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CounterWeight
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Re: Levenhuk carbon fiber 8" RC on the way! new [Re: jrcrilly]
      #6039520 - 08/22/13 02:29 AM

John - Ah... Ok, I was just going for the 'additional lensed element' in true R/C design, sorry - not wanting to muddy the waters.

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Madratter
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Re: Levenhuk carbon fiber 8" RC on the way! new [Re: CounterWeight]
      #6039693 - 08/22/13 08:03 AM

I'm definitely happy things are looking up for you.

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