GSO RC testing review

Paul Haese · #1 03-05-2009, 05:18 PM

Ok here is my initial assessment of this OTA. The images were taken with a Takahashi flattener (which is most likely not the correct piece of equipment to use with this optical system) and the images were obtained via unguided exposures. My polar alignment needs looking at, as I did my initial alignment with my TSA 102 which is half the focal length of this telescope. Tracking errors were showing up very quickly at a focal length of 1625mm.

Most of the images taken were at 180- 240 seconds duration. Stacks of 10 or more made. The individual images are not much better than the finished images. I have not done any treatment to the stars at all, however some bloating has occurred during processing.

Overall I think this scope will be a goer once I get guiding, flattener and focusor sorted. The figure of each mirror does seem to be well done given the initial results.

Click here for Review and images.

Discussion and comments welcome.

Quark (Trevor) · #2 03-05-2009, 05:31 PM

Hi Paul,
A very thorough and most informative initial review.
This scope sounds very promising and I look forward to following your progress with it.

Cheers
Trevor

dpastern (Dave Pastern) · #3 03-05-2009, 05:50 PM

Thanks for the review and images, much appreciated.

Dave

Dennis · #4 03-05-2009, 06:36 PM

Hi Paul

Some very tidy images given that you are still at the beginning of the learning curve with this optical tube – the results do look very promising indeed. Nice write up too; you’re a brave man to jump in with both feet but it looks like your adventurous streak has been rewarded – well done.

Cheers

Dennis

Paul Haese · #5 03-05-2009, 06:51 PM

Just sharing the love Dennis.

I am expecting these OTA's to be very popular in the coming years. Despite some minor defects with focusor rigidity and the need for a flattener, I hold high hopes.

Cheaper equipment is better for all of us who are not millionaires.

multiweb (Marc) · #6 03-05-2009, 07:06 PM

The Eta shot sharpness is remarkable. As you say for a $2.5k scope or so it's bang for the buck. Thanks for sharing.

Paddy (Patrick) · #7 04-05-2009, 04:09 PM

Thanks for the review Paul, I look forward to some more images and thoughts.

Moon (James) · #8 04-05-2009, 04:45 PM

Paul,

Thanks for keeping us informed - once you sort out the focuser, it would be great to see how all the various flatteners stack up.

The astronomics web site says "an optional reducer/field flattener is available"

http://www.astronomics.com/main/prod...oduct_id/AT8RC

I wonder which one they are referring to?

Paul Haese · #9 04-05-2009, 05:12 PM

James that is essentially my main question at the moment. I cannot seem to find a specific flattener on their site for that very telescope.

I am going to contact them just to find out.

leinad (Dan) · #10 05-05-2009, 09:03 AM

For the interested of those GSO RC owners and others..
The EQMOD forums had some images taken with this scope by Leonardo(I think his name was).

http://stelledelcielo.blogspot.com/2...ggio-2009.html

dpastern (Dave Pastern) · #11 05-05-2009, 10:23 AM

It looks promising, but I'm still worried about the star elongation - you can see that on these shots:

http://3.bp.blogspot.com/_gXAjcr03u5...0-h/M13Web.jpg

http://3.bp.blogspot.com/_gXAjcr03u5...0-h/M82web.jpg

http://2.bp.blogspot.com/_gXAjcr03u5...b_filtered.jpg

that there is star elongation on the left, but not towards the right hand side of the frame. Or, on some of the stars, the elongation is in a different direction. Is this typical of field curvature? I would have thought the elongation would be in the same direction, or am I (probably) wrong?

I'm really curious to see these units with the focal reducer/field flattener in play and a solid focuser without slop. Oh, and properly collimated. I don't think the scopes I've seen have been perfectly collimated yet to be honest, although most have been very close I do admit.

Dave

Paul Haese · #12 05-05-2009, 10:48 AM

Actually Dave, there are most likely a variety of errors at play in these shots. Guiding is evident in 2 shots (either poor guide parameters or differential flexure), focusor slop is another error in at least one other shot (that is the most likely cause of the elongation on one side).

If the mirrors were not well figured you would see blurry detail on the targets, these images look pretty sharp.

A flattener will only flattener the elongated stars that radiate from the centre of the image near the edge of the field. If the camera is not square to the optical line it will produce elongation on side of the image or at worst it can cause elongation on one side and out of focus stars on the other. That is what I have on my eta image. Yet when 4945 was shot it was directly over head and the camera was hanging almost vertical. Its all a matter of reasoning it out.

Thanks for the links and your thoughts.

dpastern (Dave Pastern) · #13 05-05-2009, 11:02 AM

Ah excellent! So, let's hope that once collimation, camera/focuser slop and tracking are all addressed, these scopes will be making awesome images.

Looks like my fears of optical quality were unfounded, and that's good news!

The focuser set up looks a real worry though.

Dave

edit: thanks for the explanations - much appreciated. I'm still a newbie to all of this.

rat156 · #14 05-05-2009, 12:16 PM

G'Day Paul,

Looks like these scope are going to be pretty good for imaging. I must say I admire your bravery in buying one of these, being first cab off the rank can be a bad experience.

The imaging issues look to be all in the focusser, looks like there is tilt in the camera plane, I reckon the Moonlite will fix the issue. I have a spare one for an SCT (motorised) if you need to borrow one for a while. What are the threads on the back of the OTA?

What mount do you have it on?

I still think that the Meade 8" ACF is a worthy contender at $2699 from Bintel (OTA). It's a bit longer in focal length, but already has a flat field, so the AP focal reducers work straight out of the box and don't cost the Earth. Anything of much less focal length (sub 1000mm) I reckon a quality APO is available and would be better.

