Those dreadful RC light blooms

[pmrid]

I thought others might be interested to see just how bad the internal reflections are in the GSO RCs from Andrews. These are images taken tonight of NGC4945 and 4976. They are 5 minute subs (5 in 4945 and 2 in 4976) at iso800 in a Canon 1000D with a master flat applied. The stacked elliptical bands are bad enough but note also the bright rays like sunlight through cloud, coming in from the mid left side.
I would be very interested to hear from other RC owners experiencing similar problems. It seems to me that these scopes, marketed as the imager's dream, are in fact a shocker; unfot for the purpose for which they were sold.
Peter

[leon]

Peter, that is just bloody awful, :mad2: I expect that these models are equally as bad from any distributer, :shrug: however that is just not good enough.

Leon :thumbsup:

[TheDecepticon]

A guy in our Deep Space Imaging group-a special interest group in our society-purchased one of these with great delight. It went back to Andrews in the same week for a full refund. The reason it went back is exactly this fault. Andrews sent the scope up to his repairer-cant remember his name-and he was quite surprised with the problem and thought it was a serious one, however, we have heard no more. It seems if you get a good one-great!! I did see some where else on this site that some one did mention what they thought was a fix for them-focuser flocking of something??

[Mike21]

I only know an RC to be radio control. What is an RC in astrophotography?

[Octane]

Peter,

That's just plain horrible.

What's with the curvature, too? I thought RCs were supposed to offer flat fields?

I guess you get what you pay for!

Regards,
Humayun

[Octane]

Mike,

Ritchey-Chretien. It's a specialised mirror/telescope design.

Very expensive to manufacture.

Regards,
Humayun

[MrB]

Ritchey-Chrétien
http://en.wikipedia.org/wiki/Ritchey-Chr%C3%A9tien_telescope

[DavidU]

It seems the optics are a bargin. Paul Haese (spelling) has nailed an easy fix, he is an IIS member but I can't find his post on fixing the problem. You get what you pay for however if it is an easy fix where else can you get an RC with CF tube and diaelectric coated primary for what you paid.
Pete Ward could probably explain more succinctly.

[Mike21]

I have seen these scopes on the Andrews site and thought they'd be the ducks guts. Not too sure that you do get what you pay for anymore, I'm not restricting that comment to astronomical purchases (pun or not). It seems too often to be the case that you pay what they can get out of you. No disrespect to Andrews, they've always done right by me.

[marki]

Check out the ATS website. Peter is selling them with the revised baffle tube (the cause of the problem in the original) and at a cheaper price then Andrews. All of the earlier models have displayed this fault so far but only in certain situations. There have been a number of posts about this very problem. I have seen some cracking images from the old version and Paul Haese posted a link to how he fixed the problem by flocking the baffle on his website. It is a comprehensive step by step guide. Peter Ward took a nice pic with the new version which does not seem to suffer from the reflection problem.

http://www.iceinspace.com.au/forum/showthread.php?t=48028&page=2

http://www.atscope.com/

http://paulhaese.net/gsobaffleflocking.html

Mark

[Gama]

RC's have curvature. You need a flattner to get rid of curvature.

Theo.

[Tandum]

Send it back for a refund and make a profit :)

How did those 240v network adapters go ... did it work?

[Exfso]

Paul Haese bought one of these had similar problems, he fixed it with flocking. Now fine and beaut. I daresay he will respond to this in due course.

[Alchemy]

pretty much all of the early ones had the same problem, wait until they come out consistently producing good results.
better still buy a good refractor, far less problems...
ive used most styles of scopes. refractors win hands down for me. No mirrors needing recoating, no internal dew, no secondary to block light, no star spikes..... just expensive, but a pleasure to use.

[gregbradley]

Nasty reflection. Seems to me you have 2 choices:

1. Ask for a refund and try ATS as I doubt Peter would sell gear that did that.

2. Go the route of fixing up the manufacturer's product with flocking and a new focuser.

Perhaps the focuser is better also on the later model as Peter says the focsuer was solid yet Paul's one he replaced the focuser witha Feathertouch one as it had too much flex.

I know from experience how annoying it is to do the manufacturers job for them so I'd recommend 1) above. There's no guarantee mucking around with fixes will solve it and it willl probably void your warranty or ability to refund if you start modifying it.

Most modern scopes use a series of baffles to redcue reflections.
They also coat interior surfaces with a matt black paint that is absorbing.

Greg.

[rider]

RCOS, and similar high end RC's keep field curvature to a minimum by adjusting the focus with both the secondary mirror position and the back focus. (The secondary mirror on RCOS scopes moves to change focus.)

Generally the RC design has a field that remains flat(ish) when the relationship between the primary to secondary remains proportional to the secondary to back focus position.

While I have not seen the GSO in the flesh, all focussing seems to be at the back plate. This does not matter much for visual observing, but may be the reason why the image showed curvature.

