Should I downsize my refractor?

[Mr_42tr0nomy]

Should I sell my W.O gt102mm for the W.O gt81MM?

This is a question I have been playing over and over in my head as of lately. I have a 102mm refractor from William optics that I use for imaging. I have recently come to the conclusion that a wider field of view would be more appreciated. I do use the FF/focal reducer.

Considering that my resolution would be 3.4"/pixel with an 81mm WO (which would be up from 2.33"/ pixel with a 102mm), would this be an okay swap?

3.4"/pixel sounds a bit high. Let me know your opinions so I can sort out any confounds


I dont know, just a thought!

[ollypenrice]

Obviously you lose resolution at 3.4"PP but you are certainly not in the realm of square or blocky stars. All of the widefeild images coming from our setups are shot at 3.5"PP. It's in the natrue of widefield targets, generally, to be about breadth and vista rather than fine detail so I think it works out fine. Where there are parts of an image which would benefit from finer resolution we blend in longer FL detail but that's a luxury. It only refines an image, it doesn't transform one. I wouldn't go below 3.5 but 3.5 is OK for me. Some examples without any long FL overlay shenanigans:

You should be able to click on the image and see a larger one using the button lower left.

Olly

[Pingster]

OMG Off topic... the red target... you cant even see that in the eyepiece.... so how on earth do you focus so well on it?

[DaveS]

You don't. You focus on a nearby brightish star then slew to the target and run your subs.

[ollypenrice]

2 hours ago, DaveS said:

You don't. You focus on a nearby brightish star then slew to the target and run your subs.

In fact it's only in ultra narrowband Ha (3 nanometre) that it might be hard to find a focus star in that target. I can't remember if we needed to slew away to a brighter star or not. In the other filters it is easy to use Full Width Half Max (FWHM) measurements to focus. You never focus on nebulosity, only stars, as you say.

Olly

[DaveS]

Ah yes, I guess I said brightish as I do use 3nm filters. Last time I worked on the Rosette I used either Betageuse or Procyon so I could still use short exposures with a B mask.

[Pingster]

Wow thanks for the exsplaination. Stunning photo as always olly

So just to clarify. In the close up of the nebular.... none of the stars in that picture are visible at all in the eyepiece? And its done with an 80mm to 100mm APO. Amazing.

[ollypenrice]

11 hours ago, Pingster said:

Wow thanks for the exsplaination. Stunning photo as always olly

So just to clarify. In the close up of the nebular.... none of the stars in that picture are visible at all in the eyepiece? And its done with an 80mm to 100mm APO. Amazing.

No, lots of the Rosette stars are visible at the EP. The cluster in the darker centre of the nebula always reminds me of the 6 pattern on a domino. That's what I look for in the finder.

Olly

[ronin]

Back to the topic: Personally I would keep the GT102. Don't think you would really gain that much and might lose, in others aspects, more then you gain.

However if someone said would I swap my GT-81 for a GT-102 I likely would not (I do like the 81), but if I was given a GT-102 as well as my GT-81 I would be happy.

[ollypenrice]

Have you modelled the two fields of view on a planetarium? That will show you what you'd gain.

Olly