14 replies to this topic

[rludlamjr]

Hi, after imaging with refractors for a while, I decided to step up to a GSO RC6 for some extra reach. I’ve read a lot about these, and understand that they are finicky and mechanically problematic, and also quite demanding when it comes to seeing. But I do have a couple of questions I haven’t been able to find good answers to yet, hoping you can help!

Here’s my setup: cem26 mount, 8pos 1.25” efw, zwo 5v eaf, and asi1600mm cool. ASIAIR mini for control. So far I’m averaging ~.8-1.0” rms. I am guiding with a 60mm guide scope and asi120mm, and have an astrophysics ccd67 reducer which I haven’t setup yet (so far shooting unreduced).

1. After reading about issues with the stock focuser I ordered my scope with the slightly upgraded linear bearing option that is sturdier than the one that normally comes with the rc6. This seems to work ok, but I am struggling with attaching my imaging train as it’s just got two tiny thumbscrews on the 2” tube. There doesn’t seem to be any way to screw the imaging train on and I don’t have a good way to ensure my camera is straight. Any tips / tricks for this?

2. Even with good focus, I’m surprised at how large my stars are - in general I feel the images lack detail even compared to my very inexpensive refractor. The scope appears to be close to well collimated and so I’m guessing this is maybe a combination of backfocus and tilt? I read elsewhere that if the secondary isn’t positioned the right distance from the primary it can cause this too - is that correct? My scope plate solves at 1387mm vs the indicated 1370, so is this something I should try to correct? I’m scared to adjust. Maybe just seeing? I’m in a fairly dark area (bortle 2-3), but seeing has only been slightly above average. Disappointed that my images so far appear to have about the same detail as cropped images at 1/3 the fl from my refractor (which resolve much faster due to f-stop).


Thank you for your help! I love this hobby so far and I’m excited to learn more.

[unimatrix0]

Hello! 

 

I got an RC6, you can see my images here: 

https://www.cloudyni...19233-gso-rc-6/

 

The telescope is best to be used on mid to small galaxies or entry to planetary imaging.  If you ask me the 8" version is probably a better version, since the secondary mirror obstruction vs. aperture size ratio is better. But that's just a side note. 

 

 

1. Yes, it's an issue.  I usually manage to put the camera in with the 2" extension pieces and secure enough. Another option is that since I have no clue what focuser you got, mine can have the 2" eyepiece holder screwed off, and a M 54 to M48 adapter allows me to directly screw whatever 48mm extension piece I want to add, and it's a threaded connection. This requires figuring out if you still gonna be in focus or not. 

 

2. You are oversampled. The stars will take up a lot of pixels in your camera, it's because you are at 1370mm focal length and your camera sensor has small sensors. This is typical.  That's why reducer is recommended to be used. Here is an example of an image taken at this focal length and the stars are quite large. I have to use star reduction methods in post processing. 

Scope plate solves at 1387 is fine. Not all made equal, the 1370 is a ballpark number. 

You are shooting at F9 with native focal length, compare to a typical 80mm refractor with is F6, the F9 is a huge difference in light gathering capability you are several F stops down,  integration increases by multiple numbers, meaning 5hrs with that F6 refractor translates to like 10 hrs or more with the F9 RC.  Someone can probably correct me, I just gave a modest guess, It might be more.  You are gonna be imaging a lot slower with this scope, so whatever lengths of exposing you were doing with the refractor, you gonna have to lower your expectations and think that you just sat into a slower train and DSO's will require longer integrations. The secondary mirror obstruction also makes the images look softer. 

 

3. Use the reducer. For the best results, use between 55mm-75mm  (longer backfocus= stronger reduction) to your liking. I commonly use no more than 70mm, because the corner stars start to go out of shape with stronger reduction.  Also the stronger the reduction, the F stop drops, meaning the scope gets faster. so I'm usually around F6-F7 and that makes a difference in imaging time. 
If you gonna use the 2" eyepiece holder (thumb screws) you can shove the reducer up the tube to reach focus and have the compression ring a good firm hold on the 2" extension pieces. 

Here is an example with a reducer (1060mm)  (x.77 reduction)  Follow this guide to figure out how much backspacing you need to certain reductions on this illustration with the CCDT67

Edited by unimatrix0, 03 October 2023 - 08:44 AM.