[pmrid]

Hi Rob and thanks again for your thoughtfulness:
In answer to your question, one did but the other was a dud. Something in the power train. No sparkee, no lightee!
So I have the age-old dilemma of deciding what I can do with a single powerline ethernet adapter. It is like the philosophical question "what is the sound of one hand clapping?"
Peter

[pmrid]

Thanks everyone. I've read Paul's instructions on flocking now and am tempted. If Andrews care enough about their reputation, they'll take it back or at least arrange a replacement baffle. I'm going down there in a few weeks so we'll see. Failing that, it's flocking. Anyone klnow of a source for flocking material?
Peter

[renormalised]

Got a sandpit with coarse sand?? Some matt black paint and glue??

[Peter Ward]

If you think a Cassegrain or Newt will not have the odd off-axis stray reflection, sorry... no such telescope exists...

e.g. this was taken with a $40K RCOS...note the rays from a pair of bright nearby stars top and left of field.

http://www.atscope.com.au/BRO/gallery10.html

GSO have told me their latest RC's have a redesigned baffle tube. As posted elsewhere, I took this test image a short while ago.

http://www.atscope.com.au/BRO/images/GSO/gsom8.html

If you look very closely there is a dim off-axis reflection in that image.

The scope did need colimation and apart from a mild spherical error, had no other vices that I could see.

The focuser handled an SBIG STL11k with no problems.

A bright star, just off axis, may still throw some scattered light onto the focal plane. Sorry, that's the way most telescopes work.


As for the RC design, they have no off-axis coma. This is very different to field curvature which they do indeed have.
That said it is very low, and would be hard to pick with an APS sized chip.

[pmrid]

Hello Peter. Granted, but surely the pattern exhibited in the images attached to my initial post are well beyond anything that ought to be considered 'normal', 'acceptable', 'within tolerances'. Would you consider a scope you had bought to be accetable if it displayed those features?
Peter

[Peter Ward]

Depends.

If the 'scope always showed said reflections, I would not be happy.

If it only happened with a bright star/object just off axis, then I have to say that is is nature of the (in this case, inexpensive) beast and look at a workaround....eg flocking to make it better.

[gregbradley]

Yeah I bought self adhesive backed flocking from Scopestuff. I think its scopestuuff.com

Its not very expensive and fairly easy to use. I flocked the interior of my RCOS 12.5 inch and it improved contrast a bit.

Here's the link:

http://www.scopestuff.com/ss_flok.htm

I also made a little ring out of black thin cardboard you can get from a newsagent to block the last 5mm of the primary mirro as
sometimes the primary mirror has a turned down edge right at the edge that causes unwanted effects although I don't know that it causes
large round reflections like that more extra haloes around stars.

Greg.

[Wavytone]

The images I've seen suggest these blooms are light from a bright star just outside the field of view being reflected into the focal plane, but there are several candidate sources of the reflection. In particular I suspect it comes from something with a machined surface.

1. The central baffle - both outside and inside surfaces.

2. A machined edge around the perimeter of the secondary mirror cell, with the light reflected in much the same way as glancing incident light will be reflected off the sides of those lovely "handgrenade" eyepieces from Meade, Vixen and others.

3. Circular baffles inside the OTA - assuming there are some.

4. Could even be coming from a reflecton off the inside of the last bits of tubing coupling the camera to the scope, if any of them has a machined surface - even if its black anodised its still shiny enough.

I think the vanes supporting the secondary could be ruled out.

[mick pinner]

l assume this is an early version of the GSO RC's.

[AlexN]

Mick, To my knowledge, this was the first GSO RC sold in australia... (according to the only person who stocked them at the time of purchase... As I said to you via PM Peter, Andrews will likely replace you baffle tube.. If not, flocking it yourself will be the go...

You could also try Astronomics in USA, they might be able to sell you a new baffle tube, ATS may be able to do it for you too.. All in all, these scopes are great despite a few shortcommings.. given the money, I would buy one again tomorrow..

[TrevorW]

I've asked Andrews to replace my baffle and confirm that the scope he sold me was an F/8 as advertised

These reflections did not appear in every image I've taken and were most apparent in an image I took off M83 but not apparent in anything else except maybe the recent shot I took of M30 which I'm still not convinced was an internal reflection caused by the scope.

As mentioned internal reflections as displayed are quite common in SCT's and the like.

The replacement of the focuser is a personal choice if your not happy with the rigidity of the stock one.

As far as I'm concerned they are a good scope for the price and will produce the results

[Paul Haese]

Hi there,

ok first and foremost. It is likely you have a scope from the second batch and not the third batch. I found that on bright nebula the reflections were not present, however on this galaxy and M83 it was present.

The new baffles will not fit your scope so flocking is the only solution. The optics are fine you just need to make a few mods to get the most out of the scope. I only found two galaxies that this issue arose. I have seen plenty of other galaxies imaged with this scope and no reflections.

RC are coma corrected not flat fielded, use of a flattener is mandatory if you want to correct for field curvature.

For the price, this scope is a ball tearer, it just has some spots that is all.

[multiweb]

This made a big difference for me: http://www.fpi-protostar.com/flock.htm

[pmrid]

Thanks. That's helpful. It is a hunting list, at least.
I have identified the prime candidate for the just-out-of-image reflections. See attached image.
Peter

[pmrid]

Thanks to all helpful thoughts and ideas. Lots to ponder. Going overseas tomorrow for a bit but will tackle when I get back.
Peter