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buzz

Help required 10" Truss RC or 11" Celestron Edge HD for imaging?

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I have a portable setup and my longest refractor is 916mm FL.  I need something longer.  I'm considering a new Altair Truss 10" RC or a used 11" Celestron Edge HD for about the same money. Both will have high quality motorized focusers on them.

I'm imaging with a QSI 683 on a Paramount MX, so the peripherals are up to the task.  

The scope is stored outside, so it is approximately at the ambient temperature at all times.

My initial investigations suggest the RC has the better imaging potential but I am concerned that I will be forever collimating it when I put the system together for a session. On the other hand, the Celestron is a closed tube, so more friendly to handle but with potential cool down issues.

Any strong views on pros and cons to consider?

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If it is going to be stored outside then cool down issues are already at a minimum because its constantly very close to the ambient temperature already. Yes the RC will cool quicker but you talking maybe 20-30min difference at the most. It would be different if you where taking it from inside to outside. So I personally would consider them basically equals in the regards and I wouldnt take it into consideration. 

The next thing to consider is that at first thought you might think that the SCT only has an extra 1" of aperture but it actual has more due to the fact that the RCs have a much large secondary. So if you are wanting that extra resolution the SCT is going to win out. 

The next thing to consider is what you are wanting to image. The SCT has quiet a bit for FL than the RC. It is also at F/10 vs F/8 (native). If you are looking to image small galaxies and PNs then I would suggest the SCT. If you are still wanting to do nebula and have a more wider fields of view then I would suggest the RC. The RC has some decent options fof reducers so you get still get some pretty wide FoV at around F/5-6. This SCT really only has one option for a reducer to bring it down to F/7. Also make sure that the scope matches well with your camera and you are getting the FoV you want. As well as making sure you are not over or under sampling too much.

I'm currently saving up to get into long FL imaging to image galaxies and PNs. So after my last year of research for me the perfect fit is the 11" Edge. But if galaxies arent your thing I think the RC would probably be the better fit. Not saying you cant image nebula with the SCT, I've seen ppl do it and produce excellent quality images, just I think the RC wins out a bit. 

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Hi - I have just found a technical report with some scientific basis. A 10" f/8 RC is really a f/9  and apparently, even with theoretical perfect optics, has a diffraction limited resolution equivalent to a 5" refractor. This report also suggests that the baffles in the closed tube designs are better removed and replaced by black flocking and that the cooling fans need to be reversed in direction to reduce turbulence.  Then there is the see-thru condition....  this report suggests a design that has not been fully thought out.  I need to find something on the Edge rather than Celestron's blurb.

http://interferometer-tests.blogspot.co.uk/2013/06/2542000-10-ritchey-chretien-gso.html

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Thats a good write up. But I do want to point out that you mentioned the truss RC and the RC that was tested in that paper is the metal tube version. There is a big difference between the two and though they are similar they have some big differences that shouldnt be over looked.

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I would get the 11" edge SCT. It would keep your options open as far as being able to add the Hyperstar and go F/2. So kinda the best of both worlds. The thing is when you swap the secondary and the Hyperstar you may need to collimate every time. I added a sleeve to the locating pin on the secondary and have not had to collimate that because the sleeve takes up the slop of the pin. The Hyperstar is a little more picky. A temperature change of around 10F can put it out of collimation and taking it on and off for sure. Right now I have mine dialed in at around 32F or so. It only takes me three images to collimate when it is out. 

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I have the Altair RC250-TT truss model on a PMX, very similar to one of your proposed setups. Here is the thread on my setup. I've not collimated it yet as it seemed OK out of the box. I've not tried it on planets yet.

Graham

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What about the Orion Optics 10 inch ODK? I used the 14 inch for two years. It worked. I host a 10 inch in the robotic shed and that also seems to work.

Olly

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I have the Altair RC250-TT truss model on a PMX, very similar to one of your proposed setups. Here is the thread on my setup. I've not collimated it yet as it seemed OK out of the box. I've not tried it on planets yet.

Graham

Thanks Graham -  I would be using this with a KAF8300 sensor - once you had good focus and collimation, did you get good flatness to the corners on your sensor?

Olly - I will check out the Orion. thanks for that.

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My sensor is 15.2x15.2mm, 7.4 um, so slightly smaller diagonal than yours (21.4 v 22.5) but slightly bigger pixels (7.4 v 5.4). I've enclosed a luminance image, it doesn't look too bad in the corners. I don't have any software for examining image curvature.

Graham

M27%20L%20600s%20ps3_zps5rsbpe0l.jpg

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Thanks Graham - for some reason CCDInspector could not handle the JPEG. Could you send me a link to a full frame FITS file and I can measure it for you.

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I can't get the TIFF to upload to Photobucket, maybe its too large. If you PM me your email address I'll send you a link to a OneDrive folder where its stored.

