Redcat 51 First Light tilt issue?

[XavierL]

Hi all,

 

I received my RedCat 51 a few weeks ago (from FLO) and had first light this weekend. After initial post-processing it seems there might be an issue with the scope (possibly the tilt?). 

Here are one of the RAW files, the initial post-processed stack and a few measurements using CCDInspector:

Single exposure RAW:

Processed stack:

Curvature analysis:

3D Map:

What do you guys think? Should I contact FLO? Return it? Attempt adjusting the tilt myself?

 

Thanks in advance.

Xav''

 

My target was the North America Nebula (NGC7000)

Equipment used: Redcat 51, Optolong L-Pro 2", M48-to-F-Mount ring adapter, Nikon D5300

Settings: 180sec subs, ISO 1000, 50 lights, 17 darks, 16 flats, 16 bias

[david_taurus83]

Has the camera been modified? 

[XavierL]

1 minute ago, david_taurus83 said:

Has the camera been modified? 

No, stock DSLR. I should also mention: I don't see this issue with any of my regular photo lenses.

Edited June 7, 2021 by XavierL

[david_taurus83]

Hmm are you sure focus was bang on? Looks a little soft to me. I'm not liking those star shapes either. Looks pinched..

[XavierL]

I agree, the focus might have slipped a bit (I did use the included Bahtinov mask). However the stars at the edges look elongated in all sorts of different ways. Could it be the result of bad focusing or could there really be an issue?

[david_taurus83]

I had a quad before (Altair 70EDQR) and it was very good, even with my 6D. But you absolutely have to nail focus down as it also sets the correct distance from the internal corrector to the camera sensor. You might find better focus will improve things a lot. Try again but on a patch of stars with no nebula. CCD Inspector works better if its just stars. 

[XavierL]

Thanks @david_taurus83! I will try again, hopefully tonight (the forecast looks promising ATM🤞) and report back.

[david_taurus83]

12 minutes ago, XavierL said:

Thanks @david_taurus83! I will try again, hopefully tonight (the forecast looks promising ATM🤞) and report back.

Hopefully it's better, I've just ordered one myself!

[XavierL]

Hi all,

 

I had another go last night at the same target. I made sure the focus was bang on and here are the results (RAW file below). It looks a little bit better but I still see elongated stars and the corners while the center stars are round. The analysis in CCDInspector seems to confirm that.

What do you guys think? Should I send it back?

[kirkster501]

I was also going to suggest the focus.  I have borrowed one from a very kind forum member and I notice focus is critical and not so easy to eyeball.  If I buy one I will definitely motor focus it.

[XavierL]

35 minutes ago, kirkster501 said:

I was also going to suggest the focus.  I have borrowed one from a very kind forum member and I notice focus is critical and not so easy to eyeball.  If I buy one I will definitely motor focus it.

I used the included Bahtinov mask and made sure the diffraction spike was bang on in the middle so we can probably rule out focus issues

[david_taurus83]

If that was a full frame sensor then I'd expect it to not be perfect in the corners but an APS-C should be better. Are you absolutely sure there is no play in how the camera is mated to the scope? Does is have a rotator? I have found the rotator on my Altair 0.8 reducer induces tilt if the grub screws around the outer side of it are not exactly the same depth. Slightly off topic, I know but until I get my hands on my Redcat I don't know the mechanism works. At this stage I'd at least let the vendor know you are experiencing issues. They might recommend trying to utilise the tilt adapter to see if things can be improved. Are you sure the push pull screws on the tilt adapter are all slightly loose and there's not one tighter than the others?

Edited June 8, 2021 by david_taurus83

[XavierL]

No play but the standard M48-to-F-Mount adapter is not the best quality... So maybe it's not perfectly parallel to the back of the scope. I wonder if I should have gone with the high precision version instead.

It does have an integrated field rotator, which I used to frame the target. I would hope the rotator can be used in the field without requiring a tilt adjustment afterwards. Otherwise, that makes its pretty much unusable. 🤨

I'll contact FLO and WO directly and see what they think.

Thanks

Edited June 8, 2021 by XavierL

[Adam J]

18 hours ago, XavierL said:

Hi all,

 

I received my RedCat 51 a few weeks ago (from FLO) and had first light this weekend. After initial post-processing it seems there might be an issue with the scope (possibly the tilt?). 

Here are one of the RAW files, the initial post-processed stack and a few measurements using CCDInspector:

Single exposure RAW:

Processed stack:

Curvature analysis:

 

3D Map:

 

What do you guys think? Should I contact FLO? Return it? Attempt adjusting the tilt myself?

 

Thanks in advance.