[Robert7980]

Do you have an example image, really hard to guess about optical issues. 
 

It’s hard to say for sure, but your guiding is suspect, 1.0 is too high at 1400mm… That's not helping and probably isn’t the only issue. There are ways to tweak the guiding to get it better. 
 

If you have optical issues the CCDT67 will make it worse as it’s going to expand the feild…

 

If you don’t like the thumbscrews you might look into an SCA 2” noisepeice from Hoteck. 

[rludlamjr]

Unimatrix, thank you so much for the detailed reply - this is extremely helpful!  I think that the reducer is probably the key as I am definitely oversampled.  I will see if I can unscrew the nosepiece but so far I haven't been able to.  But using the reducer and pushing it farther into the nosepiece will also probably help quite a bit.

 

Robert7980 - I'll try to post an image later today, but I think between the oversampling and the guiding not really being good enough for the focal length will definitely be improved by the reducer as well.  

 

thanks again for this info - this helps confirm what I suspected and gives me some more to think about.  I will try to get the reducer into my imaging train tonight and see how that goes.

 

best,

roland

[rludlamjr]

Ok, here are some files (jpg) from the last couple of nights.  These are stacked images, 120s and 180s subs.  This is the second time I've used the scope, so please be gentle   These are Melotte-15 in the middle of the Heart Nebula.   I use APP for stacking.  The stars definitely show some egg-shape from the tracking accuracy of the CEM26 with this FL, and maybe some collimation issues as well.  I haven't adjusted collimation since I took delivery on the scope last week. 

 

First is an RGB composite using the hubble SHO palette.  Very green due to the lack of Oiii in the data (it's super dim, and unfortunately I was competing with the moonlight).  This is 2 nights of data, about 15 hours.  Cropped a little due to stacking registration issues and some post to correct color.  I need to remake my flats for the RC so please ignore the vignetting.
https://drive.google...?usp=drive_link

 

And here's the stack of just the HA channel which is the brightest and most successful.  This is about 4.5 hours of data, again 120 and 180s subs.  

https://drive.google...?usp=drive_link

 

 

Also, here's a link to the focuser I have installed on the RC6: https://agenaastro.c...rd-focuser-86mm

 

Thanks again!

[unimatrix0]

Unimatrix, thank you so much for the detailed reply - this is extremely helpful!  I think that the reducer is probably the key as I am definitely oversampled.  I will see if I can unscrew the nosepiece but so far I haven't been able to.  But using the reducer and pushing it farther into the nosepiece will also probably help quite a bit.

 

Robert7980 - I'll try to post an image later today, but I think between the oversampling and the guiding not really being good enough for the focal length will definitely be improved by the reducer as well.  

 

thanks again for this info - this helps confirm what I suspected and gives me some more to think about.  I will try to get the reducer into my imaging train tonight and see how that goes.

 

best,

roland

Sure, no problem!  Can you show us what focuser you are using? 

[rludlamjr]

Hi Unimatrix - here's a link to the focuser: https://agenaastro.c...rd-focuser-86mm

[arbit]

I have the stock GSO focuser (works fine for me). I choose the extension tubes so that the drawtube is out maybe 5-10mm, and also the focus tension knob is pretty tight. The load is 533MM, manual rotator, EFW, OAG. No EAF. But it holds focus very well, and sometimes I don't even have to refocus between sessions. of course, temperature changes in my location are hardly anything.

 

Unfortunately, can't thread on anything. So try to make sure that there is a machined flat surface which presses evenly against the focuser 2" adaptor, to keep it as aligned as possible. Eg, if there an EFW, the EFW should be flat against the adaptor. Or, use something like this https://agenaastro.c...ad-adapter.html (I have one in M48). The barrel goes inside the tube, pressed as flat as you can, maybe with scope vertical and gravity helping you. Then the 2 knobs are turned alternately and slowly till its tight as can be. Now just let this be and use the threaded end to attach the imaging train.

 

The stars will be bigger in terms of pixels. My 480mm refractor has a pixel scale of 1.6" or so, the RC6 0.55" or so. In pixels, the fwhm in RC6 is 4-4.5, in the 480mm, 2 or so. In arc seconds, the RC6 has smaller stars. YMMV.

