OTA buying advice

[Penguin]

Hi guys, I'm after a little advice on what sort of OTA I should go for.

TL,DR: Advice wanted on the best type of used OTA at around £500 to go an EQ6-Pro for mainly deep sky photography.

I have recently bought a second-hand EQ6-Pro and am currently trying to sell my HEQ5-Pro + Explorer 150p to recoup some of the EQ6 cost / fund a bigger OTA. I fancy an 8" Schmitt or Maksutov but not for any good reason other than that they look cool! What are they each good for and what are the downsides of them? I don't think I should go for a refractor as they would be too long for the proportions of my observatory. I'd expect to have around £500 to spend and I am very much thinking of used rather than new. Should I stick with a Newtonian? I mainly do mediocre photography of galaxies, clusters and nebulae. but would also be interested in Jupiter, Saturn and Mars.

[newbie alert]

SCT do have their own weaknesses for deepsky.. mirror flop, mirror shift, if not using a oag then differential flexure, cooling to ambient, is your camera sensor too large for the scopes imaging circle, you may need a reducer for fov and less taxing on the mount.. lots to consider before jumping in 

But they have a good few positives and very rewarding when the images come in

[Penguin]

Thanks, doing a bit of Googling, which I should have done before bothering you guys, all the sites point out the longer focal length and smaller field of view and some say this rules them out for deep sky but other say they are quite versatile (possibly with a focal reducer).

I currently have one of the small guide scope/guide camera kits from FLO so that would be going on it, so not an OAG. What is Differential Flexure? what is it that is flexing in this scenario. Likewise mirror shift, is this just the need to regularly collimate? I'm happy doing that with my 150P, is the process much different with a catadioptric?

My imaging camera is an unmodified Canon 450D so pretty basic. I might go to an ASI colour CCD at some point.

Sounds like they could be a good choice, I'd like to be able get in a little closer to the Crab and Ring nebulae for example, and the planets. But I'm not intending to rush into things!

[newbie alert]

First of all Id like to say I'm not trying to put you off, I think SCT for deepsky are awesome.. had mine on the mount for 18 months now and it serves me well, but..

The whole setup of a SCT has a few weaknesses.. the mirror moves up and down a shaft to achieve focus, the mirror is big and heavy and so with gravity the mirror moves, hence the term mirror shift and mirror flop..The idea of a oag is if it shifts the whole imaging train also does.. if you have a separate guide scope setup the guide camera could be happily guiding on a star and it won't see that the mirror in the imaging has shifted, this is differential flexure... Mirror flop is where the mirror has moved down the shaft under the influence of gravity.. some SCT have mirror locks to hopefully reduce this problems

I think oag is the way to go, I tried the separate guidescope but that made issues within itself..

Collimation needs to be pretty much spot on as with any scopes, but again is fairly straightforward but then quite easy to get it wrong too

I've not tried the crab, but the ring nebula is a fairly decent size, not a frame filler, although the crescent does at 1280mm fl

 

[Adam J]

14 hours ago, Penguin said:

Thanks, doing a bit of Googling, which I should have done before bothering you guys, all the sites point out the longer focal length and smaller field of view and some say this rules them out for deep sky but other say they are quite versatile (possibly with a focal reducer).

I currently have one of the small guide scope/guide camera kits from FLO so that would be going on it, so not an OAG. What is Differential Flexure? what is it that is flexing in this scenario. Likewise mirror shift, is this just the need to regularly collimate? I'm happy doing that with my 150P, is the process much different with a catadioptric?

My imaging camera is an unmodified Canon 450D so pretty basic. I might go to an ASI colour CCD at some point.

Sounds like they could be a good choice, I'd like to be able get in a little closer to the Crab and Ring nebulae for example, and the planets. But I'm not intending to rush into things!

I would make the assumption that you will end up with a camera eventually in the 3.75um pixel size range and hence that if you want to optomise for detail you will want to have sufficient focal length to enable about ~1-1.5 arcseconds per pixel. So something like a 700mm focal length scope is about what I would be aiming for with a mono camera. Seeing is unlikely to allow higher resolution irrespective past about that point.  So honestly I would probably just switch out the 150P for a 150PDS and maybe get a ASI585mc  / ASI533mc or mm Pro to go with it if it was me and I wanted to image smaller targets depending on budget. 

Adam 

Edited August 15, 2022 by Adam J

[Shibby]

An SCT, I suggest, has an unnecessarily long focal length for deep sky imaging. I personally moved up from a 150P to a MN190 and have not regretted that decision; second hand they can be purchased (as I did) for around your budget, perhaps a little more now.

[newbie alert]

21 minutes ago, Shibby said:

An SCT, I suggest, has an unnecessarily long focal length for deep sky imaging. I personally moved up from a 150P to a MN190 and have not regretted that decision; second hand they can be purchased (as I did) for around your budget, perhaps a little more now.

MN190 has 1000mm FL? ..1m tube 

A 8 inch SCT you can reduce its native focal length with a 6.3 reducer, some people run there's at f5 , but it depends on sensor size if you can get away with it

 

[Shibby]

14 minutes ago, newbie alert said:

MN190 has 1000mm FL? ..1m tube 

A 8 inch SCT you can reduce its native focal length with a 6.3 reducer, some people run there's at f5 , but it depends on sensor size if you can get away with it

 

I can't argue that an SCT isn't versatile. But if you're just planning on deep sky imaging at shorter focal lengths, the Newtonian is going to be a less complicated affair. Not impossible with the SCT by any means, of course, but more of a challenge. This is backed up by many threads from other forum users.

I'd also like to add that short-tube refractors are also great for imaging, even simpler, and not too long for your observatory @Penguin

[Elp]

+1 for short FL refractors, much easier (sharper more contrast) than SCT imaging, in my experience anyway. Newtonian fits in between them.

[newbie alert]

16 minutes ago, Elp said:

+1 for short FL refractors, much easier (sharper more contrast) than SCT imaging, in my experience anyway. Newtonian fits in between them.

No central obstruction in a frac

The bigger the central obstruction the lower the contrast

[newbie alert]

37 minutes ago, Shibby said:

I can't argue that an SCT isn't versatile. But if you're just planning on deep sky imaging at shorter focal lengths, the Newtonian is going to be a less complicated affair. Not impossible with the SCT by any means, of course, but more of a challenge. This is backed up by many threads from other forum users.

I'd also like to add that short-tube refractors are also great for imaging, even simpler, and not too long for your observatory @Penguin

Yeah totally agree with you there, but penguin mentioned planets aswell... Not that you can't do that with a newt either..

[Varavall]

On 14/08/2022 at 14:50, Penguin said:

Should I stick with a Newtonian?

Well my twopenny worth is go with a SW 200pds and a 2.5x o 5x powermate for the planets. Works for me! However you would also need a guide camera and scope and possibly a coma corrector. My first solution was a QHY5L-ii C used with the 9x50 finder. Works well and the camera should cost around £75 secondhand . 

 

On 14/08/2022 at 14:50, Penguin said:

I mainly do mediocre photography

We all started that way! My journey started with an unmodified Nikon D5300. Just practice and as you get better invest more money when you have it for electronic focuser, etc.