Eta Carina from GSO RC

[Alchemy (Clive)]

Originally Posted by AlexN

I dont have a pic, but can get you a shot showing what it looks like at the focus position with my SBIG. There are a lot of extension tubes involved the back focus is quite long... I dont have an exact measurement at hand, but it is quite long, as is generally the case with the RC design..

yes i have read somewhere a complaint about the long back focus.... whip up a shot , i will have a squiz

re the vixen style bar ..... what is the diameter of the tube and could you just use some 8 inch rings to clamp it down rigidly, i say this as i had a vixen vc200l and thought it had allowance for movement given the attaching ( note as i was using only a dslr at the time exposures beyond 5-6 mins got too noisy for the F9 i think, and i sold it to go f5 with the newt, which has its issues too)

i am giving thought towards some sort of new scope at some stage, my personal concerns (and Paul has produced a fine shot here..) are as others that having a scope that needs almost the same again spent to bring it up to quality which is a bit of an issue. The one other factor is that eta is just about the brightest object in the sky along with M42, its easier to produce a fine image of bright objects (most of us want to move on to the tougher stuff).... given f8 with a large central obstruction makes it have almost an f10 lux gathering capacity which then means much longer exposure times to gather the photons... so how will it perform on the faint stuff. possibly out of the range of a non cooled camera for the dslr users. and of course a tighter regime for guiding . i will be watching to see what else paul produces and whether or not others with the scope can do the same..... jury is considering the verdict, but it does seem to havesome promise.

these are just musings about the scope and in no way a criticism on what paul has done.

[AlexN]

Clive, I'll get a pic tomorrow morning for you, PM me your email address and I'll email it through... I'm hoping to hit M16 as my first target for the 8" RC, this will be tackling one of the darker targets in the sky, the only issue with regards to comparing that result with say, your potential results or DSLR users is that my ST9 is VERY sensitive.. (one of the reasons I wasnt phased about the scope being F/8, with a large obstruction.) I do look forward to getting to know this scope... And sure, they do need a little money spent to bring them up to a perfectly workable imaging system, there are cheaper ways to fix these problems.. You could upgrade to a different focuser and have a flange made to fit between the focuser and the OTA.. Using something like the WO SCT focuser, you'd be looking at less than half the cost of the feather touch upgrade.. As for balancing the scope, ADM accessories sell a 3.5lb counterweight that will clamp to the vixen dovetail rail for US$59. With that fitted to the front of the dovetail I think that would provide plenty of weight to balance the scope... TrevorW had a good idea too.... He used a dovetail clamp with a 60mm finder scope converted as a guide scope using Orion SSAG attached to the front of the scopes dovetail to balance it up.. A great plan in my opinion...

I think with regards to guiding regimes, you'd find that with your G11, guiding this 8" F/8 scope would be easier than your 12" F/5 scope. Consider the fact that the RC OTA weighs in at slightly less than 7kgs, and has a 1624mm focal length, where as the 12" would weigh closer to 20~25kgs with rings etc, and a focal length of 1500mm... The focal length is similar, but the extreme weight difference would definitely make life easier... The mount would be under much less load, and therefor should perform better... wind would be less of a problem..

That was my thinking when I bought it.. its 126mm shorter F/L than my C11 @ F/6.3, but weighs less than half... making life easier all round.

[AlexN]

Oh, I'll measure the exact outer diameter of the tube tomorrow too... I was considering a set of rings and a longer dovetail... This would also be advantageous as the standard aluminium dovetail bolted to the front and rear of the scope negate the use of carbon fiber in the tube to stop thermal expansion. Using rings will detach the aluminium from the OTA itself, allowing the dovetail to expand or contract without putting any stress on the OTA... Just a thought..

[Paul Haese]

Clive, sorry I could not answer your question but Alex took care of it for me.

I still reckon that even if you have to spend another 1500 on the scope to get it right, it is still far cheaper than buying an RC from RCOS. So there is still value for money. Ok maybe it should have better back focus, and maybe it should have a better focusor, but the main thing is the optics. My C14 I changed the focusor to a microshift moonlight and that was a 8K scope. So I am not worried about that. My only concern was the mirror figure. Which looks ok at present.

Even with fainter objects you can image for longer. I know two guys in Adelaide who are imaging with an EQ6 for up to one hour subs. Stars are perfectly round. So guiding is just a matter of getting that right and the images will come.

