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Old 20-08-2015, 10:07 AM
glend (Glen)
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RCs (f8) vs Newt (f5)- Why Bother?

This thread is born of my frustration at trying to image at f8 with my RC08. Last night I was out in the Obs all night testing the RC08, which is finally approaching collimation (yes it still needs work as you can see from the image of Sculptor A below). I selected a couple of reasonably bright targets (Sculptor and M42), and one dim one (The Horsehead), as I have good library images of these from my 10" f5 Newt (and Mak-Newt MN190 f5.3). Mount is the same NEQ6 Pro for all the scopes, as is the guide camera, and other gear. Camera was my Canon 450D Full Spectrum/Cold Finger modded. Reduced sized single sub jpg images below are from the RC (Sculptor A) and from the Newt (Sculptor B). I realise both single subs have star shape problems, but it's what I have at this point for comparison purposes.

I probably shot the RC subs without enough exposure time (at 260"), but I wanted to try and eliminate any mount, guiding, influences if possible, and the sub times were greater than the F5 times (194"). I realise that f8 will always require more exposure time, which is why I included M42 which is so bright it can burn out the Trap on even short subs.

This morning stacking my subs in DSS, all failed due to poor star detection, even when I bumped it up significantly, the stack still failed. Maybe this was unrelated to the RC08 performance but the same stacking settings worked fine for the other scopes. When I examined the individual subs in Photoshop some appeared fine to me (like the M42 ones), but the Sculptor was dim and lacked detail compared to the f5 scopes. The Horsehead was barely visible.

I realise the long focal length imaging at f8 does require more time, but honestly the focal length of the RC08 (at around 1600mm) is only 20% longer than my 10" f5 newt (@ 1250mm). The 190mm Mak-newt does have an advantage at f5.3 and 1000mm fl.

When I consider the problems that I have with focuser sag on the RC08 under the weight of my cooled DSLR (which is not an issue on the Newt or Mak-Newt), I have to wonder why I am continuing to pursue refining this setup.

Perhaps I just lack the patience, skills, and time required to make this RC work well with my DSLR setup, or is there some secret RC handshake that I don't know about yet?
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Last edited by glend; 20-08-2015 at 10:25 AM.
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Old 20-08-2015, 10:20 AM
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Shiraz (Ray)
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Quote:
Originally Posted by glend View Post

I realise the long focal length imaging at f8 does require more time, but honestly the focal length of the RC08 (at around 1600mm) is only 20% longer than my 10" f5 newt (@ 1250mm). The 190mm Mak-newt does have an advantage at f5.3 and 1000mm fl.
your f8 system is only 1/3 as sensitive as your f5 system (the ratio of the square of the FNo) - that is a huge difference and way more than 20%. You could maybe consider subs that are 3-4x as long as those that you used with the Newtonian and total exposure time that is 3x as long.

The MN system will be slightly less sensitive than the Newtonian.
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Old 20-08-2015, 10:24 AM
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rustigsmed (Russell)
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sounds frustrating Glen, can you post a pic so we can see how dim they turned out? 260 seconds sounds like you should pick something up at f8, what iso? did you push the star detection threshold down to a lower percentage in DSS?

i'm currently investigating a 2x barlow for my f4 newt. If it can give a sharp and corrected image then it will be like having a long FL scope like a RC and a wider field instrument at half the price of the RC.
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Old 20-08-2015, 10:27 AM
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Glen,

DSS is notorious in many aspects regarding star recognition. If you like, send me all your RC subs and I will run them through CCDStack.

I take it the second image is the Newt shot? Both shots are showing tracking errors (elongation is probably worse in the second than the first image), and yes, the RC collimation is still quite off. Have you used the Tak collimation scope yet? This is THE ONLY way to get an RC properly collimated, and saves HOURS and headaches.
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Old 20-08-2015, 10:35 AM
glend (Glen)
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Russell, I have put sub examples from the RC and Newt. Keep in mind they are converted to jpg and downsized to fit here so they look even worse. Yes they have star shape issues but that's not the subject of the question at this time.

Lewis I do not have, nor will I be buying a TAK collimation scope. I don't see it as a necessity when so many other people seem to be getting good results without it. I am using the techniques shown in the RC collimation utube videos, Cheshire and Howie Glatter concentric ring laser, plus the advice of Wade on CN to remove the internal baffle tube during collimation to expose the whole internal structure, vanes, rings, etc for easy alignment. My collimation problem is due in part to focuser sag. Collimation looks great on an EP star test but when I put the camera on it goes to hell. Thus I am trying to collimate it via a Liveview camera screen star test, and 'seeing' impacts good results there at this time. Again this question was not about RC collimation, there is another thread on that.

