possible tilt problem Skywatcher 250PDS

andrea tasselli:
It is a problem with guiding, of which you don't say anything specific. Was it AOG or with a guide scope? It looks like either differential flexure or mirror flop.

Thanks for your reply! Sorry... My Guiding-Equipment is an ASI 120MM with a 240mm Guidescope. No OAG

Michael Wimmer:
Thanks for your reply! Sorry... My Guiding-Equipment is an ASI 120MM with a 240mm Guidescope. No OAG

So it is more likely than not mirror flop but other sorrces of differential flexure exist such as poor guider support, slack in the focuser (although this shouldn't really occur for just 60s) and overall poor support between mirror and camera. To try and remove mirror flop you'd need to take the primary cell down and put some low-modulus silicon (black!) around the points of contact between mirror and cell but in a way not to create stress at either low temperatures or high temperatures which is easier said than done, to be fairly honest.

Honestly, I looks like you bumped the mount or something changed suddenly during the exposure. Could eve be a cable snag I suppose. It's certainly not an optical defect.

Is this your only telescope? try to do a 20-30 second unguided exposure if the stars are round it is not the mount, in the past I had a similar problem it was the mount, just purchased, solved by sending the mount for assistance, otherwise perhaps a completely incorrect collimation, the defect is uniform across the entire frame, so it is not tilt. Also try a bathinov to check the focus, to see if the eaf worked properly.

masluigi:
Is this your only telescope? try to do a 20-30 second unguided exposure if the stars are round it is not the mount, in the past I had a similar problem it was the mount, just purchased, solved by sending the mount for assistance, otherwise perhaps a completely incorrect collimation, the defect is uniform across the entire frame, so it is not tilt. Also try a bathinov to check the focus, to see if the eaf worked properly.

i have a RC6 with the same Mount/Camera/Filterwheel-Combination. The images are tack sharp:

in my opinion the 250pds which weighs 12 kg, plus being very long it unbalances the setup and with a very light wind it is unusable, plus the setup which will weigh another 2 kg is too much for that mount, also the focal length of the guide is too low better an oag, it seems like a backlash of the mount try to balance it perfectly  I would not go above 11 - 12 kg all included. first change the guide then balance. the rc weighs less than half … and being compact it balances better.

I have a SW 250PDS which I use for visual.  Just recently (just over a week) I thought Id give it a whirl thinking that it was way too long, camera gear way too heavy and cumbersome on my SW AZEQ6 etc.  Same horse different jockey compared to an EQ6R.  Well I was surprised just how well it tracked.  Far better than my AskarFRA500.  Im using a ZWO2600 mono, FW, OAG-L, 174mini and televue parracor.  I do have a TS Focusor as the original could not handle the parracor and an ethos.  Only drama I have had was double spikes like yours.  Was out of focus.  The stars though were elongated due to the focus unlike the photo above.  I now have a mirror mask and the stars are fine.  I also thought that short subs would be a go…pushed them to 600sec on the Helix.  Mount is fine.

Definitely not tilt.

Hi Guys!

yesterday I had the opportunity to test the Newton again:

- balanced the tripod
- balanced the mount (with counterweight bar extension)
- Polar aligned
- run autofocus routine (double-checked with bahtinov mask)
- calibrated guiding

First Test-Object was "Altair" (60s, guided) - the image still shows the same problem:


After that i adjusted one screw at the back of the primary mirror cell i got this:

"Markab" (30s, guided)


Next object was "Tarazed" (30s, guided)


What do you guys think? Is it getting better?

Thanks!

Hi Tony,

for Markab the Guiding-Graph looked like this:


and on Tarazed it looked like this:


Looks actually quite normal.

Given your graphs I still think it's guiding. The RMS numbers match up with your images. Although your RMS numbers are ok you need to look at the excursions on the graph. For Markab you have a 2 arc/sec spike and in general your first guiding graph needs to be smoother. If you showed both graphs at the same scale you'd see how rough your graph for Markab is. I have an EQR-6 pro and I know it's much better than this. I typically get an RMS error of 0.40 to 0.35 arc/sec RMS, I've even seen it dip down to below 0.3 in periods of really good seeing but the main difference is I get a much smoother graph.

Best advice is to look at everything. I don't use the AIR but I understand it uses a variant of PHD2 for guiding. Check your balance and make sure it's reasonable. Go back to setting up your guiding using the default values, do all the usual calibrations. Double check your system for possible movement of the guide scope relative to the imaging scope. Bottom line is at 1200mm, your guiding just has to be smoother. There's not a lot of room for error. I think you can feel confident that It's not tilt or anything to do with the primary shifting around.