I will report more as I fix and image more. Stay tuned.

Hope I answered everyones questions. If not just give me a reminder.

Regards

[AlexN]

Originally Posted by Paul Haese

I still reckon that even if you have to spend another 1500 on the scope to get it right, it is still far cheaper than buying an RC from RCOS. So there is still value for money. Ok maybe it should have better back focus, and maybe it should have a better focusor, but the main thing is the optics. My C14 I changed the focusor to a microshift moonlight and that was a 8K scope. So I am not worried about that. My only concern was the mirror figure. Which looks ok at present.

Agreed.

With my C11, the scope was $4k, at the same time that I bought it, I also bought about 2000 worth of extra bits and pieces I knew I'd need in order to do what I wanted with the scope, between reducers, different dovetail setups, motorized focuser, dew control gear etc etc.. At the end of the day, no scope purchase is going to be without the need for accessories, modifications etc etc.

I figure, $2500 + $1000 give or take for extras still makes this a VERY cheap RC imaging platform.

[dugnsuz (Doug)]

Originally Posted by AlexN

Agreed.
...At the end of the day, no scope purchase is going to be without the need for accessories, modifications etc etc.

I figure, $2500 + $1000 give or take for extras still makes this a VERY cheap RC imaging platform.

I know you're right Alex, just whingeing!

But on behalf of the financially embarrassed imagers out there without the extra $1000 to upgrade, I still "would love to see an uncropped high res image from the scope that came out of the box"
How's about it Alex?

[Alchemy (Clive)]

well im interested, the points you make are all valid, im being killed by flexure in my system which i probably would not get with that "little" scope you are using i have all my gear mounted on a huge alloy plate so rings would be beneficial for my setup, (note i have ordered custom rings for the 12 inch to try to eliminate flexure, if it doesnt work i will ditch it) my scope setup is overweight but the mount handles it, the smaller scope would handle better.

i will watch to see what happens , and let you guys iron out all the bugs, if it turns out as well as potentially it could do then it is cheaper than a decent refractor.

still would like to see a pic of the camera mounted at the back to get an indication of how far back it really is, personally if its way back i would use extenders rather than wind the focuser way out.


watching........ , would like to see a low contast target thats faintish just to see what you can do


thanks for replying

clive

[Paul Haese]

Clive I don't have an image but I can tell you it is about 10 inches to the from the focal plane of the camera to the back of the scope.

[Alchemy (Clive)]

ok thats scary.

hmmm if i had it on a plate like mine i would have an extra support made basically a 50mm ring that you could lock down over the extenders just in front of the camera and brace onto the plate.

i wouldnt be game to rely soley on a focuser particularly the gso i have a qhy8 on an mpcc with 39 or so mm of spacers and the focuser is out 15mm ish and its barely up to it.

[gregbradley]

Apparently GSO make a flattener for this OTA, well at least that is what I read on CN and even on Astronomics but so far I have not found it.

Thanks so much for the feed back Greg, much appreciated.[/QUOTE]

Also keep in mind Paul that an RC has no coma. So all the flattener will do it reduce star sizes off axis. A good one will also correct for other aberrations.

But I had a 12.5inch RC for a few years and never used the flattener. My images and those of others with 12.5 inch RC using a flattener looked awfully similar. So the gain is not necessarily massive but probably worth it if not too costly.

Tak often uses 106.2mm and there is often a tolerance of a few mm either side of that figure. I think they also use 86mm as well.

Looking at the image it would seem to be more tracking error although miscollimation is a possibility also. Did you use a Tak collimating scope to get collimation? Another hint collimation may be out is you can see a little black dot in the centre of some stars which is the secondary mirror showing through or perhaps focus was slightly off??

Greg.

[Paul Haese]

I just checked the collimation with CCD inspector on one of the images of 4945 Greg and it reckons that the collimation was around 1.6" out. We used live view to collimate the scope. So I might just use CCD inspector to do the collimation next time I am down at the house and using the scope.

So maybe I will take the flattener off when I get the new focusor and rings. Then do some more images and see what transpires.

[gregbradley]

So maybe I will take the flattener off when I get the new focusor and rings. Then do some more images and see what transpires.[/QUOTE]

Did you take an image with and without the flattener? I suppose that would tell how much it affects the image.