Ray I realise the f ratio light gathering differences, and yes I would need to increase the exposure times significantly. I was trying to impart the focal length differences in terms of the ability to get the same perspective (magnification) on an object. If I can get near the same magnification from the 10" newt, without focuser sag, or collimation headaches, or the need to significantly lengthen sub times then I wonder about the wisdom of an RC for me. It might be a great scope in the right hands, with the right equipment and systems, but at my skill level it if frustrating my enjoyment of the hobby.
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Old 20-08-2015, 10:38 AM
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260 seconds for an F8 RC as Ray has pointed out seems way short.

Also CCDstack would be the software of choice. I can't comment on DSS as I have never used it. I would not have thought that dimness would be an issue for CCDstack.

Greg.
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Old 20-08-2015, 11:05 AM
glend (Glen)
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I have taken Russell's advice and played around with the DSS star detection threshold and managed to get a stack of eight subs (2080" of total time). It's actually looking much better, once stacked and with just auto leveling done in Photoshop. Please ignore the star shapes as noted before I know there are issues there.

Image is reduced jpg below, posted next to the unprocessed Newt single sub of 255" as an example for comparison - so about an 8:1 ratio there.
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Old 20-08-2015, 01:23 PM
glend (Glen)
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I have gone through and rechecked the collimation, still spot on with tools on the bench, will star test again tonight with EP and camera. I suspect that even if I get the static camera star test spot on, as soon as the mount slews and changes attitude and force of gravity applied to the focuser in another direction it will go out again. I have another DSLR (uncooled) and I will try that as well because it weighs less than the cooled camera. If it still sags out of collimation with the lighter camera I will need to make a decision about the path forward; I don't want to have to buy a new focuser so I might have to sell the scope to someone looking for a 'project'.
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Old 21-08-2015, 07:24 AM
glend (Glen)
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Did more test subs last night, shooting at 332" and things are improving. I have attached a single sub of the Lagoon. Star shape is improving. I used my other DSLR (which is full spectrum but not cooled, so is lighter than the main camera), and had much less collimation shift due to focuser flex. I suspect that ulimately, if I want to keep this scope, it will need the focuser replaced order to support the cooled DSLR. As to f8 sub timing, I suppose it's just a learning curve on my part after having shot at f5 for years I had a 'feel' for what was best for capture, and need to adjust to it.
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Old 21-08-2015, 10:08 AM
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billdan (Bill)
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Hi Glen,

Those double star spikes you are getting on the brighter stars suggest to me you may have a twisted vane. So the top and bottom edges are being diffracted.

Well that is what happens on my F5 Newt, when I get double spikes.

Regards
Bill
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Old 21-08-2015, 10:08 AM
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looks like you are getting on top of it Glen.
the extra sub length seems to have improved the signal a lot.
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Old 23-08-2015, 11:51 AM
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Stellar profiles still indicate to me that your collimation is way off. Possibly focus too.

I would think that your 10" f/5 would produce better images in much less time than the rc.

Collimation is key with those rc's, I bought the first one sold in Australia to my knowledge and to my knowledge it was the only one that arrived collimated. I never touched the collimation and never had problems. I did some modification to the focuser and it easily held my sbig st10xme and cfw9.

If you have collimation issues I personally feel that the TAK scope is the answer. It's pricey but it's a problem solver.
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Old 23-08-2015, 01:48 PM
glend (Glen)
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Alex it is slowly getting better. Have found a flex problem in the back end and tightened up the focuser rail. Visually I can get it very close but as soon as the cooled camera goes on it sags out of collimation. When I put the uncooled DSLR on it was better but stll showed some sag. As soon as I get some clear nights I will run some more tests with the stock DSLR and tightened focuser. Next step would be to move my Moonlight focuser from my iStar refractor to the RC to test that but I will need to get an adaptor for the RC flange and the Moonlight. Baby steps.
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Old 23-08-2015, 02:11 PM
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Cool. How do you find the ES 80/480 triplet? Interested as it's a well priced option for a wide field imager
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Old 27-08-2015, 09:11 PM
E_ri_k (Erik)
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Hi Glen, thought I'd share my experience with my RC10. Regarding the exposure time, I ran with 900 second exposures most of the time. A lot longer than what I would have with my old F4 Newt.

The Sculptor was my first target when I got the scope, and I remember 600 second exposures worked well. 5 minutes worked well on large bright objects.

As for the focuser, I'd recommend replacing it. I found WAY to much flex in the stock one.

Erik
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