 

The 1387 can of course be adjusted to 1370, but I wouldn't bother at this stage. First, who knows if the actual FL for your particular scope is really 1370 (each piece varies a bit, and a few % off is not uncommon),. Second, even if it is, the difference of 17mm should not be perceptible till other issues are ironed out. 

[unimatrix0]

Hi Unimatrix - here's a link to the focuser: https://agenaastro.c...rd-focuser-86mm

Yeah, you cannot unscrew that one.  I don't like those focusers. I went ahead and bought mine from China from aliexpress, because no one carries the rack and pinion one, unless you pay some extreme amount of money for a moonlight focuser. 

 

When it comes to high focal length, I like to use an EAF, and this one allows it to attach one, just like to a refractor. 

 

https://www.aliexpre...2765552057.html

 

 

 

or this one, more expensive, a little nicer, I think Robert7980 has this one: 

https://www.aliexpre...4223215199.html

Edited by unimatrix0, 04 October 2023 - 08:50 AM.

[arbit]

Yeah, you cannot unscrew that one. I don't like those focusers. I went ahead and bought mine from China from aliexpress, because no one carries the rack and pinion one, unless you pay some extreme amount of money for a moonlight focuser.

When it comes to high focal length, I like to use an EAF, and this one allows it to attach one, just like to a refractor.

https://www.aliexpre...2765552057.html




or this one, more expensive, a little nicer, I think Robert7980 has this one:

https://www.aliexpre...4223215199.html

Does the Hercules have a tilt adjustment?

Sent from my SM-S908E using Tapatalk

[rludlamjr]

Thanks for the tips on the focusers - I may need to upgrade to one of those in the future. In the meantime I’ll see about getting an adapter that has a solid flange I can push into the focus tube - that makes a ton of sense.

Last night I put the reducer in and wow, what a difference. I didn’t add the spacing to go to .67x, but the plate solve is showing 1100mm. Even with this small reduction the stars are much smaller and the overall detail is greatly improved. Thank you again for the tips.

[unimatrix0]

Does the Hercules have a tilt adjustment?

Sent from my SM-S908E using Tapatalk

No. I bought a tilt adjuster ring, but eventually stopped using it.  It's a $160 focuser, even cheaper than the junk ones they sell in USA (and yet better) .  Only 2" focuser movement. 

Edited by unimatrix0, 04 October 2023 - 01:48 PM.

[fewayne]

If you can get the guiding RMS down a little you might see a bit of improvement. Experimenting with balance, guide exposure time, and tweaking parameters is a fine way to while away an evening. I decided that an off-axis guider would probably help and it seems to have; with my AT8RC and CEM70, I'm getting 0.6" on a good night, 0.45" on a great night, and 1.0+ on a terrible one.

 

But IMO the chances of both DEC and RA being off enough to give big stars, but so close together that they're round, is not super-high. You're very likely up against seeing limits. You can run ASTAP or something similar to analyze tilt, if you're concerned about that, but again, if the stars are round, that's wasted effort.

 

I invested in SkyWave to ensure that I could quantitate on-axis coma and astigmatism, and hopefully distinguish that from the effects of tilt. It also analyzes for spherical aberration, which is what improper mirror spacing can yield. Having a solid, repeatable indication that the mirrors are spaced correctly and that the primary is properly aligned gives me a good basis on which to work on secondary alignment and tilt (I also picked up a tilt plate in an effort to get around the infamous GSO focus-tube-moves-with-the-primary problem).

 

For me it's definitely still a work in progress, but I've gotten some results that I like pretty well. If I get another scope, though, it's going to be a 130mm refractor. One labor-of-love telescope is enough, and I've pretty well established that our seeing here is not gonna give pinpoint stars much above 1000mm.

[rludlamjr]

Yeah Fewane, I think you're right - last night before the moon rose my guiding was much better and with the reducer on (still at 1100mm) things look much better.  I think I messed the collimation up a little (long story) but hopefully I can tweak that and fix it tonight.  Here's a 4.5 hour shot of the Cygnus Wall (1.5h each SHO).   With a Cem60 that refractor does sound like a great option - I don't think I could swing one on my 26 though!  

 

https://drive.google...?usp=drive_link

[fewayne]

Yeah, that's got it goin' on!