Getting started with Newtonian telescopes

[OK Apricot]

Not sure if this is the correct sub forum so mods feel free to place it somewhere more suitable if need be.

Recently bought a 250P Quattro and have been dying to get it outside to see what's what. I just wondered what exactly I'm seeing here, normal or not? I took a few 10s shots of Alkaid. In focus (or close to it - I was rushing as thicker cloud was coming in) looks OK. Out of focus I see it needs collimating - Fair enough. In past focus there are 6 "imprints" within the halo of the star. Pinching from the mirror clamps? Is it normal? Is it something else? Also managed a quick 120s shot of M51, which, while guide stars were visible, surprised me. It was breezy and this thing definitely has some surface area to pick it up, I had 10kg of counterweight on the EQ6R extension so not ideal, and the guide "star" did not resemble one in the slightest. This is fine - A counterweight is on the way and I'm fairly confident in sorting the OAG. Pleased enough from the first test. Under strenuous conditions, it can produce round stars and a decent sub frame.

What can you experienced guys/girls tell me about this?

Cheers!

[900SL]

Maybe mirror clips

Is the focuser tube projecting into the light path at focus? 

[wimvb]

What you see around the single star is the diffraction caused by the mirror clips. To get rid of this, you'll need a mirror mask, a ring which you attach on top of the clips, and which covers them. This will decrease the effective aperture by about a cm, but that isn't necessarily a bad thing. You can 3D print your own mask, or buy one.

https://www.firstlightoptics.com/misc/backyard-universe-primary-mirror-mask-for-sky-watcher-250mm-newtonian-telescopes.html

Regarding the need to collimate, the Quattro is a fast newtonian where the secondary mirror is offset. This may mean that the secondary mirror holder is offset. If so, the out of focus doughnut will not be symmetrical. Another Quattro owner should be able to clarify this. (The alternative is that the holder is mounted symmetrically, but the mirror is offset on its holder.)

[OK Apricot]

Thanks chaps, makes sense.

It doesn't look like the draw tube is protruding into the light path. I've also had a more concerted look through the bare draw tube and a Cheshire and I'm fairly confident the secondary is not concentric with the draw tube so there are adjustments to be made there. 

A mirror mask is in my cart along with a few other things. I feel like I'd like to learn to collimate properly with a Cheshire so will hold off on the OCAL for now I think. 

[wimvb]

No need for an OCAL, in my opinion. A cheshire for the secondary, and a barlowed laser (I use the cheap one that came with my scope), is all you need.

[Elp]

I don't think you even need all that. All I used was a collimation cap when I had my 130pds.

[Spile]

Sort out the offset errors (if there are any) first and then correct the tilt/rotation error. All done with your Cheshire eyepiece and sight tube combination tool.

[Spile]

4 minutes ago, Elp said:

I don't think you even need all that. All I used was a collimation cap when I had my 130pds.

Yes I can get close with just a cap. However…

The cross hairs mean it’s easier with the Cheshire eyepiece and the long sight tube makes it easier to confirm the outer edge of the secondary is concentric. 

[OK Apricot]

It's a while since I had my 200P dob and after a few attempts felt like I was getting somewhere with it, but not sure I understood properly.  I might need some more simpleton tuition 😏

Make outer edge of secondary concentric with inside edge of focus tube. Make full primary mirror visible in secondary. Align primary centre dot with Cheshire crosshair. 

Something like that? I am reading and watching about it again and feel like those are condensed steps to get there. 

[OK Apricot]

Managed to get out tonight to get a proper test image - just over an hour's worth of 60s subs. Ignoring image aberrations, I'm loving the reach! Having only shot with 274mm and 514mm focal lengths, it's great to get up closer on galaxies! Anyways, I've not yet sealed the scope against light leaks, flocked the tube, or collimated properly. Here is a very quick process in DSS and Photoshop:

 

 

Most obvious is the diffuse rainbow like ring, almost perfectly centred in the image. Firstly, as you can see from one of my pictures a few posts up, I unfortunately have a bright street lamp high above my garden. Even though I've shielded it myself as best I practically and legally can, I'm thinking that maybe stray light from that is falling on the front corrector lens and the camera is picking it up. Shooting M51, the scope is pointed loosely in the vicinity of the lamp. My primary isn't yet shielded so there could be spilled light coming in here too. Second, I see vignetting, or is this a trick on the mind due to the rainbow circle? I use an IMX533 sensor so doubt that it's actually inadequate field illumination? Third, it's a bit out of focus - Not fitted my EAF yet and haven't got a bahtinov so it was just a best guess. There was trailing all in one direction so I'm going to say this is down to the slight breeze catching the tube and making the mount struggle. Guiding was only around 0.95" for the short session. Otherwise the shape of the stars is consistent so I'd like to think that I'm pretty much spot on with backspacing. Does this look about right considering the handicaps?