Noting that the scope is for imaging only, the Meade represents good value as it's a nice visual scope as well, perhaps not as good as a big APO, but significantly cheaper.

Of course a top quality mount should be the #1 priority for imagers, but for those of us that like to look at stuff as well, nice optics make a difference. That's why I'm still battling with the Meade fork mount on the RCX, which BTW at f/8 native makes a top imaging scope and very nice visually (still for sale as I want Theo's bigger one!). Shame they're not making them anymore.

I'll be very interested to see the progress you make with this scope, the 10" or 12" might make it onto the shopping list in the future.

Cheers
Stuart

Paul Haese · #15 05-05-2009, 05:48 PM

Stuart,

the focusor on this would not screw onto an SCT or vice versa. The thread is 90x1mm pitch.

The scope is on the an EM400 mount, so I got the good mount covered. I just need to get pempro onto the polar alignment which was done with my wide field scope. I did not have any guiding because I am waiting on guide rings for the vixen plate.

Forgive me, but the ACF is not an alternative for several reasons. Corrector plates don't make for long imaging runs (dew builds up and using a dew heater cancels out the chance of ultra sharp images, which is afterall the object of an RC scope; an ACF is not an RC), cool down time is longer in a closed tube and often cannot be controlled without active measures being taken and both tubes require better focusors to maintain optical alignment.

Besides I have never been a real fan of fork mounts. When I owned my LX200 I found that flexure was a major problem and using those wedges was just a total pain for precision polar alignment.

I think these RC scopes have the potential for great images. The scopes work straight of the box but need refining like any telescope one buys.

Alchemy (Clive) · #16 05-05-2009, 06:25 PM

Quote:

Originally Posted by Paul Haese

Stuart,

Forgive me, Corrector plates don't make for long imaging runs (dew builds up and using a dew heater cancels out the chance of ultra sharp images, which is afterall the object of an RC scope; cool down time is longer in a closed tube and often cannot be controlled without active measures being taken .I think these RC scopes have the potential for great images. .

cropped the quote a little bit,

so just what do you intend to use to control dew both on the primary and the secondary, particularly if it cools down quicker, and then if you use dew heaters isnt the debate spurious.

Paul Haese · #17 05-05-2009, 06:43 PM

Clive,

that is not really the issue. My point was this thread is a discussion on the RC, not on an alternative. The other night the dew was very strong but I did not find any dew forming on the optics of the telescope. In fact the only telescope that get dew on the optics is my C14. This is at a site which dews up at the fastest possible rate.

However, even if I were to use a dew heater, it would have not even the slightest effect that dew heaters have on corrector plates or maintaining heat in the tube.

marki · #18 05-05-2009, 09:35 PM

Bit off topic I know but I use an astrozap flex dewshield on my LX200R and find I only ever need to use heating when the thing is pointing straight up on a soggy night. Perhaps Perth air is drier

. Back on topic, I am going to turn an adaptor for TrevorW so we can trial my EV1 on his OTA. Do you or anyone know what the thread depth is as I have not physically sighted the tube yet and none of my charts have this thread on them. I know its 90 x 1.0 but depth?

Mark

rat156 · #19 05-05-2009, 09:51 PM

Quote:

Originally Posted by Paul Haese

Stuart,

the focusor on this would not screw onto an SCT or vice versa. The thread is 90x1mm pitch.

The scope is on the an EM400 mount, so I got the good mount covered. I just need to get pempro onto the polar alignment which was done with my wide field scope. I did not have any guiding because I am waiting on guide rings for the vixen plate.

Forgive me, but the ACF is not an alternative for several reasons. Corrector plates don't make for long imaging runs (dew builds up and using a dew heater cancels out the chance of ultra sharp images, which is afterall the object of an RC scope; an ACF is not an RC), cool down time is longer in a closed tube and often cannot be controlled without active measures being taken and both tubes require better focusors to maintain optical alignment.

Besides I have never been a real fan of fork mounts. When I owned my LX200 I found that flexure was a major problem and using those wedges was just a total pain for precision polar alignment.

I think these RC scopes have the potential for great images. The scopes work straight of the box but need refining like any telescope one buys.

90mm is pretty close to the rear thread on the OTA of mine, when you take off the SCT threaded bit, I'd ask Astro-physics if they're planning on making an adapter to the AP2.7" threaded stuff, that way you'll definitely get a nice focusser.

A decent dewshield fixes the dewing problems on a small SCT, similar to the RC actually, you've got a dewshield essentially the length of the main tube, should work pretty well. I don't understand the comment about the corrector temperature and image sharpness, can you explain further, off-line if you want.

I don't have too many problems with cooldown, the thing is permanently bolted to the pier in the observatory. I'll turn the fans on before I observe or image to help with the day/night transition, but it's not as bad as taking from a warm house to a cold outside. Watch out for that big primary mirror you've got there, it's got a large thermal mass and will also take quite a while to cool down properly. If you can clean the mirror, I'd be installing fans to help things along.

Yeah, fork mounts aren't the best for imaging, but that's why I was comparing the scope to the OTA versions of the ACF scopes. I'll get a decent GEM one day...

Cheers
Stuart

Paul Haese · #20 19-05-2009, 11:07 AM

Well I am now in posession of both the new guide rings for the Vixen bar on the top of the OTA and the feather touchj focusor with the adapter.

Just a note on the Feather Touch. The construction of these focusors is superb, everything just fits like a glove and is very smooth too. The way the rollers and the guide bar are designed ensures that there is no flop at all. The brake seems very nicely made an would appear to hold a lot of weight. Easily as good as the Moonlite if not better. Lovely piece of engineering.

So this week I will be trying for some more images with the RC. I will no doubt need to collimate the instrument again once I fit the focusor. So with any luck I will have something to report on.