Graham

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Things to consider include internal reflections, ghosting and other peripheral issues. Given the volume of their sales I'm surprised we don't see more from the Edge series scopes on the boards. This doesn't prove anything, it's just a bit odd. Both John and Sara have done great stuff with the 6 and 8 inch RCs.

Olly

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Yes Olly - I noticed the same. A google search does not find many images taken with Edge 11's, let alone compelling ones. I think the best RC images I found to date were the ones on IKI's site that he and Nik took.

I think I'm leaning towards the RC -  I might try some planetary but my aim is to image small galaxies and nebula. I passed on the used Edge 11 and you will see it on Astrobuysell in the near future. It belongs to a mate of mine and it has been kept in an indoor obsy and it is fully kitted out with moonlight focuser and accessories.  He has just bought an Officina Stellare modified DK. Drool.

I did some further digging into the effect of the central obstruction, the differences between the Edge and RC are not as significant as some adverts would leave one to believe. I might even stretch to the 12" RC. 20Kg is about my weight limit. The PMX can handle more, but I can't, trying to slide that into the dovetail every session.

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Have you looked through Astrobin? Thats what i usually go to for comparisions. Since my plan is for small galaxies and PNs too I'm aiming for every bit of FL my budget will allow. Give me the most resolution and detail...well until my local skies mess that all up lol. I know there are plenty of people producing great images with both so its a tough choice indead. The main two resons I've decided on the 11" Edge is 1: because of the extra focal length and 2: because of the native flat field. This will allow me to get the most FL and not have to worry about needing to crop the image or mess with a FF/R. Plus there is always the option of a reducer if I want a bit wider FoV for some of the galaxy groups/chains. I personally have no desire to mess with the Hyperstar option so that wasnt even in the comparison for me. Maybe if I feel like a really want to challenge myself I might get one....but i doubt it lol. I dont like frustration much. Plus if I want that wide of a FoV I would prefer a smaller triplet/quad refractor. Sorry for the hyperstar tangent lol.

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Yes Olly - I noticed the same. A google search does not find many images taken with Edge 11's, let alone compelling ones. I think the best RC images I found to date were the ones on IKI's site that he and Nik took.

I think I'm leaning towards the RC -  I might try some planetary but my aim is to image small galaxies and nebula. I passed on the used Edge 11 and you will see it on Astrobuysell in the near future. It belongs to a mate of mine and it has been kept in an indoor obsy and it is fully kitted out with moonlight focuser and accessories.  He has just bought an Officina Stellare modified DK. Drool.

I did some further digging into the effect of the central obstruction, the differences between the Edge and RC are not as significant as some adverts would leave one to believe. I might even stretch to the 12" RC. 20Kg is about my weight limit. The PMX can handle more, but I can't, trying to slide that into the dovetail every session.

Have a care. I remember reading that the focuser on the 12 inch and up GSO based RCs is fundamentally flawed. I can't remember all the details but I'd dig into this if I were you. The thrust of the conversation was that the 6,8 and 10 were all sortable but not the 12. I'm sorry not to remember more about this but someone will know.

Olly

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Have a care. I remember reading that the focuser on the 12 inch and up GSO based RCs is fundamentally flawed. I can't remember all the details but I'd dig into this if I were you. The thrust of the conversation was that the 6,8 and 10 were all sortable but not the 12. I'm sorry not to remember more about this but someone will know.

Olly

Thanks Olly. I think this might be it...http://stargazerslounge.com/topic/187671-gso-12-rc-dissambly/

I know that the truss designs now have a revised backplate to alleviate the issues of the focuser hanging off the mirro assy. Adverts refer to a V2 design. I need to find out if these issues are post or pre 'fix'

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IKI checked with Altair -  GSO have since revised the rear cell and say they have resolved all the issues on the 12" Truss RC.   I think I will take a look at the 10" (Ian has them in stock) and then work out if the 12" will be just too heavy to lift into place each time I want to use it.

Luckily the Paramount MX has a 41Kg payload, so this 20Kg scope is well within the normality limit.

Graham - I tried the TIFF file, but had no luck.  I am not certain but I think CCD inspector needs an image before stretching.

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I think the comparison is very close. The hyperstar is not really an interest to me at the moment - I would use one of my three refractors for that.

I have found very different views on the ability of a GSO Truss RC to maintain its collimation after careful handling. I will be assembling / disassembling my rig each nightand and carefully storing the RC in an unheated building. Some views suggest it will be OK, but if left on the mount and others that the truss is rigid enough to not distort with handling. I have seen other views that suggest that 3 mirror supports are insufficient (especially in the 12" version) to prevent mirror pinching at all orientations. Certainly GSO did use 3 (judging from pictures from SGL forum posts) but I don't know if that is one of the improvements that GSO have made.  I did talk to Altair - mentioned my concerns and that I would simply return the telescope if it does not work without continual collimation. They were not concerned, they have existing customers with temporary setups and said they were not aware of issues.