I never used a flattener with mine but as I mentioned comparing my images with others with same scope but a flattener seemed very similar so the gain was minor.

Greg.

[gregbradley]

[QUOTE=Peter Ward;440509]I've got a RCOS flattner. Seems to fit just fine.

Cerevolo (a mate of mine) does make them. Not mass produced...Each are hand figured....but probably didn't have a clue (not!)

Packaging is important...goods need to arrive in one piece.



Sorry didn't mean to tread on your dealers toes. But I did find RCOS very unpleasant to deal with. They also had to be chased and chased to even sell the item, like they didn't care or incompetent staff. Then they sent me the wrong one and then they dropped the whole thing after I sent them the correct spacings for the scope back. Not good for a US$1500 item. I am glad your flattener fits. I am sure Peter Ceravolo is very good at his work and his astrograph seems awesome.

Greg.

[TrevorW]

Hey guys see me post under equipment it shows the back focus length with a Canon 350d attached and I sorted the balance issue all I need to do now is get the collimation exact.

PS: the focuser worked fine with the Canon no slippage once locked down although I'll still probably update it to a Moonlite who are currently working on one too fit the GSO RC's and should have available in 4 weeks by the way.

[Peter Ward]

Originally Posted by gregbradley

Sorry didn't mean to tread on your dealers toes.....
Greg.

Nothing to do with it Greg. My "other" day job pays well enough for me not to have the slightest worry about dealerships

It simply read to me like you were phoo-phooing the rig I personally use and have had a good of success with for some years....guess we'll leave it at that

That said the GSO looks very interesting.

Field flattners (IMHO) are not a "near-enough-good enough" affair.

They generally need to be within +/- 1mm of the design spec, or you simply make matters worse.

[Paul Haese]

So Peter, can you tell me more about the RCOS flattener? I know it is expensive, but how far does it need to in terms of metal back distance?

[Peter Ward]

Originally Posted by Paul Haese

So Peter, can you tell me more about the RCOS flattener? I know it is expensive, but how far does it need to in terms of metal back distance?

I wish I could give you a good answer to that Paul, in my case it's with a with a secondary spacing to give a BFD of around 9".

There is a great write up here:
http://www.astrosurf.com/antilhue/rcosff.htm

RC's are optimized for zero spherical error at only one secondary/primary spacing...yet to focus an RCOS you need to move the secondary (!)

This is not the case with the GSO (should be there already) plus the FL is vastly different to a 12/14/16 RCOS, so I suspect the RCOS FF will be an (expensive) dead end for GSO users.

William Optics now make an adaptable FF
http://www.williamoptics.com/accesso...4_features.php

This I would expect to be very promising for GSO users!

[strongmanmike (Michael)]

Originally Posted by Paul Haese

Clive here is a 1:1 crop of the area.

As you can see the stars look triangular and are a bit blobby with flares. The triangular comes from not guiding with the EM400 and the blobby and slightly flaring is from the focusor.

It is not pretty but once these issues are sorted I am sure it will produce fine images.

All taken with a cooled 40D

First of all, that's an excellent first light image with your new budget RC .

You may be right regarding the possible explanations for the triangular stars but it could also be astigmatism or pinched optics too triangular stars are hallmarks of astigmatism. Try racking the focus in and out slightly and watch the star image, do the star images look slightly oval (or linear even) inside focus and then shift to be slightly oval at 90deg to the first orientation outside focus? If so you have astigmatism. take a shot very slightly inside focus then another very slightly outside focus, if you have some astigmatism you probably show star images that are little lines oriented one way inside focus and then oriented 90deg to these when outside focus.

Once you have taken more shots you will have a better idea I am sure.

Very nice work Paul

Mike

[Paul Haese]

Thanks Peter I will chase that up.

Mike definitely not pinched optics. If you take a look at the other images I posted in the equipment section the stars are not triangular. Nothing to worry about there.

I checked for astigmatism during my first star test and all diffraction rings were smooth and concentric at that stage. I upset the collimation when I put the new dove tail on and I had to readjust collimation The mirror cannot be pinched because it is glued into position. Appreciate your advice, but this is not the case here. Just crummy guide errors mate. No guiding and being slack on the PA when I first did it.

[strongmanmike (Michael)]

Great, glad about that!

Mike