[AstroMuni]

12 hours ago, OK Apricot said:

Does this look about right considering the handicaps?

Thats a great image. Several of the issues you mention above should get sorted if you run a background extraction in Siril or use GraXpert.

[OK Apricot]

4 hours ago, AstroMuni said:

Thats a great image. Several of the issues you mention above should get sorted if you run a background extraction in Siril or use GraXpert.

I did try a few different combinations of Gradientxterminator and reverse vignette in camera raw filter but no real joy. 

[alacant]

On 14/04/2024 at 01:13, OK Apricot said:

Quattro 

 

Edited April 16 by alacant
Bad translation.

[alacant]

On 14/04/2024 at 01:13, OK Apricot said:

250P Quattro

A proper telescope. Nice!

Excellent m51. The aperture makes it a lot easier, but did you take flat frames? They're not really optional and you'll find it much easier to process. 

Cheers.

Edited April 16 by alacant

[OK Apricot]

No flat frames taken for this, nope. If I understand the cause of the ring I can understand how to remedy it, but at the moment I'm not sure. This cam has been used many times on my smaller rig from the garden and I've always managed to produce something decent even without flats. I guess the only thing to do is try things and see what happens. 

[alacant]

On 16/04/2024 at 14:58, OK Apricot said:

how to remedy

Loafsa stuff to do to a budget Newtonian to bring it up to imaging standard. All of it quick, cheap and easy.

As well as take flat frames, to address the ring artefact you could start with... velvet lined tube and focuser. Shower cap covering the 1° end of the tube, darken the edges of the secondary, tape the focuer drawtube... Some would advise a dew shield, but since you have a 800mm model already fitted, any longer may be overkill.

The next stage is modifying the the tube and its elements so that collimation is retained at all tube angles.

The next stage, correction of the star shapes etc etc. It just depends upon where you decide enough is good enough.

Cheers and HTH

Edited April 18 by alacant

[ollypenrice]

You absolutely do need flats. They are truly at the heart of things in AP.

Olly

[OK Apricot]

A3 tracing panel it is - eye-watering prices for flat field generators at FLO! I'm wondering if this is reflection of stray light, well when doing flats the panel will then block that light and that light won't be there to correct, and instead will have the opposite effect in stacking... If that makes sense? How best to do flats in the mean time with this? White t-shirt over the aperture and pointed at a white light? 

Edited April 17 by OK Apricot

[Clarkey]

3 hours ago, OK Apricot said:

White t-shirt over the aperture and pointed at a white light? 

Probably your best option for now. White T-shirt and overcast sky should work - and it is unlikely you will need to wait too long🤣

[ONIKKINEN]

You can also take flats indoors by pointing the scope at an evenly lit wall or a tv showing a white screen (and a t-shirt).

I did the latter a few years back and it worked ok.

[OK Apricot]

Naturally I'm going to struggle to bring my whole rig inside to point it at the telly, it's got to be in the region of 60kg, don't fancy that with a bulging disc. Could I use the t-shirt and the street light? It's a bright light source albeit more "orangey" than white? 

[wimvb]

1 hour ago, OK Apricot said:

Naturally I'm going to struggle to bring my whole rig inside to point it at the telly, it's got to be in the region of 60kg, don't fancy that with a bulging disc. Could I use the t-shirt and the street light? It's a bright light source albeit more "orangey" than white? 

Unless the telly is an antique, it may be easier to take it out to the scope.

Not completely unrelated; many years ago a colleague built his own large format camera around the lense from an airplane surveillance camera, a really massive piece of glass. The camera was so heavy that he mounted it on a truck to do nature photography. When he told me that, he sighed that it would be easier to keep the camera indoors, and take nature to the camera.

More related: a street light, even with t-shirts isn't good for flats. A street light is more of a point source than a flat surface.

Edited April 17 by wimvb