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I have decided on the 10" RC. I think the 12" is too heavy for me to lift safely each time I want to use it. I am not in an obsy, so the Edge HD is going to be a dew magnet, or with its dew shield, a sail.  The actual central obstruction in f/stops is marginal between the designs.

I have researched all the various ingenious RC alignment procudures - they make very interesting reading. Some challenge the premise that the optical surfaces's optical center is the physical center of the mirror, a 'fact' used by many procedures to align both mirrors, another one, from a University, looks very interesting indeed - creating a cross hair in the middle, between the mirrors, to act as an independent optical center for each mirror. Another uses aberrations and their symmetry to set each mirror (again, assuming they are centered). There is the Holtech multiple laser device and another, that uses platesolving to determine the focal length and hence the mirror separation.

Sounds like enough material to write a chapter for the 2nd edition of the book! I'm going to have to dig out my MSc Optics notes from 25 years ago.

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The 10" carbon fiber truss RC arrived last month. I found a way of an almost invisible secondary heater attachment using copper tape. (There is a picture of the completed assembly below and the method is on my website)

The scope was not collimated and I spent the next weeks researching every possible collimation technique. It was very interesting. So far, I have managed to discard about half the proposals, since they rely upon mechanical characteristics that may or may not be good. I have done good bench alignment of both mirrors and the focuser with just a Howie Glatter laser and using a telephoto lens on a SLR, confirmed by star testing (outside of focus). I borrowed a Tak scope but discovered I did not need it. I could reproduce the Tak blot view using a laser with faint diffraction rings (like the Howie).  I also discovered that my mirror separation was wrong, something that is not often mentioned and which caused excessive star elongation in the corners of the KAF8300 chip.  I found a website that suggested the focal length increases by 9 mm for every 1 mm reduction in mirror separation. I unscrewed the secondary mirror baffle by about 2.5 mm (it has a fine thread ) and did up the knurled lock ring. My focal length is now exactly 2000 mm and the stars are much better in the corners.

I still have to evaluate and compare the sensitivity of the Howe Glatter holographic attachment and I have just ordered the GoldAstro collimation tool too. You cannot write about this stuff unless you have carefully evaluated for yourself.  It also gives me something to do over the summer.

First Light:

I couldn't resist capturing a few hours of M3 before it sets too early - the collimation is about 95% complete for this shot. Processing was done quickly in PixInsight but I think I should have used masked stretch to avoid blowing out the core. One other interesting thing, I need to take more care with Sequence Generator Pro autofocus with a RC. The autofocus algorithm does not like donuts.

Thanks for all the help and advice. I think the truss scope is very good, no sign of pinched optics and it keeps the collimation after multiple handling on and off the mount. I probably could have gone for the 12" - I think the weight would just be OK for me.

post-16414-0-63582000-1434466802_thumb.j

post-16414-0-99200500-1434467230.jpg

Edited by buzz
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Hi,

interesting thread.  I have had an Edge 11" for about 3 years and it has been a great performer.  However, I needed a dew shield and for most DSO objects used a Optec Lepus FR with a Kaf8300 as the fl is too long and the F ratio is really too slow.  I had Hyperstar for this scope, but sold it when the scope went in my obs, as the Hyperstar would hit the obs wall if the scope slewed and I didn't want to risk breaking the corrector.  I didn't use the Hyperstar very much, but when I did t was mega fast.

12 months ago I bought a RC250 and like you it took me weeks of adjusting to get t aligned.  I know it is now fairly close, I got some half way decent images of Jupiter with a 2x Barlow and ASI120mm.

Anyway my questions, how did you know that the mirror separation was wrong?  Or were you able to measure the distance?  How did you set it correctly?

I also have a rather poor flat field, I use a focal reducer on a Kaf8300 so it might be that, but get a doughnut of light around the centre of the frame.  The centre third of my Kaf8300 looks okay, then a bright doughnut then back to about the same brightness.  My FR is giving about 0.69x, well within the spec.  Taking a flat sorts it out but I can't help feeling it is not right.  Could this be caused by incorrect mirror spacing?

i am going to remove the FR to see if that fixes it, but any advice on mirror spacing would be greatly received.

Robin

 

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DrRobin - the mirror separation affects focal length. I simply did a platesolve using PinPoint and noted the imaging scale. That goes to 3 sig figures and is sufficient to alter the mirror spacing. Bringing them closer together lengthens the focal length - by about a 10:1 ratio.

 

I am using a KAF8300 too - I do not have vignetting and I sometimes don't bother with flats. I am using Astrodon filters that fit into the 1.25" housing but do not have a metal threaded ring.

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