Xav''

 

My target was the North America Nebula (NGC7000)

Equipment used: Redcat 51, Optolong L-Pro 2", M48-to-F-Mount ring adapter, Nikon D5300

Settings: 180sec subs, ISO 1000, 50 lights, 17 darks, 16 flats, 16 bias

looks to me that the focus is poor across the entire image and so its difficult to make an assessment of tilt or colimation. Really you need to repeat the test with better focus. 

[Adam J]

2 hours ago, XavierL said:

Hi all,

 

I had another go last night at the same target. I made sure the focus was bang on and here are the results (RAW file below). It looks a little bit better but I still see elongated stars and the corners while the center stars are round. The analysis in CCDInspector seems to confirm that.

What do you guys think? Should I send it back?

 

 

 

This looks more like a colimation issue to me but the only way is to rotate the camera and see of the worst corner moves or stays orientated to the sensor. 

[PadrePeace]

38 minutes ago, Adam J said:

This looks more like a colimation issue to me but the only way is to rotate the camera and see of the worst corner moves or stays orientated to the sensor. 

Try reducing your backfocus slightly. If you can focus using the HFR in your capture software that might give you a sharper image to work with. At F4.5 your focus window will be small so do expect that when focusing. Bat masks are not the best tool with fast scopes in this range in my experience. Go for the smallest/lowest HFR reading you can get. 
It’s not too far away given some. Good luck.  

[XavierL]

3 minutes ago, PadrePeace said:

Try reducing your backfocus slightly. If you can focus using the HFR in your capture software that might give you a sharper image to work with. At F4.5 your focus window will be small so do expect that when focusing. Bat masks are not the best tool with fast scopes in this range in my experience. Go for the smallest/lowest HFR reading you can get. 
It’s not too far away given some. Good luck. 

Thanks! I'll try that next time 🙂

[david_taurus83]

59 minutes ago, PadrePeace said:

Try reducing your backfocus slightly.

I don't think you can do that on these. The internal corrector is fixed so the correct distance is when you reach focus.

[PadrePeace]

Of course, it’s a DSLR isn’t it. 

[PadrePeace]

My personal experience with DSLRs and T adaptors is that they never connect to the camera as well as a lens does so you’ll always have some droop or misalignment in the rig. I tried all sorts to sort out my DSLR/WO71 stars issues and in the end I used strong rubber bands to pull my canon up tight to the T adapter to stop it slopping about. 

Edited June 8, 2021 by PadrePeace

[geeklee]

2 hours ago, david_taurus83 said:

I don't think you can do that on these. The internal corrector is fixed so the correct distance is when you reach focus.

This is correct.  The only limit you have is not having too long a train between the rear of the scope and the sensor.  Too much and you can't bring the focus in enough - it's still a generous distance though.  You can bring the camera in close and the focus should compensate by having to move it outwards more.  A DSLR with a standard setup would be the "typical" distance by default.

This is why filters shouldn't make a difference either (as long as your train isn't too long!)

I've typically aimed for "traditional" spacing and get the infinity mark (I think it is) close to the focus marker but I've been either of side of this with no difference seen (I use small sensors though).

There's some good info (and diagram) on this site highlighting the flexibility.  Number 4.  

Edited June 8, 2021 by geeklee

[XavierL]

Thanks for your help everyone! Really appreciated. 👍👍

I have contacted FLO. Like @Adam J, they think it is a mis-collimation issue. Since RedCats are currently out of stock everywhere, they can't get me a replacement but offered to get it collimated again with their specialist. I think that's a fair alternative. I will keep you posted after the repair (don't know how long it will take).

 

Thanks again.

[XavierL]

Hi everyone,

I heard back a week ago from FLO. They sent to scope for collimation/calibration to Es Reed. Unfortunately, after inspection, it turned out to be an uncorrectable astigmatism (probably from the factory) so FLO sent me a new replacement unit that I received last week. I haven't tested the new scope yet (clouds...) but I will try to update this post with a new shot of the North America Nebula for comparison as it might be useful to others in a similar situation.

[XavierL]

I did manage to get first light on the new scope tonight and shot the same target for comparison.

I quickly processed a stack of 10x2min exposures and did the same thing with the old scope in order to compare. The new scope looks a lot better across the field of view but stars still aren't round in the corners. Note that the noise in the new version is higher than in the old version since I was in a more light polluted area tonight, so please ignore that when looking at the images.

This is the new scope:

 

The same image with the old scope:

 

Center/Corner details on the new scope:

 

Center/Corner details on the old scope:

 

What do you think? Do you think the new RedCat is good enough or should it also be adjusted?

[licho52]

IMO that's probably good enough for 50-60mm category, much better than the 1st.  If you're going to zoom corners on most telescopes you'll see some funny stuff.  The full picture looks good.